BBC Relay Station Singapore
I heard your description of the echo on your podcast and before listening I knew the cause - but I think you know that now!
Yes, it's from two separate transmitters, and quite common, though not usually noticed.
It has nothing to do with path length differences - the longest round-the-world echo via the ionosphere is only about 0.15 seconds - so anything more has a different cause.
It's from the audio feed to the transmitter. Your regen receiver picked up two transmitters on different frequencies. It was very noticeable before transmitters used digital land-line feeds, just analogue and satellite.
On a BBC SW frequency (forget the which one now) one tx was in UK and the other in Singapore, on the same frequency with the same programme to completely different service areas. When propagation was right and listening in Europe, the UK signal fed by analogue audio from Bush House came first and the Singapore tx came with two geostationary satellite delays later, plus the tiny bit of UK-Singapore ionospheric path difference.
Now it's worse because there are all sorts of digital delays via land-line and satellite, although using the same frequency for the same service in not common.
In the UK Absolute Radio on AM medium-wave has multiple transmitters (mostly 1215 kHz and 1197 kHz) on the same frequency which are audible at night. If you listen carefully you can often hear multiple (up to FOUR!) echoes from different transmitters all being fed by different internet feeds/satellite links with varying delays.
As an ex-BBC engineer, I can tell you that in the old days not only were these AM medium-wave group stations all synched to within 0.05Hz, but the phase of the modulation was adjusted so all tramsitters were modulating in phase! Now the commercial boys have taken over most of these syched groups, not only are the frequencies all over the place, but the modulation isn't even time delayed to match, let alone synchronized! Some even put diferent commercials in the breaks so if you're geographically between stations you get a complete, unlistenable-to mess. Apparently these days that's ok.
Why did we bother...?
Anyway, I hope this adds to and confirms your findings.
Ian Liston-Smith, G4JQT
(A grumpy old retired BBC Engineer)