Chapter 13
Communal and closed-circuit TV systems
If all the tenants in a block of flats had their own TV aerial on the roof of the building,
the result would be very unsightly. In the case of a 24-storey tower block having 4 flats
on each floor, the roof would probably not be large enough to accommodate 96 separate
aerials. Even a low-rise development looks ugly if every house has its own aerial;
sometimes a four-storey development consists of two flats and a maisonette above each
other so that each ‘house’ would need three aerials. In fringe reception areas, one needs
large aerials mounted very high up. This makes it difficult to equip each dwelling with its
own aerial. In built up areas, large buildings shield smaller ones, so that if an estate
consists of a number of small blocks and one or two towers, occupants of the small
blocks left to provide their own aerials might find it desirable to put them on masts rising
as high as the top of the tower block.
For these reasons, it is an advantage to receive television and radio signals at one
suitably sited aerial array and relay them to individual dwellings by cables or
transmission lines. Very large relay systems exist, serving whole towns, sometimes from
a mast receiver several miles away. The building services engineer is more likely to be
concerned with community systems serving a single block of flats or one small estate of
houses and maisonettes. In this book, we confine our attention to such community
systems.
Radio signals are electro-magnetic waves in space. They cover a range of frequencies
which are classified as shown in Table 13.1. Since the product of frequency and
wavelength is always equal to the velocity of light, which is constant, the frequency is
inversely proportional to the wavelength. Radio broadcasts have in the past usually been
identified by the wavelength, but at the frequencies used for television the wavelength
becomes so small that it is more convenient to use the frequency. The properties of
aerials and transmission lines depend very much on frequency and different types have to
be used for different frequencies. It is convenient to subdivide the VHF and UHF
frequencies into five bands, which makes it possible for commercial equipment to be
manufactured for one or two selected bands only. This subdivision is shown in Table
13.2.
Table 13.1 Frequency bands
Designation Abbreviation Frequency range
Low frequency LF 30kHz−300kHz
Medium frequency MF 300mHz−3MHz
High frequency HF 3MHz−30MHz
Very high frequency VHF 30MHz−300MHz
Ultra high frequency UHF 300MHz−3000MHz
Super high frequency SHF 3000MHz−30000MHz
Table 13.2 Broadcasting services
Range Band Channel numbers Frequency Service
LF – – 150–285kHz AM sound long wave
MF – – 535–1605kHz AM sound medium wave
HF – – 2.3–26.1MHz AM sound short wave
VHF I 1–5 41–68MHz TV Band I
II – 87.5–100MHz FM sound (VHF)
III 6–13 175–215MHz TV band III
UHF IV 21–34 470–582MHz TV band IV
V 39–68 614–854MHz TV band V
If a signal consists of one frequency only, the only way it can convey information is by
varying in amplitude. As soon as several adjacent frequencies are present, they can
combine to form complicated waveshapes and the total number of distinguishable
patterns increases rapidly. This is a simplified explanation of why the band of frequencies
required for a transmission increases as the amount of information to be conveyed
increases. Television provides much more information than sound broadcasting, and
therefore each service requires a large band-width. For sound broadcasting, each station
needs a bandwidth of only 10kHz. A station broadcasting on 1500m (which corresponds
to 200kHz) actually uses all wavelengths between 1457m and 1543m. Provided the next
station has a nominal wavelength of 1587m or more there will be no interference between
them; it is obvious that there is no difficulty about keeping stations separate from each
other under these conditions.
A 625 line TV picture, on the other hand, requires a bandwidth of 5.5MHz. It is
immediately obvious from Table 13.1 that this cannot be transmitted at less than HE and
that the HF range would only accommodate five different stations. This is the reason that
TV is transmitted in the VHF and UHF ranges. Even within these, care has to be taken
about separation of stations. The five bands of frequency are, therefore, further divided
into a number of channels each of which covers a bandwidth of 6MHz. These channels
Communal and closed-circuit tv systems 189
are also indicated in Table 13.2. Each station is allocated one channel, and neighbouring
stations are thus prevented from interfering with each other.
Since the distance over which VHF and UHF waves can be propagated is quite
limited, two stations more than a certain minimum geographical distance apart can safely
use the same channel.
Aerials
If an e.m.f. is placed in the centre of a short cable (Figure 13.1), the two halves of the
cable act as capacitor plates, one becoming positively charged and the other negatively.
Each charge produces an electric field. Suppose now that the e.m.f. is alternating; there is
then an alternating charging current in the cable. When the current is a maximum the
positive and negative charges occupy the same place and produce equal and opposite
fields. When the current is zero the positive and negative charges are at opposite ends of
the cable and produce a resultant electric field. Thus there is an electric field which
alternates with the charging current in the cable.
The current also produces a magnetic field which spreads out from the cable with the
velocity of light. The motion of this magnetic field induces a further electric field.
Now the oscillating charges in the cable have not only a velocity, but also an
acceleration. This acceleration is propagated outwards in the electric field at a finite
velocity and, therefore, the field further out is moving with a lower velocity than that
closer in. Since the charges oscillate the acceleration is alternately forward and backward,
and the result is that the complete field radiated forms closed loops which travel out from
the cable and expand (Figure 13.2). It can be shown that this radiated field is appreciable
only if the length of the cable is of the same order as the wavelength.
Figure 13.1 Principle of dipole
Design of electrical services for buildings 190
Figure 13.2 Radiated field
The total electric field thus contains three terms. The first two are the induction fields and
the third is the radiation field. Near the aerial the induction fields predominate, but they
become negligible at a distance greater than about five wavelengths. At larger distances,
the radiation field is the only important one.
By a converse mechanism, a cable placed in an alternating electric field and suitably
orientated to that field will have an e.m.f. induced in it. In fact, it turns out that receiving
aerials are identical to transmitting aerials. In practice, receiving aerials can be made
simpler than transmitting aerials because high efficiency is not so important when
receiving as when transmitting.
The simple aerial of the type shown in Figure 13.1 is known as a dipole and its total
length is half the wavelength radiated or received. The description given above is an
attempt to explain in simple physical terms how a dipole radiates and receives. Any
electric current is associated with a magnetic and an electric field, the relationship always
satisfying Maxwell’s equations. The field radiated by an aerial is thus the solution of
Maxwell’s equations for the boundary conditions given by the current distribution in the
aerial and the geometry of the aerial. The mathematical difficulties of calculating such
solutions are so great that theoretical solutions are not given in even the most advanced
textbooks on radio propagation, and aerials are designed on the result of experimental
investigations. The general principle remains that the dimensions of the aerial are
approximately equal to the wavelength. It follows from this that different aerials are
required for different frequency bands.
Aerials are used for services ranging from telegraphy through navigation, broadcasting
and telephony to radar and space communications. There is a very large number of types
of aerial in use to cover the wide range of
Communal and closed-circuit tv systems 191
Figure 13.4 Strengthening elements
applications and frequencies employed, but the following are the types most likely to be
encountered by the building services engineer. They are all simple variations of the basic
dipole described above.
For band I, a single dipole as shown in Figure 13.3a can be used for reception at a
short range from the transmitter. If the receiver is some distance from the transmitter a
modification is introduced to increase the strength of the dipole. It has been found that the
addition of elements not connected to the receiver cable can strengthen the signal at the
dipole. An element slightly longer than the dipole and spaced about a quarter of a
wavelength behind it has the effect of reflecting waves onto the dipole and thus
strengthening the signal to it. This is illustrated in Figure 13.4 and results in the well
known H aerial shown in Figure 13.3b. A modification of this is the X aerial, also shown
in Figure 13.3b. The principle is illustrated in Figure 13.5 from which it is seen that the
dipole and reflector are partially folded on themselves. It has been found that they then
operate like a straight pair of elements occupying the mean positions shown dotted in
Figure 13.5.
Further improvement in reception can be obtained by adding directors in front of the
dipole at distances of about a quarter of a wavelength, and progressively shorter than the
dipole (Figure 13.4). It is found that the effect of these is to increase the sensitivity of the
dipole to radiation from the direction in which the array is pointing. An arrangement such
as that of Figure 13.4 is known as a Yagi array.
For long-range reception on band I, an aerial consisting of dipole, reflector and
director (Figure 13.3c) is used.
As explained later in this chapter, maximum power is transferred from a source to a
load when their impedances are equal. The half wave dipole has an impedance of about
75ohms, which matches the characteristic impedance of the coaxial aerial cable used.
