Developments in long-distance H.F. point-to-point communications

The paper surveys technical developments in the point-to-point h.f. communication system of the British Post Office. In the first part, the development of radio-telegraph systems is discussed, and it is shown that these are tending to standardize on multichannel voice-frequency systems, using 2-tone for normally difficult routes (e.g. long routes subject to severe multipath effects) and narrow-band frequency modulation on the easier routes. Similarly, the development of radio-telephone systems is considered, and it is shown how the efficiency of these can be improved by the use of 4-wire privacy, with consequent simplification. The successful application in radio-telephony systems of a syllabic constant-volume amplifier, in conjunction with a complementary device at the receiving end regulated by a separate control channel, should-further increase their efficiency and may enable the radio terminals to be regarded as simple repeater stations. The second part describes modern systems which are to be provided at the rebuilt Leafield transmitting and Bearley receiving stations of the Post Office. The Leafield station will have two groups of six 30kW linear self-tuning transmitters, remotely controlled to radiate any of 25 synthesized frequencies and to have access to 10 aerial systems, each comprising two or three rhombic aerials. The Bearley station will have capacity for a large number of remotely controlled receivers with synthesized frequency control. New receivers will be of transistorized design, and the closer frequency tolerance provided by synthesized frequencies should, eventually, enable automatic frequency correction (a.f.c.) to be eliminated. It is planned to extend the control of the stations to the traffic terminals in due course, and this, together with the increased reliability of equipment which the fullest use of semiconductor devices is expected to bring, could lead to substantial economy in staffing.