An experiment to test the reciprocal radio transmission conditions over an ionospheric path of 740 km

Two-way pulse transmissions made simultaneously on the same frequency (5.1 Mc/s approximately) between Slough and Inverness (740 km) are described. An aerial common to transmitter and receiver was used at each terminal, and the fading patterns displayed at each end were compared visually: the Inverness display was relayed to Slough over a Post Office trunk line for this purpose. The fading of corresponding echoes was found to be non-reciprocal for about 1% of the time during a total period of observation of about 15 daylight hours spread over 13 days in May, 1954. The precautions taken to ensure that the effects were ionospheric and not instrumental are outlined.