When other elements, that is to say reflectors and directors, are added the impedance of
the aerial is reduced, and much of the power received is lost at the mismatch between
aerial and cable. This can
Communal and closed-circuit tv systems 193
Figure 13.5 Aerial
be overcome by folding the dipole, as shown in Figure 13.6. It is still half a wavelength
long but its impedance is higher, and when it is used in an array its impedance matches
that of the cable. Yagi arrays, therefore, generally use a folded dipole.
Similar aerials are used for the reception of FM radio on band II, but they are put
horizontally instead of vertically. This is simply because the radiation broadcast for this
service is polarized in the horizontal plane whereas that for television services is
polarized in the vertical plane. The same types of aerials are used for band III, but
because of the shorter wavelengths the strength of the signal decreases more rapidly and,
therefore, a larger number of elements is used in the array. Typical arrays are shown in
Figure 13.3d, e and f, and combined band I and band II aerials are shown in Figure 13.3g,
h and j.
At UHF the range of transmitters becomes much less, although there is less
interference between neighbouring stations. Therefore, the receiving aerials have a larger
number of elements. These are readily accommodated because as a result of the smaller
wavelength they are shorter and more closely spaced. In fact a six-element UHF array
can present a neater and more compact appearance than a single dipole for band I. Also
as a consequence of the shorter wavelength, the reflector can more effectively take the
form of a square mesh. Typical arrays are shown in Figure 13.3k, l and m.
Figure 13.6 Folded dipole
Design of electrical services for buildings 194
Figure 13.7 Polar diagrams
All the aerials we have described are directional. The voltage induced in them depends on
the angle between the axis of the array and the plane of the wave of radiation. It is a
maximum when the axis of the array is perpendicular to the wavefront and is zero when
the axis of the array is parallel to the wavefront. The sensitivity can be represented by the
length of a line drawn in the direction of the advancing wavefront. The locus of the ends
of all such lines forms the polar diagram. This is illustrated in Figure 13.7.
Greater sensitivity in the axial direction can be obtained by altering the shape of the
polar diagram as shown in Figure 13.7c. One way of achieving this is to have a pair of
Yagi arrays mounted side by side in a broadside arrangement, as shown in Figure 13.8. It
will be appreciated that in all cases
Figure 13.8 Twin arrays
the axis of the aerial array must point as closely as possible towards the transmitter.
Communal and closed-circuit tv systems 195
Transmission lines
The signal received by the aerial is sent to the television outlets along an aerial cable or
television transmission line. The design of the line is an important part of the design of a
communal TV system, and we must learn something about transmission engineering to
understand it. Transmission engineers are very much concerned with loss of power,
which is usually measured in decibels. A decibel is one-tenth of a bel, and a bel is the
logarithm to base 10 of the ratio of two powers. If the power at the sending end is Ps and
that at the receiving end is Pr the loss of the line is
The decibel is convenient because of the very large losses and amplifications encountered
in communications engineering; for example, if an amplifier has an output 10000 times
the input it is more convenient to say that is has a gain of 40dB. Since the gain, or loss, in
decibels is a ratio, the input level should also be stated.
Power ratios are proportional to the square of the voltage or current ratios. Therefore:
Thus when measurements are made in volts or amps the loss is
If the logarithms to base ‘e’ are used instead of to base 10, the unit is the neper instead of
the bel. This is not used so often, but it is sometimes convenient because the attenuation
of a transmission line per unit length is always a power of ‘e’.
There is a loss of power in all transmission lines. At very low frequencies this is
largely due to the resistance of the line, although inductance and capacitance are
important for exact calculation of long power lines. At high frequencies inductance and
capacitance become much more important, and it can readily be shown that the losses
increase rapidly with frequency. At frequencies above 3000MHz the losses in cables are
so high that transmission by cables is no longer possible and the only way of conveying
energy at these frequencies is by waveguides.
The power engineer distributing power at 50Hz is interested in supplying the power
taken by the load with the minimum loss in the line. The communications engineer
sending signals to a receiver has a rather different outlook. Here much smaller quantities
of energy are being handled, and amplifiers can be installed along the line which feed in
energy from an independent source without distorting the wave shape of the signal. The
main concern is to provide a strong enough signal to the receiver at the end of the line. It
can be shown that maximum power is taken by a load when that power equals the power
lost in the generator. Although at this condition maximum power is taken by the load, the
efficiency of the transmission is only 50 per cent. This is clearly uneconomic for power
Design of electrical services for buildings 196
transmission but is practicable in telecommunications where the magnitude of the signal
is much more important than transmission efficiency. The difference between the
operating points of power and communications systems is shown in Figure 13.9.
Figure 13.9 Operating points of
transmission lines
When an alternating voltage is applied to the sending end of an infinite line, a finite
current flows because of the capacitance and leakage inductance between the two cables
forming the line. The ratio of voltage applied to current flowing is the input impedance.
The input impedance for an infinite length of line is known as the characteristic
impedance and is denoted by Zo. It should be noted that the characteristic impedance
varies with frequency.
Since the line is infinite, no current waves reach the far end. Therefore, there is no
reflection and no reflected waves return to the sending end. For the same reason, the
current flowing depends only on the characteristic impedance (Zo) and is not affected by
the terminating or load impedance (Zr) at the far end. Although an infinite line is
obviously a purely hypothetical object, in practice the state of affairs we have just
Communal and closed-circuit tv systems 197
described is approximately fulfilled by many long lines. Furthermore, a short line
terminating in a load impedance equal to the characteristic impedance of the line (i.e.
Zr=Zo) behaves electrically as if it were an infinite line.
The characteristic impedance is the ratio of voltage to current at any point in an
infinite line or in a correctly terminated line. However, the current and voltage are not the
same at all points; because of the ordinary impedance of the line, they become
progressively less along the line.
Let
Is=current at sending end
Il=current one kilometre down line
Then where y=propagation constant per kilometre of line.
y is a complex quantity so that Il is both less than Is and also different in phase. In
general,
In=Ise–ny where In=current n kilometres along the line.
Similarly, En=Ese–ny
y is a complex quantity which can be written y=α+jβ where α is the attenuation
constant and β is the phase constant.
The four quantities:
Zo=characteristic impedance
y=propagation constant
α=attenuation constant
β=phase constant
are characteristic of the particular cable being used. They are known as the secondary
line constants and can be calculated theoretically from the four primary line constants
which are
R=resistance per kilometre (ohms)
G=leakage per kilometre (mhos)
L=inductance per kilometre (henries)
C=capacitance per kilometre (farads)
Whilst the primary constants are independent of frequency the secondary constants in
general vary with frequency.
Design of electrical services for buildings 198
Figure 13.10 Travelling and standing
waves
Now a communication signal carries information in its waveshape. It is most
important to preserve this shape while the signal is being sent along the line. There are
three main causes of distortion:
Communal and closed-circuit tv systems 199
1 Characteristic impedance varies with frequency. If the line is terminated in an
impedance that does not vary with frequency in the same manner as that of the line,
distortion will result.
2 Attenuation varies with frequency.
3 The velocity at which the wave shape travels along the line varies with frequency, so
that waves of different frequencies arrive at different times.
It can be shown that the condition for minimum attenuation is LG=CR. It can also be
shown that this condition makes Zo independent of frequency. This is called the
distortionless condition. On some communication lines the inductance is artificially
increased to bring the line nearer to this condition, but this is not found necessary on
communal TV systems.
If a line of characteristic impedance Zo is joined to an impedance having a value other
than Zo part of the wave travelling down the line will be reflected back at the point of
discontinuity. The reflection is a maximum when the line is either open circuited or short
circuited (Zr=∞ or Zr=0) and is zero when Zr=Zo The current in the line is always the sum
of the incident wave and the reflected wave.
This way of looking at matters will seem unfamiliar to power engineers. The rigorous
mathematics of transmission lines is the same for power lines as for communication lines,
but differences arise from differences in the frequencies at which they are operated. At
50Hz a line 500km long is less than one-tenth of a wavelength. Although the stationary
distribution of current along it is mathematically equal to the sum of two waves travelling
in opposite directions, this fact is of only academic interest to the power engineer. At
3000MHz, however, the wavelength is only a few metres and the two travelling waves
have a physical meaning which is easily visualized.
