[GreenKeys] Frequency diversity radioteletype experiments
Duncan Brown
duncanancy at earthlink.net
Mon Oct 12 20:03:18 EDT 2026
i knew that the military used HF multiplexing of TTY signals, but never
realized it was using 85Hz shift. I guess I assumed it was more like
terrestrial microwave FDM, with 4 Khz channels. (But thinking about it,
that would have been pretty wasteful!)
Thanks,
Duncan
K2OEQ
On 12-Oct-26 17:48, Nick England wrote:
> Another place for similar technology was in the Navy’s HF fleet
> broadcast. Eight streams of different information were transmitted
> within a 3kHz band. Each stream comprised two 85Hz shift channels
> separated by 1360 Hz for frequency diversity (so 16 channels total).
> And space diversity could be added in by having two antennas and
> receivers as well.
> https://www.navy-radio.com/rtty/ucc1/ucc1-sys-101.jpg
>
> Some additional info at
> https://www.navy-radio.com/rtty-mux-ucc1.htm
>
>
> *Channel * *Designation * *Use*
> 1 FASW Destroyer force primary traffic channel
> 2 FSPC 1 hour delayed rebroadcast of channel 1 or overload as necessary
> 3 FALD Service, Amphibious and Mine force primary traffic channel
> 4 FUSN 1 hour delayed rebroadcast of channel 3 or overload as necessary
> 5 FNSC Major warships (carriers, cruisers, large destroyers,
> including flagships) primary traffic channel
> 6 FOPI Special traffic channel (NAVSECGRU)
> 7 FHIC 1 hour delayed rebroadcast of channel 5 or overload as necessary
> 8 FMET Weather traffic channel
>
>
>
>
> Nick England K4NYW
> Chapel Hill NC
> www.navy-radio.com <http://www.navy-radio.com>
>
> On Mon, Oct 12, 2026 at 5:07 PM Duncan Brown via GreenKeys
> <greenkeys at mailman.qth.net> wrote:
>
> 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 <http://www.navy-radio.com>
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>>>>
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