A centimetre-wavelength beam telephone

The paper describes the principles of operatipn and some constructional details of a light-weight radio-telephone link operating in the 3-cm wavelength band. It is suitable for operation over optical paths of about 10 miles in length, and can be used to bridge a gap in a telephone line. Duplex operation is achieved by the use of one valve simultaneously as transmitter and as local oscillator for the receiver, making send/receive switching unnecessary. The aerial system is highly directional, and this gives a degree of security and freedom from jamming, which permits several sets to be operated in close proximity to one another. The operating power can be drawn either from a.c. mains or from a 12-volt battery, the drain being approximately 100 watts per set in either case. Longer-range links using the same system could be developed by using increased transmitter power and higher aerial gains, but it is suggested that for longer ranges a longer wavelength of about 6 cm would be preferable.