At the sending end, the current can be represented by a phasor of length Ismax rotating
at an angular frequency w where w=2πf and f is the frequency of the applied voltage. At a
point x down the line, the current Ix differs from Is because of the attenuation of the cable
and can be represented by a vector of length at an angle of βx to
the original phasor, but still rotating at an angular frequency of w. The instantaneous
current along the line at successive intervals is then as shown in Figure 13.10a. For
simplicity in drawing this figure has been taken as zero, so that the current is the same
in magnitude along the line. It will be seen that at all times the envelope of instantaneous
current along the line is a sine wave. Moreover, the sine wave moves down the line. The
wavelength is that length at which βx=2π
Since velocity=frequency×wavelength
Design of electrical services for buildings 200
This is the velocity at which the signal travels down the line. It is not the same as the
velocity at which energy is transferred.
A wave travelling in the opposite direction will be as shown in Figure 13.10b. When
the incident and reflected waves are equal in magnitude they combine as shown in Figure
13.10c. We can see that there is a wave on the line of frequency f and wavelength λ=w/β
but it does not travel along the line. Whereas with the travelling wave the rotating phasor
has the same magnitude along the line and varies only in phase along the line, in this case
the vector changes in magnitude along the line. As a result there are nodes where the
current is always zero and anti-nodes where there is a maximum. This type of wave is
called a standing wave, and is produced by the combination of equal incident and
reflected waves. If only part of the incident wave is reflected then there is a standing
wave with a travelling wave superimposed on it. Power is transferred only by the forward
travelling wave, so that standing waves represent a loss. Reflected waves also originate at
junctions where one cable branches into two or three.
Complete reflection occurs when a line is either open circuited or short circuited.
Under these conditions, there is no forward travelling wave, and no power is transferred.
This is readily understandable, because neither an open circuited nor a short circuited line
feeds a load.
For maximum transfer of power the internal impedance of the generator must be
matched to the characteristic impedance of the line and this must in turn be matched to
the input impedance of the load. In the case of the communal TV system, the generator is
the aerial. If the matching is not correct, standing waves are formed in the line, and power
is dissipated in the line. Some of this dissipation takes the form of radiation which causes
interference to neighbouring aerials.
In general, the impedance of the generator is not the same as that of the line, and that
of the line is not the same as that of the receiver. Therefore, at each of these points of
discontinuity some form of impedance transformer is required. Such a transformer can be
made from a four pole network which
Figure 13.11 Impedance transformer
has different input impedances between the two pairs of terminals. In Figure 13.11, the
line with characteristic impedance Zl sees the impedance Zl between terminals a and b
and is therefore, correctly terminated. The load with impedance Zr sees itself supplied
from an impedance Zr between terminals c and d and is, therefore, also correctly
matched. There is a small loss of power in the four pole network, but this is preferable to
the large losses that would occur in the line if the mismatching were permitted to remain.
A similar problem has to be solved where there is a branch. In Figure 13.12a, the input
cable sees the continuation and branch cables in parallel and is in effect terminated by an
impedance of Zl/2. A network has to be provided as in Figure 13.12b, so that each of the
three cables sees an impedance of Zl. There are a number of devices which have the
necessary characteristics, but many of them are sensitive to frequency and operate
Communal and closed-circuit tv systems 201
correctly only over a limited band of frequencies. In communal TV practice, a simple
arrangement of resistances is used, as shown in Figure 13.13. These junction boxes also
perform another function which we shall come to later in this chapter.
Figure 13.12 Branch network
Figure 13.13 Junction box
Cables
Design of electrical services for buildings 202
Figure 13.14 TV cables
Having discussed the theory of a transmission line, we are now in a position to say
something about the cable which actually forms the line. We start by noting that any
cable carrying current tends to act as a radiating aerial. At low frequencies the power
radiated from an ordinary cable is so small that it can hardly be detected, but at high
frequencies it can become significant. Not only is there a loss of power from the line
itself, but the radiation will cause interference in neighbouring receivers. Similarly, any
cable acts as a receiving aerial, and a line feeding a television or radio set can pick up
unwanted high-frequency radiations. Both these effects can be suppressed by efficient
screening, and radio and TV services therefore always use screened cable.
A line may be a single conductor using the earth as a return. This has capacitance to
the earth and is termed an unbalanced line. If a conductor is provided for the return, it can
be arranged so that either the two conductors have different capacitances to earth or that
Communal and closed-circuit tv systems 203
they have the same capacitance to earth. The former arrangement is termed unbalanced
and the latter balanced. Typical arrangements are shown in Figure 13.14.
The energy conveyed by a transmission line is in fact held in the electric and magnetic
fields associated with the current and voltage. In the case of open lines, these fields
extend infinitely into space. At high frequencies the energy is rapidly dissipated into
space, and the losses from the transmission system become unbearably high. If a screen is
placed round the conductors, the fields are confined within the screen and the losses are
reduced. Typical forms of radio frequency cable are shown in Figure 13.15. The cable of
Figure 13.15a has an inner conductor of copper cable and an outer conductor of seamless
lead tube. It is suitable for high frequency transmission and high power aerial feeders.
Figure 13.15b shows a cable with the inner conductors supported in the centre of a tube
of polyethylene and an outer conductor of cable braid with PVC or lead alloy sheath.
Figure 13.15d is a screened and balanced twin feeder. Figure 13.15e shows a coaxial
cable relying mainly on air as the insulation between the conductors with an insulating
helical thread supporting the inner conductor. Figure 13.15f is similar to Figure 13.15d,
but does not have the two conductors wound over each other; it is often used in radio
relay systems.
Figure 13.15 Radio frequency cables
Design of electrical services for buildings 204
As the frequency increases the losses in the dielectric become more important than the
resistance loss, and for high frequency cables the properties of the dielectric must be
carefully considered. The best performance can be obtained by air-spaced cables, but
cables with good solid dielectrics are used where economics outweigh purely technical
considerations. Communal TV systems normally employ screened coaxial cable with
polyethylene dielectric.
Frequency translation
In communications, information is contained in a complete waveform covering a band of
frequencies. This band of frequencies is evenly spaced about the carrier frequency, but
the wave can be transferred to any other band of frequencies of equal width, without any
loss of information. It can be transmitted in this form and later transferred back to its
original frequency band, or the information can be read out in the new band.
The losses in a line increase with frequency and in the early days of communal TV
systems it was impracticable to send signals along cables at UHF frequencies because the
losses were too high. UHF broadcasts were therefore translated near the aerial masthead
to suitable channels in the VHF range and transmitted along the communal distribution
system in this form. They were not translated back at the receiving points and the
receiving sets had to be adjusted accordingly. The tuner in a TV set contains a number of
pre-cabled tuning or filter circuits, each set to a particular channel. The station selector
switch connects the appropriate filter, leaving the others out of circuit. All that was
necessary therefore was for a service technician to take out one pre-cabled circuit and
replace it with another pre-cabled to the new channel. This was a very simple operation.
The frequency changing equipment accepted the broadcast signals and translated them
to the required channels by reference to high-stability local oscillators.
Modern amplifiers have made it possible to send UHF signals along aerial distribution
cables so that frequency translation is no longer necessary, but it may still be encountered
on some existing systems.
Mixers and splitters
Because the signal received by the aerial is attenuated as it travels along the cable, it must
be amplified. There are difficulties in designing amplifiers which work equally well over
a large range of frequencies and, therefore, two or more amplifiers are used, each
operating on a particular band of frequencies. The output impedance of each amplifier
must be matched to the characteristic impedance of the cable. Also the output of one
amplifier must not feed back into another amplifier to distort the output of that one. It is,
therefore, necessary to insert a mixer unit between the amplifiers and the line. The mixer
unit has to accept two or more different frequencies and combine them, but at the same
time isolate their sources one from another. It achieves this by suitable filtering networks
of inductances and capacitances.
The layout of a scheme sometimes makes it necessary to take two cables away from
one amplifier. The output impedance of one amplifier must then be matched to the
Communal and closed-circuit tv systems 205
Figure 13.16 Attenuation graph
Table 13.3 Standard junction attenuators
Reduction of outlet signal relative to
input
Reduction of continuation signal relative to
input
Ratio dB dB Ratio
100 to 1 40 0.1 1.01 to 1
50 to 1 34 0.2 1.02 to 1
20 to 1 26 0.46 1.05 to 1
10 to 1 20 0.9 11.11 to 1
5 to 1 14 1.9 1.25 to 1
3 to 1 9.6 3.5 1.50 to 11
characteristic impedances of two cables working in parallel. This is done by a splitter unit
which divides the output from an amplifier and distributes it between two or more lines.
