By C.Gillam (Marconi's Wireless Telegraph Company) Orignally published in Wireless World March 1950 FEEDING THE AERIALS Radiation from the aerial is unaffected by whether this pair of loads is connected in parallel across a source or in series. The first method means that energy is fed to them in identical phase, or push-push and the second method feeds energy in push-pull i.e. with a 180° phase difference. For both methods, the effect of the extra 90° length of the feeder to one aerial plane ensures a 90° difference in the phase of radiation from the two planes. Reference to the vector diagrams of Fig 3 (a) and Fig 3 (b) makes this clear. It also brings out the fact that for the parallel connection Fig 3 (a) vector B lags Vector A by 90 °, while for the series connection Fig 3 (b) vector B lags vector A by 270° or what in effect is the same thing, leads Vector A by 90° The eventual result of this is that, in the rotating field system radiated by the aerials, the direction of rotation when the aerial feeders are fed in parallel is opposite to that resulting when the aerial feeders are fed in push-pull. Now, let us consider Fig 4 (a), which shows a pair of resistance loads AE and BE, having a common earthed terminal E and a transmission line MN, of which the outer conductor is earthed. Energy is supplied to the transmission line by the source. We can feed the two loads in parallel from the transmission line without difficulty by joining the two terminals A and B to the inner conductor of the transmission line and the earthed terminal E to the outer conductor as shown in Fig 4 (b) If the surge impedance of the transmission line is Zo and the resistance value of each load is equal to Zo the resistance of the two loads in parallel is Zo/2 and this would represent a serious mis-match with a consequent standing wave on the transmission line. This can, however, be avoided by providing some form of impedance transformation, say by enlarging the diameter of the inner conductor of the transmission line for the last quarter-wavelength so that the surge impedance of this part is Zo/√2 The input impedance of this last quarter-wavelength then becomes That is, the line will be matched on the generator side of the last quarter-wavelength. If we wish to feed the two loads in push-pull, we can connect terminal B to the inner conductor of the transmission line, and terminal A to the outer conductor as in Fig 4 (c) This puts them in series between inner and outer, but a difficulty at once appears in that the mid-point E is earthed. If the outer conductor of the transmission line is earthed at point M then resistance AE is completely short-circuited and no power would be supplied to it. If instead the outer conductor is earthed at some other point N, then the resistance AE is shunted by the path MNE, which would not in general be a complete short-circuit, and some power would reach the load AE. If we could interpose a high impedance along the path MN, this shunting effect would be avoided, irrespective of the existence of the earth connection at E and N and resistance AE would receive its fair share of the available power. In order to appreciate how this is possible, it must be understood that currents from the generator G flow into the transmission line only along the inner surface of the outer conductor, and correspondingly along the outer surface of the inner conductor. Thus an interruption in the current path along the outer surface of the outer conductor will not intefere with the supply of power from generator G to the loads AE and BE. In Fig 5 (a) there is shown a cylinder PQ arranged coaxially with the transmission line MN. The length PQ is made an exact quarter-wavelength, and the cylinder is joined to the outer conductor of the transmission line at the end Q while they are seperated at the end P. We thus have a quarter-wavelength of transmission line formed by the inner surface of PQ and the outer surface of the outer conductor of line MN and this line is short circuited at the end Q so that its impedance has a very high value at the frequency in use. In fact, the cylinder PQ in conjunction with the outer surface of the transmission line MN behaves like a a parallel resonant circuit. This high impedance appears in series between the points M and N, and so makes negligible the shunting effect of the branch MNE across the resistance AE. The equivalent circuit arrangement is shown in Fig 5 (b) Provided we have only a single frequency to transmit, there is no objection to the use of the arrangement illustrated in Fig 5 (a) Television, however requires the transmission of a wide band of frequencies, and it is once apparent that as the transmission frequency deviates from that for which the cylinder PQ is and exact quarter-wavelength the input impedance of the short circuited line becomes less and less, and there is an increasing shunting effect across resistance AE. clearly power division between AE and EB will not be equal at frequencies off resonance.
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