[GreenKeys] HoffNet on half-kHz frequencies

[Eric Scace K3NA]

Hi --

   I am still the custodian of a AN/GRC-106A (with a dead oscillator, but that's another story), which has seen many years of
service on autostart frequencies in the Navy MARS program.  So I have a little experience with this generation of military radios.
And, as a result, I'm puzzled about the concern over older radios with step tuning that only handles exact 1 kHz steps.

   The GRC-106A has a vernier tuning control to allow the transceiver to operate between kHz steps.  The original version did not.
This radio is USB and CW only -- no FSK nor LSB.

   So let's assume we wish to accommodate transceivers like this:
-- exact kHz step tuning
-- no vernier controls
-- no FSK mode
-- USB only for sideband modes.
-- TU such as an ST-6 accepts standard tones of 2125 and 2295 Hz, but can be reversed so that 2295 is interpreted as mark.  The
reversed tones compensate for the use of USB.

   In this case, the proposal to use 10127.0 as the mark frequency instead of 10127.5 doesn't help at all.  There will be no way to
tune such a transceiver to place a mark signal on 2295 Hz.

   Therefore, we have two choices:
   a) place the mark frequency on x + 2.295 kHz, where 'x' is an exact kHz.  But many modern ham rigs with true FSK transmit ability
do not synthesize down to the 1 Hz level.
   b) place the mark frequency on x + 2.300 kHz.  That means a radio with 1 kHz step tuning will be 5 Hz off, which is within the
error tolerance of the filters in most TUs.
   b) forget about supporting radios with 1 kHz step tuning.

   Taking into account other monitoring results and comments, we could choose a mark frequency of 10127.300 kHz (and space frequency
of 10127.130 kHz).  That seems relatively clear, according to past reports.  And it seems to allow folks to choose a wide range of
radios to play with.

   And I apologize for muddying the waters further on this project!

-- Eric K3NA