Electronics > RF, Microwave, Ham Radio

Strange echos heard on broadcast AM radio.

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Delta:
I've pretty much always got a radio tuned to my favourite mediumwave station.  They use a network of TXs across the country on 1053 and 1089 kHz. (We use 9 kHz channel spacing in the UK)  My local TX is on 1053.

I sometimes get an echo - a quieter sound follows the "main" sound.  I completely understand this:  Under certain atmospheric conditions the signal from a distant 1053 TX can reach me.  The delay is due to the longer time-of-flight, and the sound is quieter as the signal is much weaker than my local TX.  I also understand that multipath can cause the same effect.

However last night I had the opposite - I would hear a quieter sound BEFORE I heard the main sound!  This was consistent, and persisted all night.

How can this be?  :-/O

Delta:
Reading back my own post I had a lightbulb moment!

What if (for some reason) the local and distant TXs weren't synchronised, and the distant TX was running slightly ahead of the local one...?

edpalmer42:
Or, they might have had trouble with the connection between the studio and your local transmitter.  So they had to scramble to quickly put in a backup link and ended up with something that has more delay than usual.  As a result, your local signal is delayed with respect to the distant one.

German_EE:
A difference in frequency between two AM transmitters would not produce the effect that you describe, how do I know? I just tried it using two signal generators that were locked to the same 10 MHz source but outputting AM at 1.000000 MHz and 1.000001 Mhz. The result was a combined signal that faded in and out at a 1Hz rate.

What would be causing your echo are differences in the audio path length between the studio and the two AM transmitters. The audio will have been sent by either microwave or fiber to the transmitters and then used to modulate the carrier on site to produce AM. Any equipment differences in the two audio chains would produce a delay.

German_EE:
P.S.

It's 837 miles or 1347 Km between Lands End and John O Groats, the longest distance that you can travel in the UK. A radio signal will travel that distance in 4.493 milliseconds so 'time of flight' for the two radio signals will not be the source of your echo as the delay will be too short.

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