[aprssig] HF APRS Transceiver

Robert Bruninga bruninga at usna.edu
Mon Mar 10 09:20:51 EDT 2008

One of the original applications of APRS was on HF for tracking
boats back in the early 90's.  The original HF.TXT file contains
the concept of a $50 50W HF transmitter using just an Xtal
oscillator, buffer, driver, amp, drive and PA all to fit in a
box the size of a cigarette pack..

But the issue was always stability.  It was just not achievable
with simple XTAL oscillators.  And no sense sending a boat to
sea if it could not stay on channel without constant attention.
(Better than +/- 10 Hz).

> Overkill.....
> Most Ham rigs are crystal stabilised anyway, even 
> if they use a synthesised VFO, the base clock is 
> always a free running crystal.

True, but it is a very special calibrated, high-stabilty
frequency reference grade xtal produced on a standard frequency.
Such a xtal custom made on a single APRS frequency would cost
more than the cost of the rest of the transmitter.

> ... most of the old style land line modems were only 
> crystal clocked at best, and not a high spec xtal either! 

Yes, because 20 parts-per-million at a moden tone of 1200 hz is
only 0.02 Hz which is extremely accurate for a modem tone.  But
20 ppm for a 10 MHz APRS transmitter is 200 Hz which is
completely out of the passband.

The original poster was correct:

> > Crystal control is not sufficient.  20ppm at 10MHz 
> > is 200Hz which is an order of magnitude or more 
> > too high (consider that the shift of the FSK is 
> > only 200Hz).  You probably need around 1ppm accuracy.

So, even though a 50 or even 100W HF tranmitter for APRS could
be made with only a few transistors and pair of cheap MOSFETs
and be no larger than a cigarette pack (no heat sink needed for
2 second packets once every 10 minutes), the frequency stability
was always the issue on HF.

Running 1200 baud HF on 10 meters of course greatly eliminates
the problem because it can tolerate much higher frequency drift,
but then what good is an APRS signal on 10 meters where no one
is listening..

HUMMH>>>>>  ????  But arent we going to be coming out of the
solar minimum now?  And don't we now have Igates all over the
world?  Hummh... Maybe that is exactly what we DO need.  Is a 25
Watt APRS transmitter XTAL controlled on an APRS channel on 10
meters, and then lots of HF Igates listening on 10 meters.
Size, about the size of a cigarette pack.


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