[aprssig] HF APRS Transceiver

Ray Wells vk2tv at exemail.com.au
Mon Mar 10 16:49:38 EDT 2008

Dave Baxter wrote:

>Most Ham rigs are crystal stabilised anyway, even if they use a
>synthesised VFO, the base clock is always a free running crystal.
>Though some "base station" radios offer a high stability option of an
>ovened oscillator, many do not.   They all seem to work OK on AFSK RTTY,
>PSK, APRS etc etc...  So overall accuracy is not what's required, just

Agreed, that stability is the prime requirement. Who cares what the dial 
says. Given the number of different tone pairs in use with 0k3 HF 
packet/APRS, there are many different dial readings anyway.

>In any case, most 300bd modem chips, or software implementations I've
>come across, are surprisingly tolerant of any slight frequency offset,
>after all, most of the old style land line modems were only crystal
>clocked at best, and not a high spec xtal either!  And some even used a
>ceramic resonator to keep costs down.  They still worked OK...
Define slight frequency offset. With an MFJ1270B, +/- 30Hz starts to 
make a noticeable difference, depending on s/n ratio of received signal. 
Apart from static crashes, Mother Nature's phase distortion is a far 
more significant problem than stability or absolute frequency setting, 
and she's been having a ball on 20m and 40m lately.

When one considers the division ratio from crystal (ceramic resonator) 
frequency down to baud rate, errors become insignificant. Regardless, 
the components that set the TNC modem tones on both tx and rx are only R 
and C, and it's the modem tones that count, and their drift would be 
insignificant compared with the radio's stability.

>There are direct conversion phone radio kits out there, but I'd expect
>the image issue to be a limiting factor, plus you'd transmit two sets of
>tones too.  Unless you implemented something like the "Third Method" (or
>I/Q modulation/mixing) of SSB generation.  That'd probably work quite
>well at a guess, for a limited audio bandwidth, but is the effort really
>worth the payback?
A cheap transmitter is easy, just FSK a crystal and forget about audio 
tones. A half decent rx is another issue.

>As someone else said, otherwise sick early 706's (no working 2m)
>FT100's, Alinco's etc, would all do the job.   Even (at risk of getting
>flamed!) a converted multimode CB set?
Transverting from a 27Mhz CB is not beyond possibility. Inject on the 
high side (would mean sideband inversion, but not a problem) Great image 
rejection :-). Many SSB CB radios use an IF of around 10.7/11MHz which 
might be a tad close to 10147.6 APRS but it's worth looking into (The 
Digicom 100 has an IF of 10.2MHz).Getting the rx to work shouldn't be 
too difficult, nor should producing a few watts from the tx be too hard, 
if there's room for the extra L. Some synthesiser division ratios can't 
be changed outside of that needed to produce the 40 channels, so a radio 
with a PLL02A, or similar, would be the best bet there, unless you can 
get away with changing mixer injection crystals. With some makes you 
can, some you can't. There seems to be almost as many VCO frequency 
ranges as there are brands of radio, some as high as 38MHz. Some 
circuits look as if the manufacturer worked overtime to find the most 
complicated method to implement.

There's a book called The CB PLL Data Book, by Lou Franklin. It was 
published by C.B. City International in Phoenix, Az. My copy was printed 
in 1991. It's worth a read if you're contemplating conversion of an old 
SSB CB radio.

>Even better... Military surplus kit?  Extremely high spec
>accuracy/stability wise, often chanelized & with internal ATU's etc, and
>ideal for remote/mobile/rugged use.  Also often available at good prices
>too, look on eBlag (and other sites) for starters...    Expect much of
>it to work on 24/28V though, rarely 12V....   You could possibly fit a
>tracker inside some of them, just needing a GPS "puck" on the outside to
>make a self contained HF Tracker.
Cost could be an issue, and maybe also physical size. Look, honey, the 
radio fitted, by taking out the back seat :-)

>This is "Amateur" radio of course, so get the tools out and get in
>Dave G0WBX.
Ray vk2tv

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