Many European stations run multiple modes as well as additional packet ’channels’ for BBS interlinks etc, these can be found only on the USB frequency as the multi modes such as VARA, ARDOP and Robust Packet are sideband sensitive and sit at a fixed center frequency of 1500hz.
28.1371 USB (Updated 11th November 2022)
The USB dial frequency of 28.1371 has been selected to help keep backwards compatibility with the older hardware TNCs with an easy dial frequency for them on the new “Net10” 28.140 LSB at 1700hz 300 baud packet tone center frequency (KAM tones). The 1000hz port equivalent on the USB dial frequency retains compatibilty because AX300 AFSK tones are sideband insensitive.
The various new-skool multimodes run at a fixed center frequency at 1500hz, with the packet channels described sitting on either side, depicted in this screenshot:
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