[GreenKeys] Frequency diversity radioteletype experiments

Mon Oct 12 17:07:52 EDT 2026

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 donâ€™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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