Reducing sky wave interference of an AM signal

[gary]

Does anyone know of a technique (e.g. antenna design, etc) for reducing the interference caused when (at night) a sky wave signal interferes with it's own ground wave signal - thus causing fading etc.

[Klaas Robers]

Which frequencyband Gary? Medium wave?

[gary]

Which frequencyband Gary? Medium wave?

Yes! sorry should have stated that.

[Don Tomkins]

Gary,
Reminds me of growing up in the UK in the 1950's/1960's and listening to Radio Luxemburg and the off-shore pirates. AM fade was terrible in the evenings but it was the only source of pop music then until the BBC finally restructed their broadcasting format.

There was a design for a special 208 (Radio Luxemburg) tuner in Practical Wireless in the early 1960's that was claimed to overcome fading but I cannot find reference to that any more.

One way to cobat the fade which I am sure you know is due to the loss of AM carrier due to in-phase destructive mixing of the two received signals is to use ECSS demodulation (exhaulted carrier single sideband). A number of short wave communication receivers have this mode built in.

You can simulate this by using SSB mode on AM and tuning the the BFO very carefully onto the upper or lower sideband but then the bandwidth is somewhat restricted and you need a delicate touch on the tuning.

There is a very good off-board unit is made by Sherwood Engineering, Denver CO, USA which gives higher fidelity but it is very expensive at US$500+. Several years ago a kit for a unit was sold by a radio engineer in Virgina based on a Sony chip. If I can find the circuit diagram, I will post it on the stream.

Antenna wise, a loop design is definitely a good starting point. There are many designs around and a good source would be one of the late Joe Carr's antenna handbooks. I came across one free to down load on the internet a couple of weeks back.

Another approach that can be effective is a phasing unit at the front end to couple a signals from a loop with that from a vertical long wire or dipole.

regards
Don

[gary]

Thanks for that Don, I haven't replied until now because I wanted to research the approaches you have mentioned. For instance, I had never heard of "exhalted carrier single sideband" demodulation. It makes sense and since it just requires some form of BFO it seems a worthwhile experiment - it's times like this that I wish I had expended the money on a decent (or any) signal generator. I have always gotten by using a PC and sound card since most of my work is in the audio frequencies.

I had been using a loop antenna but had not completely appreciated it's directional characteristic so more investigation required there.

Again, not being a "ham", I had heard of phasing units but did not really appreciate exactly what they were doing. I am currently investigating an active component implementation of same (I have a horror of inductors).

I must admit, I had been told of an antenna design that would combat fading caused by skywave interference and was hoping someone would know of it, none-the-less your suggestions are interesting, worth investigating, and very much appreciated. Thanks again.

[Klaas Robers]

Gary, the best trick is to use diversity reception. Then you use two radio's, almost identical, and connect them to two antennas spaced a quarter to a half wavelength apart. You will see that in most cases when one radio suffers from deep fading, the other plays optimal.

If you compare the S-meter readings you may switch (by a relay) your loudspeaker to the highest S-meter-reading receiver.

If you want to be even better use three radios and place the antennas in an L shaped position.

This works better than all phasing circuits of the world.