TopBand: diversity

Peter Chadwick
Mon, 22 Dec 1997 14:34:40 -0000

Bill, K0HA says

> Tonight brings home a point that's been in my mind for some time--
>dual frequency receivers are nice, but when is a HF diversity
>reception receiver going to show up?  

One of the difficulties is to decide what form of combining should be
used. Historically, you tied the audio outputs of the rx's together, and
the AGC lines, so that the strongest signal produced enough AGC to cut
back the noise from the receivers which had the weakest signals. But we
have the situation that none of the signals are generally very strong,
so it's quite likely that there will be insufficient AGC to be very
helpful. In any case, you need a separate receiver as far as the audio
for each diversity channel. Other methods use automatic switching
looking for best SNR; I guess this wouldn't be too hard to do with
suitable DSP on the audio, looking for the best coherent signal in the
noise. It should be possible to do it at IF with modern DSP, too. Or you
could use a PLL at IF, and look for the one with the least noise on it.
But the biggest problem is that of getting suitable RF combining without
too many separate rx front ends. If it was just a case of combining
antennas for maximum signal, you could use hybrids, but I think that you
also need to look at minimising inputs from antennas which will just add
noise. The best bet is probably a PIN diode switch, processor
controlled, looking for the antenna with the best SNR, although under
QRM condx, it might choose the best QRM SNR, rather than the best wanted
sig SNR. I guess you really need the best possible selectivity ahead of
it to make it work really well.

In cellular, you don't generally have a noise problem, so hybrid
combining works, although I believe that many cellular bases use voting.
Not too easy under topband conditions.........................

This morning, best rx conditions for W were on the 80m dipole.......


Peter G3RZP

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