The splitter is a network of resistances, inductances and capacitances chosen according to
the conditions under which the division is to be made.
Design of electrical services for buildings 206
Power loss and amplification
As we have already said, because of losses in the transmission system, the signal received
at the aerial has to be amplified either at the aerial or along the line or both. Now as the
gain of an amplifier is increased the noise it introduces also increases, and this sets a limit
to the gain which can be used. In practice, amplifiers with a gain of 30 to 60dB are used.
If a 30dB amplifier is used, then the distribution system can be allowed to attenuate the
signal by 30dB before a repeater amplifier has to be installed. Similarly a 60dB amplifier
permits losses of 60dB to be incurred before a repeater is necessary.
Attenuation occurs at a uniform rate along the length of the cable, but at each branch
there is a sharp loss in the junction unit. Consequently, the graph of signal strength
against cable run appears as in Figure 13.16. It will be seen that the signal level at each
branch decreases as one goes along the cable. A TV set must receive a signal not less
than about 1mV but will distort the picture if the signal is more than about 6dB higher
than this minimum. The signal level at a junction must be high enough to accommodate
the losses in the length of line continuing from the junction to the next amplifier. The
attenuation in the shortest branch from the junction must be large enough to bring the
signal strength down from that at the junction to less than the maximum acceptable to the
receiving set at the end of the short branch. The branch cable is quite short and in any
case its length cannot be adjusted to yield the required attenuation. It is, therefore,
necessary to build in some extra attenuation, and this is done in the junction unit itself.
The junction unit attenuates the signal to the branch outlet terminals by a given amount
whilst keeping the attenuation to the line continuation terminals as low as possible. This
is the second function of the junction unit which we referred to above and it is achieved
by a suitable network of resistors.
It will be seen from Figure 13.16 that the attenuation required to produce a given
output signal level is different at each junction. It would be most inconvenient to make a
special unit for every junction, but fortunately this is not necessary. A good TV set has a
certain tolerance in the input voltage it can accept, so that a standard attenuator can be
used for several successive junctions giving a small range of outputs within the limits
acceptable to the receivers. Table 13.3 shows a standard range of ratios which have in
practice been found adequate in a large number of cases. The resulting signals available
at the outlets in a typical case are also shown in Figure 13.16.
Typical systems
It is generally found that up to about 50 dwellings can be served from one repeater
amplifier. Two typical schemes are shown in Figures 13.17 and 13.18.
Figure 13.17 indicates a housing development consisting of two blocks of dwellings.
Each block has 17 single storey flats on the ground floor (intended for old people) and
three layers of maisonettes above them. Each of these layers consists of three floor levels;
the entrance to all maisonettes is on the
Communal and closed-circuit tv systems 207
Figure 13.17 Typical scheme
middle layer, and alternate maisonettes have the bedrooms below and above the entrance
and living rooms. Access corridors thus occur only on the ground floor and floors 3, 6
and 9, the other floors containing rooms reached by internal stairs within the maisonettes.
All services follow the same distribution pattern, that is to say, they run in the ceiling of
the access corridors and rise and drop into alternate maisonettes.
There is an aerial array which can receive three existing television channels and which
also has provision for the reception of future services on three other channels. The array
is mounted on the tank room on the roof of one of the blocks. The receiving equipment is
fixed inside the tank room and consists of amplifiers and splitters. Two cables are taken
from this main station, one to serve each block. They run along a duct in the roof and
then drop in a duct alongside the main stairs. One cable drops to the ground and then
continues inside a 50mm plastic conduit under an open space to the other block where it
rises in a duct alongside the main stairs. The other cable drops in the same duct of the
first block, but has a junction box at level 9. From this two branches run along the
ceilings of the access corridors with further junction boxes outside each front door. A
third branch from the
Design of electrical services for buildings 208
Figure 13.18 Typical scheme
junction box continues down the duct and feeds repeater amplifiers at levels 6, 3 and 1.
From each of these, outgoing cables run along the ceilings of the access corridors feeding
junction boxes outside each flat.
The cable entering the second block serves that block in an identical manner except
that it works from the bottom up instead of from the top down. The reapeaters are
therefore at levels 3, 6 and 9, whilst the ground floor is served directly from the main
mast-head amplifier.
Figure 13.18 illustrates an estate consisting of one 24-storey tower block and 18 low
blocks. Of the low blocks, numbers 1–8 and 17–18 are built on top of a podium covering
a ground-level car park. They are of two storeys and alternate blocks contain maisonettes
and a pair of single-floor flats. Blocks 9–16 start at ground level and are four storeys
high. Here alternate blocks contain one maisonette and two flats above each other and
two maisonettes above each other. Each floor of the tower block has four flats.
The aerial array is on the tank room of the tower block. The receiving equipment is
just inside the tank room and consists of amplifiers and splitters together with a power
unit. Eight of the outgoing cables cross the roof and drop inside conduit in the corners of
the tower block. In each corner one cable serves the living rooms in the upper half of the
block with a junction box at each level from which a short stub cable leads to the aerial
outlet. The other cable in each corner drops past these levels without junctions and then
serves the living rooms in that corner in the lower half of the block in a similar manner.
Two other cables from the receiving equipment drop in trunking in the central service
duct of the tower block and then continue underground in 25mm polythene conduit. One
of them runs along blocks 8–2 receiving amplification at points in blocks 8, 7 and 5 and
terminating in a final amplifier in block 2. From each of these amplifiers a final outlet
cable runs at high level in the car park under each block. Under each living room there is
a branch going to the living room above.
From the amplifier at block 5 there is another branch taking the main cable to further
amplifiers in blocks 10, 12 and 13. From each of these there is a cable running within a
Communal and closed-circuit tv systems 209
polythene conduit outside the block. There is a junction box in the wall of each bay of
these blocks from which an aerial cable runs to each living room outlet. Where there are
two or three living rooms above each other two or three branches come off next to each
other and run in separate 20mm conduit to the several outlets. This arrangement makes it
unnecessary to enter the lower flat if the cable to one of the upper flats has to be renewed.
The other cable from the tower block goes to block 17 where it feeds an amplifier. The
cable branches at this amplifier; one branch goes to amplifiers in blocks 17 and 18 and
the other to an amplifier in block 16. Blocks 17 and 18 are fed from their amplifiers in the
same way as blocks 1–8 and block 16 is fed in the same way as blocks 10–13.
On the whole scheme all the junction units are contained in conduit boxes accessible
from outside so that any repairs or replacements can be done without technicians having
to enter flats. This is an important consideration because it is always difficult to get
workmen to a job at a time when all the tenants are there to let them in.
The power to the amplifiers is supplied from the receiving power unit and is fed at
mains frequency along the aerial cable itself. This method of line-feeding the amplifiers
makes it unnecessary to provide power points at each amplifier position and this results in
a significant saving in cost.
A smaller scheme requiring no repeater amplifiers is shown in Figure 13.19. This
development consisted of a four-storey block A of four maisonettes and four blocks of
terraced town houses B, C, D and E. The aerial was mounted on the roof of block A and
there was an amplifier with a splitter and a power unit in a cupboard at high level on the
common staircase of this block. Four cables ran through 20mm conduit within this block
to serve the four living rooms in it.
Two other cables dropped in 20mm conduit in the block and continued in 20mm
polythene conduit in the ground outside. One ran along blocks C, D and E whilst the
other ran along block B. In both cases there was a junction
Design of electrical services for buildings 210
Figure 13.19 Small scheme
box in the wall of each house from which a short stub aerial cable ran in conduit to the
outlet in the living room. The longest cable on this scheme served only 13 dwellings and
it was therefore possible to avoid the use of repeater amplifiers altogether. On a small
scheme it is better to have a splitter at the masthead with several distributing cables than
to run a single cable round the whole site with several repeater amplifiers along it.
