BBC Relay Station Singapore
Hi Bill,
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.
73,
Ian Liston-Smith, G4JQT
(A grumpy old retired BBC Engineer)
A similar but different HF delay effect used to be noted in the good old days of high power SW broadcast, where an identical programme transmitted from the sane site on two bands had a noticeable delay between the two frequencies. Couldn't be feed latency... it turned out that a deliberate delay was introduced so that both transmitters didn't hit modulation peaks at the same time, thus smoothing out the load on the local power supply. When you're talking multiple fifty- or hundred-kW AM outputs, this sort of thing soon adds up to real power...
ReplyDeleteRupert, G6HVY