[GreenKeys] Frequency diversity radioteletype experiments

[John Vendely]

Howdy,

The military sets with which this waveform were used all have a nominal 
3 kHz BW (e.g., audio passband 250-3250 Hz), so the 2805 Hz subcarrier 
passes with no problem.  This certainly could be a problem with some 
non-SDR ham-type transceivers.  Regarding transmitter power, each 
subcarrier is at 6 dB below PEP, giving a peak/average power ratio of 
2:1.  Linear amplification is required, of course.

We ran some informal tests comparing standard 850 shift and the MD-522 
frequency diversity mode and observed substantial reduction in error 
rates in diversity mode, even though the single-channel 850 shift FSK is 
transmitted at 4X the power of either diversity mode subcarrier.  The 
very narrow demodulator bandwidth of 85 shift relative to 850 shift 
gives a signficant improvement in SNR and reduced detection threshold.  
However, the problem is that during fades, the two closely spaced tones 
virtually always fade together, negating some of the advantage of narrow 
shift FSK. This is more than compensated for by adding a second  85 
shift subcarrier, widely spaced from the first.  The spacing is 
sufficiently wide that the two subcarriers tend to fade in a nearly 
uncorrelated manner, much less correlated than the mark or space fades 
encountered with 850 shift.  Error rates are therefore reduced relative 
to 850 shift despite the reduced power per subcarrier.  In effect, you 
get the SNR/threshold advantages of NSK, but mitigate the flat fade 
problem.

The system does work well.  There were times in which usable copy was 
obtained when the signals were barely audible.  Substantial immunity 
from interference from SSB, CW, and other digital stations which often 
start transmitting on top of us was observed.

73,

John K9WT



On 10/12/2026 1:29 PM, Harold Hallikainen via GreenKeys wrote:

> That sounds great! The 2805 Hz is pretty high for an SSB transceiver.
> Also, it seems that the levels of the simultaneous tones have to be kept
> down since their amplitudes add. I wonder how this compares with just
> running wide shift (say 2 kHz) with dynamic threshold control.
>
> I want to experiment some with space diversity. It will not be as fancy as
> having two receivers and a diversity TU. Instead, I'll just switch
> antennas whenever the TU is in mark hold. There will be errors, but
> ideally there will be better copy in a long fade.  A TV satation I worked
> with did this on a microwave receiver. There were two dishes on the tower.
> During a fade, they would switch to the other dish.
>
> The 8:30 AM ET is a bit early for me being on MST. But I'm almost always
> on 14.086 MHz LSB (14.083,875 MHz Mark, 14.083,705 Space) autostart during
> the day.
>
> Harold
> https://w6iwi.org
>
>
> On Mon, October 12, 2026 7:32 am, Nick England via GreenKeys wrote:
>> Ron K3TZJ, John K9WT, and I had very nice QSO/test session last week and
>> today using the MD-522 diversity mode for radioteletype. It worked very
>> well indeed at overcoming fading and QRM. This mode sends identical 85 Hz
>>   shift tones centered at 425Hz and 2805Hz. Come join us or listen in.
>> Monday 0930 EDT. 7087kHz USB. Mark low. 45 baud.
>>
>>
>> And don’t forget our regular Saturday morning 850 shift net starting
>> around 0830 EDT also on 7087 USB (2000 Hz center). A couple of the fellows
>> have used spatial diversity for receiving on that net, with two different
>> antenna/receiver channels feeding a diversity combiner TU.
>>
>> Remember, if you don't have a short wave receiver that you can always
>> listen in via web SDR http://www.websdr.org/
>> http://kiwisdr.com/public/
>>
>>
>> Have fun,
>> Nick England K4NYW
>> Chapel Hill NC
>> www.navy-radio.com
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>