[GreenKeys] Space diversity receive antenna

[John Vendely]

Tone frequency spacing is large enough with 850 shift to provide 
significant diversity effect, and many military and commercial FSK 
modems are designed to take advantage of this, automatically providing 
copy on one tone in the absence of the other.  When this is combined 
with another similarly capable modem in space diversity, really 
excellent fade immunity is obtained.

Combination time and frequency diversity modems such as the MD-1142 
mentioned by Brooke Clarke are even more effective.  These were 
available surplus for a time, and several of us have them and have 
experimented with them.  THese 8-channel modems provide remarkably low 
error-rate performance for non error-correcting baudot teleprinters, 
even in disturbed conditions.  The simpler two channel frequency 
diversity mode of the MD-522, which provides two 85-shift channels 
separated by about 2.4 kHz, is also effective.  The more modern military 
serial and parallel tone PSK modems all have bit-interleaving, thus 
providing a time-diversity effect.

I would be surprised if the current sound card based modems designed for 
amateur use include in-channel tone diversity capability, as most are 
oriented towards 170 shift, where little diversity effect occurs.  
Perhaps someone on the list familiar with the inner workings of these 
sound card modems can tell us whether any are capable of in-channel 
diversity on 850 shift...

73,
John K9WT

On 1/5/2025 10:43 PM, Harold Hallikainen via GreenKeys wrote:
> On frequency diversity, as shown at
> https://w6iwi.org/script/csvgraph.html?CsvUrl=https%3A%2F%2Fw6iwi.org%2Frtty%2FTuNotes%2Fms220517a.csv
> , there is some time offset in the fades of mark and space with 170 Hz
> shift, but not enough to keep one of them out of the noise. I need to run
> the experiment with 850 Hz shift.
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/greenkeys/attachments/20250106/ecd9a0bb/attachment.html>