Standards relevant to this chapter are:
BS
3041
Radio frequency connectors
BS
6259
Code of practice for the design, planning, installation, testing and maintenance of sound
systems
BS
6330
Code of practice for reception of sound and television broadcasting
Communal and closed-circuit tv systems 211
Communal and closed-circuit TV systems
If all the tenants in a block of flats had their own TV aerial on the roof of the building,
the result would be very unsightly. In the case of a 24-storey tower block having 4 flats
on each floor, the roof would probably not be large enough to accommodate 96 separate
aerials. Even a low-rise development looks ugly if every house has its own aerial;
sometimes a four-storey development consists of two flats and a maisonette above each
other so that each ‘house’ would need three aerials. In fringe reception areas, one needs
large aerials mounted very high up. This makes it difficult to equip each dwelling with its
own aerial. In built up areas, large buildings shield smaller ones, so that if an estate
consists of a number of small blocks and one or two towers, occupants of the small
blocks left to provide their own aerials might find it desirable to put them on masts rising
as high as the top of the tower block.
For these reasons, it is an advantage to receive television and radio signals at one
suitably sited aerial array and relay them to individual dwellings by cables or
transmission lines. Very large relay systems exist, serving whole towns, sometimes from
a mast receiver several miles away. The building services engineer is more likely to be
concerned with community systems serving a single block of flats or one small estate of
houses and maisonettes. In this book, we confine our attention to such community
systems.
Radio signals are electro-magnetic waves in space. They cover a range of frequencies
which are classified as shown in Table 13.1. Since the product of frequency and
wavelength is always equal to the velocity of light, which is constant, the frequency is
inversely proportional to the wavelength. Radio broadcasts have in the past usually been
identified by the wavelength, but at the frequencies used for television the wavelength
becomes so small that it is more convenient to use the frequency. The properties of
aerials and transmission lines depend very much on frequency and different types have to
be used for different frequencies. It is convenient to subdivide the VHF and UHF
frequencies into five bands, which makes it possible for commercial equipment to be
manufactured for one or two selected bands only. This subdivision is shown in Table
13.2.
Table 13.1 Frequency bands
Designation Abbreviation Frequency range
Low frequency LF 30kHz−300kHz
Medium frequency MF 300mHz−3MHz
High frequency HF 3MHz−30MHz
Very high frequency VHF 30MHz−300MHz
Ultra high frequency UHF 300MHz−3000MHz
Super high frequency SHF 3000MHz−30000MHz
Table 13.2 Broadcasting services
Range Band Channel numbers Frequency Service
LF – – 150–285kHz AM sound long wave
MF – – 535–1605kHz AM sound medium wave
HF – – 2.3–26.1MHz AM sound short wave
VHF I 1–5 41–68MHz TV Band I
II – 87.5–100MHz FM sound (VHF)
III 6–13 175–215MHz TV band III
UHF IV 21–34 470–582MHz TV band IV
V 39–68 614–854MHz TV band V
If a signal consists of one frequency only, the only way it can convey information is by
varying in amplitude. As soon as several adjacent frequencies are present, they can
combine to form complicated waveshapes and the total number of distinguishable
patterns increases rapidly. This is a simplified explanation of why the band of frequencies
required for a transmission increases as the amount of information to be conveyed
increases. Television provides much more information than sound broadcasting, and
therefore each service requires a large band-width. For sound broadcasting, each station
needs a bandwidth of only 10kHz. A station broadcasting on 1500m (which corresponds
to 200kHz) actually uses all wavelengths between 1457m and 1543m. Provided the next
station has a nominal wavelength of 1587m or more there will be no interference between
them; it is obvious that there is no difficulty about keeping stations separate from each
other under these conditions.
A 625 line TV picture, on the other hand, requires a bandwidth of 5.5MHz. It is
immediately obvious from Table 13.1 that this cannot be transmitted at less than HE and
that the HF range would only accommodate five different stations. This is the reason that
TV is transmitted in the VHF and UHF ranges. Even within these, care has to be taken
about separation of stations. The five bands of frequency are, therefore, further divided
into a number of channels each of which covers a bandwidth of 6MHz. These channels
Communal and closed-circuit tv systems 189
are also indicated in Table 13.2. Each station is allocated one channel, and neighbouring
stations are thus prevented from interfering with each other.
Since the distance over which VHF and UHF waves can be propagated is quite
limited, two stations more than a certain minimum geographical distance apart can safely
use the same channel.
Aerials
If an e.m.f. is placed in the centre of a short cable (Figure 13.1), the two halves of the
cable act as capacitor plates, one becoming positively charged and the other negatively.
Each charge produces an electric field. Suppose now that the e.m.f. is alternating; there is
then an alternating charging current in the cable. When the current is a maximum the
positive and negative charges occupy the same place and produce equal and opposite
fields. When the current is zero the positive and negative charges are at opposite ends of
the cable and produce a resultant electric field. Thus there is an electric field which
alternates with the charging current in the cable.
The current also produces a magnetic field which spreads out from the cable with the
velocity of light. The motion of this magnetic field induces a further electric field.
Now the oscillating charges in the cable have not only a velocity, but also an
acceleration. This acceleration is propagated outwards in the electric field at a finite
velocity and, therefore, the field further out is moving with a lower velocity than that
closer in. Since the charges oscillate the acceleration is alternately forward and backward,
and the result is that the complete field radiated forms closed loops which travel out from
the cable and expand (Figure 13.2). It can be shown that this radiated field is appreciable
only if the length of the cable is of the same order as the wavelength.
Figure 13.1 Principle of dipole
Design of electrical services for buildings 190
Figure 13.2 Radiated field
The total electric field thus contains three terms. The first two are the induction fields and
the third is the radiation field. Near the aerial the induction fields predominate, but they
become negligible at a distance greater than about five wavelengths. At larger distances,
the radiation field is the only important one.
By a converse mechanism, a cable placed in an alternating electric field and suitably
orientated to that field will have an e.m.f. induced in it. In fact, it turns out that receiving
aerials are identical to transmitting aerials. In practice, receiving aerials can be made
simpler than transmitting aerials because high efficiency is not so important when
receiving as when transmitting.
The simple aerial of the type shown in Figure 13.1 is known as a dipole and its total
length is half the wavelength radiated or received. The description given above is an
attempt to explain in simple physical terms how a dipole radiates and receives. Any
electric current is associated with a magnetic and an electric field, the relationship always
satisfying Maxwell’s equations. The field radiated by an aerial is thus the solution of
Maxwell’s equations for the boundary conditions given by the current distribution in the
aerial and the geometry of the aerial. The mathematical difficulties of calculating such
solutions are so great that theoretical solutions are not given in even the most advanced
textbooks on radio propagation, and aerials are designed on the result of experimental
investigations. The general principle remains that the dimensions of the aerial are
approximately equal to the wavelength. It follows from this that different aerials are
required for different frequency bands.
Aerials are used for services ranging from telegraphy through navigation, broadcasting
and telephony to radar and space communications. There is a very large number of types
of aerial in use to cover the wide range of
Communal and closed-circuit tv systems 191
Figure 13.4 Strengthening elements
applications and frequencies employed, but the following are the types most likely to be
encountered by the building services engineer. They are all simple variations of the basic
dipole described above.
For band I, a single dipole as shown in Figure 13.3a can be used for reception at a
short range from the transmitter. If the receiver is some distance from the transmitter a
modification is introduced to increase the strength of the dipole. It has been found that the
addition of elements not connected to the receiver cable can strengthen the signal at the
dipole. An element slightly longer than the dipole and spaced about a quarter of a
wavelength behind it has the effect of reflecting waves onto the dipole and thus
strengthening the signal to it. This is illustrated in Figure 13.4 and results in the well
known H aerial shown in Figure 13.3b. A modification of this is the X aerial, also shown
in Figure 13.3b. The principle is illustrated in Figure 13.5 from which it is seen that the
dipole and reflector are partially folded on themselves. It has been found that they then
operate like a straight pair of elements occupying the mean positions shown dotted in
Figure 13.5.
Further improvement in reception can be obtained by adding directors in front of the
dipole at distances of about a quarter of a wavelength, and progressively shorter than the
dipole (Figure 13.4). It is found that the effect of these is to increase the sensitivity of the
dipole to radiation from the direction in which the array is pointing. An arrangement such
as that of Figure 13.4 is known as a Yagi array.
For long-range reception on band I, an aerial consisting of dipole, reflector and
director (Figure 13.3c) is used.
As explained later in this chapter, maximum power is transferred from a source to a
load when their impedances are equal. The half wave dipole has an impedance of about
75ohms, which matches the characteristic impedance of the coaxial aerial cable used.
