Long feeders for transmitting wide side-bands, with reference to the Alexandra Palace aerial-feeder system

Abstract

The paper is concerned with reflection phenomena in long feeders used to transmit modulated high-frequency carrier waves such as television signals, in which the time of transmission of a wave along the feeder is comparable with the time periods of the modulation frequencies. The nature of the various impedance irregularities giving rise to reflection is discussed, and their effect on a received television picture is indicated. Methods forreducing the effect of these irregularities are considered. These reflecting irregularities may be studied in terms of the variation with frequency ofthe input impedance of the terminated feeder, and examples are given of themagnitude of the effects for various feeders. The vision aerial feeder system of the London Television Station is described, including the aerial-to-feeder matching arrangements and a device for reducing the residual mismatchdue to the variation of the aerial impedance over the frequency range of the carrier and side-bands. Apparatus for measuring the input impedance and the methods adopted for establishing a standard resistance for terminating the feeder are described. In the Appendix consideration is given to the permissible limits of eccentricity of an approximately terminated but irregularly eccentric feeder.