> Most applications of diversity reception are on the MF and low HF bands. I
> believe that most fading on these bands is selective fading, where signals
> traveling slightly different paths, the low-frequency equivalent of
> picket-fencing at VHF/UHF. It might be worth studying propagation on the
> MF/HF bands before assuming that the two receivers can be added. I could be
> wrong, but I strongly suspect that signals at the two receivers will be
> displaced in time, and that the displacement can be a variable by virtue of
> the variability of those paths.
>
Jim,
symbol transmitted by FT8, JT65 and so on are long hundreds milliseconds.
On HFs, such time interval is usually 1) shorter than the channel coherence
time (which is seconds) and 2) still much longer than the channel delay
spread (which is milliseconds).
Therefore, for signals few Hertzs wide, the channel appears as a
slow-varying frequency-flat medium whether you use diversity or you don't.
In these cases space diversity helps only because the probability that two
uncorrelated signals exhibit a given amount of fade at the same time is
much smaller than the probability that a single copy fades out.
Frequency selectivity can be almost safely neglected. You don't need OFDM
systems to cope with selective fading if you are transmitting at few bits
per second in the HFs.
73
Nico / IV3NWV
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