Abstract :
The paper deals principally with the work done by the author during the period May 1940 to January 1941 in the development of a system permitting the same aerial to be used for transmission and reception in the C.H.L. stations then operating in various parts of the British Isles. A brief description of the various features of the C.H.L. equipment relevant to the problem is first given; then several alternative methods are discussed, and their relative merits compared; and finally, a detailed description is given of the spark-gap switching system actually used. A special case for a diode-switching system for common-aerial working is also taken and analysed in some detail, since diodes may have certain advantages over spark-gaps for low power. The spark-gap switching system described uses transmitter-operated spark-gaps, in conjunction with quarter-wave line sections (or their equivalent lumped circuits), to isolate the receiver from the aerial during transmission, and from the transmitter circuits during reception. The relatively short pulses characteristic of radar systems have appreciably reduced the difficulties of design, but considerably longer ones could undoubtedly be dealt with if required.