When other elements, that is to say reflectors and directors, are added the impedance of
the aerial is reduced, and much of the power received is lost at the mismatch between
aerial and cable. This can
Communal and closed-circuit tv systems 193
Figure 13.5 Aerial
be overcome by folding the dipole, as shown in Figure 13.6. It is still half a wavelength
long but its impedance is higher, and when it is used in an array its impedance matches
that of the cable. Yagi arrays, therefore, generally use a folded dipole.
Similar aerials are used for the reception of FM radio on band II, but they are put
horizontally instead of vertically. This is simply because the radiation broadcast for this
service is polarized in the horizontal plane whereas that for television services is
polarized in the vertical plane. The same types of aerials are used for band III, but
because of the shorter wavelengths the strength of the signal decreases more rapidly and,
therefore, a larger number of elements is used in the array. Typical arrays are shown in
Figure 13.3d, e and f, and combined band I and band II aerials are shown in Figure 13.3g,
h and j.
At UHF the range of transmitters becomes much less, although there is less
interference between neighbouring stations. Therefore, the receiving aerials have a larger
number of elements. These are readily accommodated because as a result of the smaller
wavelength they are shorter and more closely spaced. In fact a six-element UHF array
can present a neater and more compact appearance than a single dipole for band I. Also
as a consequence of the shorter wavelength, the reflector can more effectively take the
form of a square mesh. Typical arrays are shown in Figure 13.3k, l and m.
Figure 13.6 Folded dipole
Design of electrical services for buildings 194
Figure 13.7 Polar diagrams
All the aerials we have described are directional. The voltage induced in them depends on
the angle between the axis of the array and the plane of the wave of radiation. It is a
maximum when the axis of the array is perpendicular to the wavefront and is zero when
the axis of the array is parallel to the wavefront. The sensitivity can be represented by the
length of a line drawn in the direction of the advancing wavefront. The locus of the ends
of all such lines forms the polar diagram. This is illustrated in Figure 13.7.
Greater sensitivity in the axial direction can be obtained by altering the shape of the
polar diagram as shown in Figure 13.7c. One way of achieving this is to have a pair of
Yagi arrays mounted side by side in a broadside arrangement, as shown in Figure 13.8. It
will be appreciated that in all cases
Figure 13.8 Twin arrays
the axis of the aerial array must point as closely as possible towards the transmitter.
Communal and closed-circuit tv systems 195
Transmission lines
The signal received by the aerial is sent to the television outlets along an aerial cable or
television transmission line. The design of the line is an important part of the design of a
communal TV system, and we must learn something about transmission engineering to
understand it. Transmission engineers are very much concerned with loss of power,
which is usually measured in decibels. A decibel is one-tenth of a bel, and a bel is the
logarithm to base 10 of the ratio of two powers. If the power at the sending end is Ps and
that at the receiving end is Pr the loss of the line is
The decibel is convenient because of the very large losses and amplifications encountered
in communications engineering; for example, if an amplifier has an output 10000 times
the input it is more convenient to say that is has a gain of 40dB. Since the gain, or loss, in
decibels is a ratio, the input level should also be stated.
Power ratios are proportional to the square of the voltage or current ratios. Therefore:
Thus when measurements are made in volts or amps the loss is
If the logarithms to base ‘e’ are used instead of to base 10, the unit is the neper instead of
the bel. This is not used so often, but it is sometimes convenient because the attenuation
of a transmission line per unit length is always a power of ‘e’.
There is a loss of power in all transmission lines. At very low frequencies this is
largely due to the resistance of the line, although inductance and capacitance are
important for exact calculation of long power lines. At high frequencies inductance and
capacitance become much more important, and it can readily be shown that the losses
increase rapidly with frequency. At frequencies above 3000MHz the losses in cables are
so high that transmission by cables is no longer possible and the only way of conveying
energy at these frequencies is by waveguides.
The power engineer distributing power at 50Hz is interested in supplying the power
taken by the load with the minimum loss in the line. The communications engineer
sending signals to a receiver has a rather different outlook. Here much smaller quantities
of energy are being handled, and amplifiers can be installed along the line which feed in
energy from an independent source without distorting the wave shape of the signal. The
main concern is to provide a strong enough signal to the receiver at the end of the line. It
can be shown that maximum power is taken by a load when that power equals the power
lost in the generator. Although at this condition maximum power is taken by the load, the
efficiency of the transmission is only 50 per cent. This is clearly uneconomic for power
Design of electrical services for buildings 196
transmission but is practicable in telecommunications where the magnitude of the signal
is much more important than transmission efficiency. The difference between the
operating points of power and communications systems is shown in Figure 13.9.
Figure 13.9 Operating points of
transmission lines
When an alternating voltage is applied to the sending end of an infinite line, a finite
current flows because of the capacitance and leakage inductance between the two cables
forming the line. The ratio of voltage applied to current flowing is the input impedance.
The input impedance for an infinite length of line is known as the characteristic
impedance and is denoted by Zo. It should be noted that the characteristic impedance
varies with frequency.
Since the line is infinite, no current waves reach the far end. Therefore, there is no
reflection and no reflected waves return to the sending end. For the same reason, the
current flowing depends only on the characteristic impedance (Zo) and is not affected by
the terminating or load impedance (Zr) at the far end. Although an infinite line is
obviously a purely hypothetical object, in practice the state of affairs we have just
Communal and closed-circuit tv systems 197
described is approximately fulfilled by many long lines. Furthermore, a short line
terminating in a load impedance equal to the characteristic impedance of the line (i.e.
Zr=Zo) behaves electrically as if it were an infinite line.
The characteristic impedance is the ratio of voltage to current at any point in an
infinite line or in a correctly terminated line. However, the current and voltage are not the
same at all points; because of the ordinary impedance of the line, they become
progressively less along the line.
Let
Is=current at sending end
Il=current one kilometre down line
Then where y=propagation constant per kilometre of line.
y is a complex quantity so that Il is both less than Is and also different in phase. In
general,
In=Ise–ny where In=current n kilometres along the line.
Similarly, En=Ese–ny
y is a complex quantity which can be written y=α+jβ where α is the attenuation
constant and β is the phase constant.
The four quantities:
Zo=characteristic impedance
y=propagation constant
α=attenuation constant
β=phase constant
are characteristic of the particular cable being used. They are known as the secondary
line constants and can be calculated theoretically from the four primary line constants
which are
R=resistance per kilometre (ohms)
G=leakage per kilometre (mhos)
L=inductance per kilometre (henries)
C=capacitance per kilometre (farads)
Whilst the primary constants are independent of frequency the secondary constants in
general vary with frequency.
Design of electrical services for buildings 198
Figure 13.10 Travelling and standing
waves
Now a communication signal carries information in its waveshape. It is most
important to preserve this shape while the signal is being sent along the line. There are
three main causes of distortion:
Communal and closed-circuit tv systems 199
1 Characteristic impedance varies with frequency. If the line is terminated in an
impedance that does not vary with frequency in the same manner as that of the line,
distortion will result.
2 Attenuation varies with frequency.
3 The velocity at which the wave shape travels along the line varies with frequency, so
that waves of different frequencies arrive at different times.
It can be shown that the condition for minimum attenuation is LG=CR. It can also be
shown that this condition makes Zo independent of frequency. This is called the
distortionless condition. On some communication lines the inductance is artificially
increased to bring the line nearer to this condition, but this is not found necessary on
communal TV systems.
If a line of characteristic impedance Zo is joined to an impedance having a value other
than Zo part of the wave travelling down the line will be reflected back at the point of
discontinuity. The reflection is a maximum when the line is either open circuited or short
circuited (Zr=∞ or Zr=0) and is zero when Zr=Zo The current in the line is always the sum
of the incident wave and the reflected wave.
This way of looking at matters will seem unfamiliar to power engineers. The rigorous
mathematics of transmission lines is the same for power lines as for communication lines,
but differences arise from differences in the frequencies at which they are operated. At
50Hz a line 500km long is less than one-tenth of a wavelength. Although the stationary
distribution of current along it is mathematically equal to the sum of two waves travelling
in opposite directions, this fact is of only academic interest to the power engineer. At
3000MHz, however, the wavelength is only a few metres and the two travelling waves
have a physical meaning which is easily visualized.
