[OZAPRS] Low cost HF Modem

[Noel Rowe]

Bob made this comment:

>My recommendation would be to NOT output the
>APRS to 300 baud TONES (which needs an SSB
>transmitter) but convert it to simply digital TTL levels
>to drive a crystal and generate FSK directly!

I have done exactly this in a prototype HF beacon that I was commisioned
to
make for a marine (commercial, not amateur) situation.  With some L and C
and a switching transistor, I made an existing crystal controlled three
transistor low powered transmitter (about 2 MHz) into an FSK 200 Hz shift
beacon.

Its origonal configuration was a floating buoy using a CW ident
(controlled
by a PIC chip) which was followed by several seconds of solid carrier to
allow for a boat to DF it.  After I modified it, the CW ID was followed by
an FSK transmission.  This was generated by setting a Garmin "mouse" GPS
to
output just one type of NMEA sentence at a 300 baud comms setting.   The
PIC
chip simply gated that to the FSK switching transistor, with the correct
start/finish timing.

Because it was a standard 200 hz shift/300 baud FSK transmission, normal
gear could be used to receive it.  What was received was a replica of the
actual GPS sentence.  Once I had demonstrated to the company that the
principle worked, the project was passed over to others for further
development (FSK ID etc) and I lost track of it.

If we took up Bob's suggestion, we could have a PIC chip sending the data
as
mic-e instead of NMEA, which would get a much shorter beacon than the one
I
had.

Noel, ZL3GR.

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