8 April 2020

F7B with mixed offsets mode (likely Ukr nets)

Some days ago my friend Mike (mco) sent me an interesting record about a seeming MFSK-4 signal (fig. 1) originating north of Rivne, Ukraine; such (duplex) transmissions are frequently heard on 13873.55, 15833.55, 15863.55, 16354.55, 17412.55, 17442.55 & 17469.55 KHz (Rivne TX).

Fig. 1
Actually, it's an F7B mode [1] which is actively used by Ukrainian Nets. SA program parses these transmissions as classic MFSK-4 therefore the bitstream after MFSK demodulation does not make sense. F7B is a FSK modulation technique with four modulation frequencies, the two transmission channels, termed V1 and V2, usually transfer T-207 ciphered data and are obtained through six possible combinations of the four frequencies and this leads to 6 different modes F7B-1 to F7B-6.
Hoka Code300 demodulates such transmissions using the mode "Baudot F7B" and the right offsets: since the output will always be something like garbage (contents are encrypted) will be quite difficult detect the correct mode.
This sample I think it's a  combined "free modes" in which the used offsets and speeds are: -1200 -400 +400 +1200 400Bd, -750 -250 +250 +750 250Bd, and -600 -200 +200 +600 200Bd; anyway, it's to be noticed a change of the center frequency between the first and the second mode (Figs 2,3).

Fig. 2
Fig. 3
"They use a wide range of frequencies and I've not noted all of them. This list seems ok but it certainly include Main stn freqs and also side B stations ones and some are missing. They are working full duplex with abroad stations according to multiple TDoAs. So far I've seen them using MFSK-4 with 50/100 80/160 100/200 125/250 160/320 200/400 250/500 400/800 500/1000 settings" my friend Linkz say.

Fig. 4 - TDoA Tx site (thanks to Mike)
[1] http://resources.rohde-schwarz-usa.com/c/manual-of-transmissi-2

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