[GreenKeys] Space diversity receive antenna

[Harold Hallikainen]

On Mon, January 6, 2025 8:59 am, John Vendely wrote:
> 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.
>


Thanks for the extensive comments! I do think 850 Hz should help. I need
to do more testing. https://w6iwi.org/rtty/DspTU2/ has "dynamic threshold
control". The code looks at the peak level during each mark or space and
sets the threshold to half way between these two levels. This approach is
discussed in "A New Approach to TU Design Using a Limiterless Two-Tone
Method, Frank Gaude, K6IBE, RTTY, Volume 11, Number 6, June 1963. " (
http://plaintext.w6iwi.org/rtty/TuReferences.html#:~:text=A%20New%20Approach%20to%20TU%20Design%20Using%20a%20Limiterless%20Two%2DTone%20Method%2C%20Frank%20Gaude%2C%20K6IBE%2C%20RTTY%2C%20Volume%2011%2C%20Number%206%2C%20June%201963.
).

This article describes an analog method adjusting the mark/space threshold
using analog circuitry (diode peak detectors with slow RC filters).

I'll do more experiments on both 850 Hz shift and diversity antennae. QSB
is really frustrating! I often receive a perfect CQ from KE3BK but have to
abandon the QSO due to QSB.

Thanks for the comments!

Harold
https://w6iwi.org



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