Aberystwyth picked up the signals on revolutions 5, 21, 22, and 23. (On rev 5 the spacecraft passed right over Aberystwyth and the signals on the chart below were recorded). On all occasions the signals were transmitted in bursts: 30 seconds on and 90 seconds off. This behaviour enabled Geoff to identify Kosmos-186 as a Soyuz-type spacecraft during the automatic docking mission Kosmos-186/188. Similar behaviour was observed with the Kosmos 212/213 mission and, with this, it was discovered that the telemetry was actually transmitted on 15.008 MHz during part of the off period on 20.008 MHz.
In
connection with ASTP, detailed information about the command-signalling
device in the Soyuz cockpit showed that the Soyuz short-wave
transmitter
could be cycled through four different frequencies, presumably to cover
all possible HF bands so that at least one 30 second burst would get
through
- independent of spacecraft location - a very clever emergency
signalling
system.
What frequencies were used? Well, I think they were 20.008 MHz, 15.008 MHz as observed and also 18.060 MHz (originally 18.035 MHz) and one more frequency which is not known..! Or perhaps the fourth slot was to be used for voice? 20.008 MHz, 15.008 MHz and 18.060 MHz (originally 18.035 MHz) were the only Soyuz frequencies ever given in official TASS announcements!
Signals on shortwaves from Soyuz also were often observed all along the orbit which explains the interest in the frequency hopping exercise to try to find the frequency that best propagated around the world. The simple keyed beacon signal propagated all around the earth in what is called "ionospheric wave ducts" giving an effect often called "the whispering gallery effect". A peculiarity observed during "whispering gallery propagation" was that the signal from the Soyuz increased abruptly as the spacecraft reached the antipode. At that point all HF signal energy radiated horizontally reached the antipode! We used to call this the "antipode effect".
