How to mitigate fading of SW reception

[prabhatkarpe4]

I would like to know what can be done to mitigate fading of SW reception?

I have noticed such fading on MW too but not on FM. Why is that?

[Grandchuck]

SW sigs reflect off the ionosphere.  FM direct waves.

[Andy Chee]

I would like to know what can be done to mitigate fading of SW reception?

I have noticed such fading on MW too but not on FM. Why is that?

Radio waves can travel in three main modes;

1) sky wave
2) ground wave
3) line-of-sight

[A.Z.]

https://en.m.wikipedia.org/wiki/Radio_propagation

[TimFox]

The standard method to reduce "fading" in the HF frequency range is "diversity reception".
https://play.fallows.ca/wp/radio/shortwave-radio/diversity-reception-primer-improving-radio-reception/

[Dave Wise]

Another tool for AM SW is a synchronous demodulator.  Some fades are frequency-selective, and if the fade happens to take out the carrier, the sidebands turn to mush in an envelope detector.  A synch generates a local copy of the carrier that can carry you across the fade.  My Sherwood SE-3 is always connected to my SP600's IF OUT jack.

[fourfathom]

Ionospheric refraction usually contains multiple copies of a given signal, each with a different delay and polarization.  Apparently polarization is a big factor.  A friend is studying the polarization of sky waves, and is using some method of rotation with orthogonal E-field probe antennas.  He has shown that with polarization control he can reduce typical fading depths by well over 20 dB -- sometimes a lot more.

However I don't think this really helps those of us with the typical receive setup.

[KK6IL]

With FM (Frequency Modulation), the information is carried by changes in the transmitted frequency, not the signal level as in AM (Amplitude Modulation). FM receivers include a limiter which keeps the signal level constant going into the demodulator (which converts the incoming RF signal into the original audio signal), regardless of the incoming signal level. Thus signal level fading has little effect. Only the lowest of incoming signal levels will not generate clean audio.

A side effect of this is that if two transmitters are operating on the same frequency, the strongest signal will capture the receiver, unless the signals are coming in at the identical signal strength.  This often occurs on the VHF and UHF ham bands when two people transmit at the same time and only the stronger signal is heard.

John   KK6IL

[radiolistener]

The standard method to reduce "fading" in the HF frequency range is "diversity reception".
https://play.fallows.ca/wp/radio/shortwave-radio/diversity-reception-primer-improving-radio-reception/

This is interesting, is there any document which describes in short matrix math for diversity reception?

I mean practical information, not abstract talking. Something that allows using these matrix formulas in a Matlab/Octave script, inputting a WAV file with a multichannel synchronized antenna recording, and getting real results.

[radiolistener]

Another interesting question - I’ve noticed that Chinese broadcast stations often transmit the same AM signal from different transmitters on different frequencies. This is quite common now - two, sometimes three frequencies simultaneously broadcasting the same AM signal. They also experience fading. Theoretically, for three stations, the signal could be recovered by discarding one of the three. But more often, it’s just two stations. I think that’s enough, as each station duplicates both the LSB and USB spectra, so with two synchronized stations (with calibrated signal delay between them), we get four copies of the spectrum, which should be enough for high-quality signal recovery.

Is there any established mathematics for such operations?

An even more interesting question - is it possible to recover an AM signal damaged by fading using only the LSB and USB parts of the spectrum for single AM station? This assumes that the fading affects only one part of the spectrum. I think it might be impossible, as two copies of the spectrum may not be enough to reliably recover the information. But could it still be possible for AM signals using their specific properties?

[coppercone2]

Ionospheric refraction usually contains multiple copies of a given signal, each with a different delay and polarization.  Apparently polarization is a big factor.  A friend is studying the polarization of sky waves, and is using some method of rotation with orthogonal E-field probe antennas.  He has shown that with polarization control he can reduce typical fading depths by well over 20 dB -- sometimes a lot more.

However I don't think this really helps those of us with the typical receive setup.

what is polarization control with this context? At the receiver, transmitter or both at the same time?

[fourfathom]

Ionospheric refraction usually contains multiple copies of a given signal, each with a different delay and polarization.  Apparently polarization is a big factor.  A friend is studying the polarization of sky waves, and is using some method of rotation with orthogonal E-field probe antennas.  He has shown that with polarization control he can reduce typical fading depths by well over 20 dB -- sometimes a lot more.

However I don't think this really helps those of us with the typical receive setup.

what is polarization control with this context? At the receiver, transmitter or both at the same time?

At the receiver. There are often two or more components of the ionospheric refraction and these experience different polarization rotation which can be quite different from the horiz or vertical transmit polarization. Being able to adjust the receive antenna polarization for best vector summation can help a lot.
As I said, this isn't really helpful information for most of us, but my friend is an antenna specialist and builds antennas and gear for this kind of research. Many of his antennas are short probe types.

[coppercone2]

short probes might not be receiving cuban propaganda making them more interesting to research then actual communications equipment

I think I heard a on air recording of fidel a few years ago when doing loop antenna experiments and suddenly I got way more interested in near field then far field. the loop antenna seems to concentrate communist fields

[iMo]

https://www.eevblog.com/forum/beginners/newbies-please-read-before-posting-24542/