[aprssig] HF APRS Transceiver

Bob Donnell kd7nm at pugetsound.net
Fri Mar 7 15:43:04 EST 2008

It's possible that the MFJ rig could be made to do APRS transmissions on HF.
It would be necessary to modify the transmitter to allow direct
frequency-shift keying, of 200 Hz.  It would be necessary for the oscillator
to be quite stable.  This implies a pretty stable frequency source -
referenced to or accomplished with a crystal oscillator.  The requirement to
be able to send the data is that the APRS encoder has to provide a digital
signal to the FSK circuit that would be added to the transmitter.  So it's
definitely not a plug and play situation.

Another writer stated that an SSB phone rig was required - that's not
correct.  The emmission of a properly operating SSB phone rig transmitting
AFSK packet audio is an FSK signal on the air, hence my comments above.

So in the case of adapting a "Cub" transceiver, you'd probably want to
replace the VFO with a crystal oscillator, or replace the transmit local
oscillator with a crystal for the higher of the two tones for the desired
operating frequency, and bypass around the transmit mixer, which wouldn't be
needed.  Then build up an FSK keyer to add a small amount of capacitance
(adjustable) across the existing trimmer, to set the amount of shift it
imposes on the carrier frequency.  The FSK keyer capacitance can be switched
with a diode or transistor isolation.  Alternately, a varicap diode could be
used, but then it becomes more necessary to insure that no extraneous signal
components (noise, etc) are present on the keying signal.  You'd want to use
a tracker that has an external modem chip - and support for 300 baud in the
firmware - the digital signal used to tell the modem chip to change audio
tones is the same signal you'd pick off and route to the FSK keyer described

So that's the nature of the challenge!  Hope that helps, and 73

Bob, KD7NM

-----Original Message-----
From: aprssig-bounces at lists.tapr.org [mailto:aprssig-bounces at lists.tapr.org]
On Behalf Of Geri.Norris
Sent: Friday, March 07, 2008 12:22 PM
To: TAPR APRS Mailing List
Subject: Re: [aprssig] HF APRS Transceiver

CW is a continuous wave and for APRS you will need a modulated signal for
the packet data (ones and zeroes) to travel on.
 CW is a single tone transmitter instead of several tones (voice
modulated) if you want to look at it this way.
Geri N5RIG

Eric H Christensen wrote:
> I've been trying to find the answer to this question for a few days but
you can't find good electronics answers anymore.  I'm hoping that there will
be someone on here that has more/better electronics experience than I.
> MFJ makes a "cheap" CW-only HF transceiver that is for single bands but
does allow a small amount of tuning within that band.  What extra circuitry
is needed to connect something like an OpenTracker+ to this type of
transmitter so it will transmit 300 baud packet?
> 73,
> Eric W4OTN
> _______________________________________________
> aprssig mailing list
> aprssig at lists.tapr.org
> https://lists.tapr.org/cgi-bin/mailman/listinfo/aprssig

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