Co-axial cables

Radio and TV system are now frequently built as part of the engineering services of new buildings . One aerial is made to serve a number of outlets at each of whicha receiving set can be plugged in . The single aerial is usually at the highest point of the buildings or group of buildings in the system and is connected by cable to each outlet .The cable used for this does not carrry large current and is not subjected to large voltages but it does carry a weak signal at high freqencies . The signal must not be lost and to avoid loss of the signal , the cable must have a low impedance at the frequency being used . It must also be constructed so that it does not pick up unwanted high  frequency signal , for example by capacitance or inductance between itself and nearby main cables .
  To satisfy these requirements , radio and tV distribution system are cabled with radio frequency cables . These normally have a single insulated conductor . A metal cover is placed over the insulation to screen the conductor from unwanted signals , and this cover is in its turn protected by an overall sheath of non-conducting material . Thus the single conductor is surrounded by a circular cover and a circular sheatrh which are concertrica and have the conductor on their axis . hence the cables comes to be knwon as coaxial cable . Such a cable is shown in Figure 2.10 ; it has its single inner conductor cased in polythene insulation with a cable braid outer conductor and a final sheath of PVC .
  The inner conductor can be either solid or stranded , while the screen can take several forms . It can be a one -piece sheath in either aluminium or copper , or it can be of cable braid , which in turn can be either single or double , or it can be of steel tape or of lead , for the insulation , the commonest materials used ae polythene  ,PTFE and polypropylene . The last part of the construction  is the outer sheathing and this may be of metal tape , cable , metal braid  PTFE , lead alloy  , PVC , nylon or polythene . Evidently , there is a large variety of coaxial cables and properties differ somewhat . The choice of what to use in any particular case is determined by the electrixal characteristic required for that particular application and this depends on what equipment the cable is to be used for . SOund broadcasting operates on lower frequencies than televisyen and cables for sound olny do not need to meet quite such stringent condition as those for televisyen transmission . Audio-frequency cables suitable for microphones and loudspeaker connections for public address systems and for broadcast relay systems are similar to high-frequency cables but are not made to such exacting specifications . Examples of audio-frequency cables are shown in Figure 2.11