At the sending end, the current can be represented by a phasor of length Ismax rotating
at an angular frequency w where w=2πf and f is the frequency of the applied voltage. At a
point x down the line, the current Ix differs from Is because of the attenuation of the cable
and can be represented by a vector of length at an angle of βx to
the original phasor, but still rotating at an angular frequency of w. The instantaneous
current along the line at successive intervals is then as shown in Figure 13.10a. For
simplicity in drawing this figure has been taken as zero, so that the current is the same
in magnitude along the line. It will be seen that at all times the envelope of instantaneous
current along the line is a sine wave. Moreover, the sine wave moves down the line. The
wavelength is that length at which βx=2π
Since velocity=frequency×wavelength
Design of electrical services for buildings 200
This is the velocity at which the signal travels down the line. It is not the same as the
velocity at which energy is transferred.
A wave travelling in the opposite direction will be as shown in Figure 13.10b. When
the incident and reflected waves are equal in magnitude they combine as shown in Figure
13.10c. We can see that there is a wave on the line of frequency f and wavelength λ=w/β
but it does not travel along the line. Whereas with the travelling wave the rotating phasor
has the same magnitude along the line and varies only in phase along the line, in this case
the vector changes in magnitude along the line. As a result there are nodes where the
current is always zero and anti-nodes where there is a maximum. This type of wave is
called a standing wave, and is produced by the combination of equal incident and
reflected waves. If only part of the incident wave is reflected then there is a standing
wave with a travelling wave superimposed on it. Power is transferred only by the forward
travelling wave, so that standing waves represent a loss. Reflected waves also originate at
junctions where one cable branches into two or three.
Complete reflection occurs when a line is either open circuited or short circuited.
Under these conditions, there is no forward travelling wave, and no power is transferred.
This is readily understandable, because neither an open circuited nor a short circuited line
feeds a load.
For maximum transfer of power the internal impedance of the generator must be
matched to the characteristic impedance of the line and this must in turn be matched to
the input impedance of the load. In the case of the communal TV system, the generator is
the aerial. If the matching is not correct, standing waves are formed in the line, and power
is dissipated in the line. Some of this dissipation takes the form of radiation which causes
interference to neighbouring aerials.
In general, the impedance of the generator is not the same as that of the line, and that
of the line is not the same as that of the receiver. Therefore, at each of these points of
discontinuity some form of impedance transformer is required. Such a transformer can be
made from a four pole network which
Figure 13.11 Impedance transformer
has different input impedances between the two pairs of terminals. In Figure 13.11, the
line with characteristic impedance Zl sees the impedance Zl between terminals a and b
and is therefore, correctly terminated. The load with impedance Zr sees itself supplied
from an impedance Zr between terminals c and d and is, therefore, also correctly
matched. There is a small loss of power in the four pole network, but this is preferable to
the large losses that would occur in the line if the mismatching were permitted to remain.
A similar problem has to be solved where there is a branch. In Figure 13.12a, the input
cable sees the continuation and branch cables in parallel and is in effect terminated by an
impedance of Zl/2. A network has to be provided as in Figure 13.12b, so that each of the
three cables sees an impedance of Zl. There are a number of devices which have the
necessary characteristics, but many of them are sensitive to frequency and operate
Communal and closed-circuit tv systems 201
correctly only over a limited band of frequencies. In communal TV practice, a simple
arrangement of resistances is used, as shown in Figure 13.13. These junction boxes also
perform another function which we shall come to later in this chapter.
Figure 13.12 Branch network
Figure 13.13 Junction box
Cables
Design of electrical services for buildings 202
Figure 13.14 TV cables
Having discussed the theory of a transmission line, we are now in a position to say
something about the cable which actually forms the line. We start by noting that any
cable carrying current tends to act as a radiating aerial. At low frequencies the power
radiated from an ordinary cable is so small that it can hardly be detected, but at high
frequencies it can become significant. Not only is there a loss of power from the line
itself, but the radiation will cause interference in neighbouring receivers. Similarly, any
cable acts as a receiving aerial, and a line feeding a television or radio set can pick up
unwanted high-frequency radiations. Both these effects can be suppressed by efficient
screening, and radio and TV services therefore always use screened cable.
A line may be a single conductor using the earth as a return. This has capacitance to
the earth and is termed an unbalanced line. If a conductor is provided for the return, it can
be arranged so that either the two conductors have different capacitances to earth or that
Communal and closed-circuit tv systems 203
they have the same capacitance to earth. The former arrangement is termed unbalanced
and the latter balanced. Typical arrangements are shown in Figure 13.14.
The energy conveyed by a transmission line is in fact held in the electric and magnetic
fields associated with the current and voltage. In the case of open lines, these fields
extend infinitely into space. At high frequencies the energy is rapidly dissipated into
space, and the losses from the transmission system become unbearably high. If a screen is
placed round the conductors, the fields are confined within the screen and the losses are
reduced. Typical forms of radio frequency cable are shown in Figure 13.15. The cable of
Figure 13.15a has an inner conductor of copper cable and an outer conductor of seamless
lead tube. It is suitable for high frequency transmission and high power aerial feeders.
Figure 13.15b shows a cable with the inner conductors supported in the centre of a tube
of polyethylene and an outer conductor of cable braid with PVC or lead alloy sheath.
Figure 13.15d is a screened and balanced twin feeder. Figure 13.15e shows a coaxial
cable relying mainly on air as the insulation between the conductors with an insulating
helical thread supporting the inner conductor. Figure 13.15f is similar to Figure 13.15d,
but does not have the two conductors wound over each other; it is often used in radio
relay systems.
Figure 13.15 Radio frequency cables
Design of electrical services for buildings 204
As the frequency increases the losses in the dielectric become more important than the
resistance loss, and for high frequency cables the properties of the dielectric must be
carefully considered. The best performance can be obtained by air-spaced cables, but
cables with good solid dielectrics are used where economics outweigh purely technical
considerations. Communal TV systems normally employ screened coaxial cable with
polyethylene dielectric.
Frequency translation
In communications, information is contained in a complete waveform covering a band of
frequencies. This band of frequencies is evenly spaced about the carrier frequency, but
the wave can be transferred to any other band of frequencies of equal width, without any
loss of information. It can be transmitted in this form and later transferred back to its
original frequency band, or the information can be read out in the new band.
The losses in a line increase with frequency and in the early days of communal TV
systems it was impracticable to send signals along cables at UHF frequencies because the
losses were too high. UHF broadcasts were therefore translated near the aerial masthead
to suitable channels in the VHF range and transmitted along the communal distribution
system in this form. They were not translated back at the receiving points and the
receiving sets had to be adjusted accordingly. The tuner in a TV set contains a number of
pre-cabled tuning or filter circuits, each set to a particular channel. The station selector
switch connects the appropriate filter, leaving the others out of circuit. All that was
necessary therefore was for a service technician to take out one pre-cabled circuit and
replace it with another pre-cabled to the new channel. This was a very simple operation.
The frequency changing equipment accepted the broadcast signals and translated them
to the required channels by reference to high-stability local oscillators.
Modern amplifiers have made it possible to send UHF signals along aerial distribution
cables so that frequency translation is no longer necessary, but it may still be encountered
on some existing systems.
Mixers and splitters
Because the signal received by the aerial is attenuated as it travels along the cable, it must
be amplified. There are difficulties in designing amplifiers which work equally well over
a large range of frequencies and, therefore, two or more amplifiers are used, each
operating on a particular band of frequencies. The output impedance of each amplifier
must be matched to the characteristic impedance of the cable. Also the output of one
amplifier must not feed back into another amplifier to distort the output of that one. It is,
therefore, necessary to insert a mixer unit between the amplifiers and the line. The mixer
unit has to accept two or more different frequencies and combine them, but at the same
time isolate their sources one from another. It achieves this by suitable filtering networks
of inductances and capacitances.
The layout of a scheme sometimes makes it necessary to take two cables away from
one amplifier. The output impedance of one amplifier must then be matched to the
Communal and closed-circuit tv systems 205
Figure 13.16 Attenuation graph
Table 13.3 Standard junction attenuators
Reduction of outlet signal relative to
input
Reduction of continuation signal relative to
input
Ratio dB dB Ratio
100 to 1 40 0.1 1.01 to 1
50 to 1 34 0.2 1.02 to 1
20 to 1 26 0.46 1.05 to 1
10 to 1 20 0.9 11.11 to 1
5 to 1 14 1.9 1.25 to 1
3 to 1 9.6 3.5 1.50 to 11
characteristic impedances of two cables working in parallel. This is done by a splitter unit
which divides the output from an amplifier and distributes it between two or more lines.
