John,

Thanks for the explanation of of the MD-522 diversity.  I had wondered how well the MD-522 diversity worked, with the two signals being so close together (at 2.4Khz).  But I had not thought about the improvement from the narrower bandwidths.  Better than a single 850Hz shift signal, but probably not as good as two 850Hz signals a few hundred Khz apart.  

Apparently the Army decided that the simplification of only one transmitter & antenna was worth it for tactical communications.

Thanks for the info,

Duncan
K2OEQ
31J30


On 12-Oct-26 16:40, John Vendely via GreenKeys wrote:
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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