The splitter is a network of resistances, inductances and capacitances chosen according to
the conditions under which the division is to be made.
Design of electrical services for buildings 206
Power loss and amplification
As we have already said, because of losses in the transmission system, the signal received
at the aerial has to be amplified either at the aerial or along the line or both. Now as the
gain of an amplifier is increased the noise it introduces also increases, and this sets a limit
to the gain which can be used. In practice, amplifiers with a gain of 30 to 60dB are used.
If a 30dB amplifier is used, then the distribution system can be allowed to attenuate the
signal by 30dB before a repeater amplifier has to be installed. Similarly a 60dB amplifier
permits losses of 60dB to be incurred before a repeater is necessary.
Attenuation occurs at a uniform rate along the length of the cable, but at each branch
there is a sharp loss in the junction unit. Consequently, the graph of signal strength
against cable run appears as in Figure 13.16. It will be seen that the signal level at each
branch decreases as one goes along the cable. A TV set must receive a signal not less
than about 1mV but will distort the picture if the signal is more than about 6dB higher
than this minimum. The signal level at a junction must be high enough to accommodate
the losses in the length of line continuing from the junction to the next amplifier. The
attenuation in the shortest branch from the junction must be large enough to bring the
signal strength down from that at the junction to less than the maximum acceptable to the
receiving set at the end of the short branch. The branch cable is quite short and in any
case its length cannot be adjusted to yield the required attenuation. It is, therefore,
necessary to build in some extra attenuation, and this is done in the junction unit itself.
The junction unit attenuates the signal to the branch outlet terminals by a given amount
whilst keeping the attenuation to the line continuation terminals as low as possible. This
is the second function of the junction unit which we referred to above and it is achieved
by a suitable network of resistors.
It will be seen from Figure 13.16 that the attenuation required to produce a given
output signal level is different at each junction. It would be most inconvenient to make a
special unit for every junction, but fortunately this is not necessary. A good TV set has a
certain tolerance in the input voltage it can accept, so that a standard attenuator can be
used for several successive junctions giving a small range of outputs within the limits
acceptable to the receivers. Table 13.3 shows a standard range of ratios which have in
practice been found adequate in a large number of cases. The resulting signals available
at the outlets in a typical case are also shown in Figure 13.16.
Typical systems
It is generally found that up to about 50 dwellings can be served from one repeater
amplifier. Two typical schemes are shown in Figures 13.17 and 13.18.
Figure 13.17 indicates a housing development consisting of two blocks of dwellings.
Each block has 17 single storey flats on the ground floor (intended for old people) and
three layers of maisonettes above them. Each of these layers consists of three floor levels;
the entrance to all maisonettes is on the
Communal and closed-circuit tv systems 207
Figure 13.17 Typical scheme
middle layer, and alternate maisonettes have the bedrooms below and above the entrance
and living rooms. Access corridors thus occur only on the ground floor and floors 3, 6
and 9, the other floors containing rooms reached by internal stairs within the maisonettes.
All services follow the same distribution pattern, that is to say, they run in the ceiling of
the access corridors and rise and drop into alternate maisonettes.
There is an aerial array which can receive three existing television channels and which
also has provision for the reception of future services on three other channels. The array
is mounted on the tank room on the roof of one of the blocks. The receiving equipment is
fixed inside the tank room and consists of amplifiers and splitters. Two cables are taken
from this main station, one to serve each block. They run along a duct in the roof and
then drop in a duct alongside the main stairs. One cable drops to the ground and then
continues inside a 50mm plastic conduit under an open space to the other block where it
rises in a duct alongside the main stairs. The other cable drops in the same duct of the
first block, but has a junction box at level 9. From this two branches run along the
ceilings of the access corridors with further junction boxes outside each front door. A
third branch from the
Design of electrical services for buildings 208
Figure 13.18 Typical scheme
junction box continues down the duct and feeds repeater amplifiers at levels 6, 3 and 1.
From each of these, outgoing cables run along the ceilings of the access corridors feeding
junction boxes outside each flat.
The cable entering the second block serves that block in an identical manner except
that it works from the bottom up instead of from the top down. The reapeaters are
therefore at levels 3, 6 and 9, whilst the ground floor is served directly from the main
mast-head amplifier.
Figure 13.18 illustrates an estate consisting of one 24-storey tower block and 18 low
blocks. Of the low blocks, numbers 1–8 and 17–18 are built on top of a podium covering
a ground-level car park. They are of two storeys and alternate blocks contain maisonettes
and a pair of single-floor flats. Blocks 9–16 start at ground level and are four storeys
high. Here alternate blocks contain one maisonette and two flats above each other and
two maisonettes above each other. Each floor of the tower block has four flats.
The aerial array is on the tank room of the tower block. The receiving equipment is
just inside the tank room and consists of amplifiers and splitters together with a power
unit. Eight of the outgoing cables cross the roof and drop inside conduit in the corners of
the tower block. In each corner one cable serves the living rooms in the upper half of the
block with a junction box at each level from which a short stub cable leads to the aerial
outlet. The other cable in each corner drops past these levels without junctions and then
serves the living rooms in that corner in the lower half of the block in a similar manner.
Two other cables from the receiving equipment drop in trunking in the central service
duct of the tower block and then continue underground in 25mm polythene conduit. One
of them runs along blocks 8–2 receiving amplification at points in blocks 8, 7 and 5 and
terminating in a final amplifier in block 2. From each of these amplifiers a final outlet
cable runs at high level in the car park under each block. Under each living room there is
a branch going to the living room above.
From the amplifier at block 5 there is another branch taking the main cable to further
amplifiers in blocks 10, 12 and 13. From each of these there is a cable running within a
Communal and closed-circuit tv systems 209
polythene conduit outside the block. There is a junction box in the wall of each bay of
these blocks from which an aerial cable runs to each living room outlet. Where there are
two or three living rooms above each other two or three branches come off next to each
other and run in separate 20mm conduit to the several outlets. This arrangement makes it
unnecessary to enter the lower flat if the cable to one of the upper flats has to be renewed.
The other cable from the tower block goes to block 17 where it feeds an amplifier. The
cable branches at this amplifier; one branch goes to amplifiers in blocks 17 and 18 and
the other to an amplifier in block 16. Blocks 17 and 18 are fed from their amplifiers in the
same way as blocks 1–8 and block 16 is fed in the same way as blocks 10–13.
On the whole scheme all the junction units are contained in conduit boxes accessible
from outside so that any repairs or replacements can be done without technicians having
to enter flats. This is an important consideration because it is always difficult to get
workmen to a job at a time when all the tenants are there to let them in.
The power to the amplifiers is supplied from the receiving power unit and is fed at
mains frequency along the aerial cable itself. This method of line-feeding the amplifiers
makes it unnecessary to provide power points at each amplifier position and this results in
a significant saving in cost.
A smaller scheme requiring no repeater amplifiers is shown in Figure 13.19. This
development consisted of a four-storey block A of four maisonettes and four blocks of
terraced town houses B, C, D and E. The aerial was mounted on the roof of block A and
there was an amplifier with a splitter and a power unit in a cupboard at high level on the
common staircase of this block. Four cables ran through 20mm conduit within this block
to serve the four living rooms in it.
Two other cables dropped in 20mm conduit in the block and continued in 20mm
polythene conduit in the ground outside. One ran along blocks C, D and E whilst the
other ran along block B. In both cases there was a junction
Design of electrical services for buildings 210
Figure 13.19 Small scheme
box in the wall of each house from which a short stub aerial cable ran in conduit to the
outlet in the living room. The longest cable on this scheme served only 13 dwellings and
it was therefore possible to avoid the use of repeater amplifiers altogether. On a small
scheme it is better to have a splitter at the masthead with several distributing cables than
to run a single cable round the whole site with several repeater amplifiers along it.
Standards relevant to this chapter are:
BS
3041
Radio frequency connectors
BS
6259
Code of practice for the design, planning, installation, testing and maintenance of sound
systems
BS
6330
Code of practice for reception of sound and television broadcasting
Communal and closed-circuit tv systems 211
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