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[aprssig] ARPS-HF Frequency Stability Question

Ka2upw at aol.com Ka2upw at aol.com
Tue Oct 4 04:01:03 UTC 2005


Jon Adams wrote:

>Good morning all -
>
>I've been running HF APRS for over 10 years and as an HF Gate for nearly
>that long. When I listen to the traffic, I hear stations high and low of any
>one frequency, and if they're more than a few dozen Hz off, they don't
>decode very well. (running Kam+).

Or, alternatively, instead of a Kam+ you could use a 
PC soundcard DSP program that can search across 
a chunk of RF spectrum.  Then you wouldn't have to
worry about their signal being exactly on frequency,
just reasonably close.  And you can define "close"
yourself.

I don't know if anyone has done this already, but it 
wouldn't be too hard.  With a typical SSB receiver you 
could search for any AX.25 packets in the receiver's 
bandpass (let's say 3 kHz). If you wanted to get fancy
you could even use something like an SDR-1000
or a Softrock-40 type receiver.  This could digitize a
large (let's say 48 kHz) piece of the HF band and then 
you could search over whatever range of frequencies you
wanted ( +/- a few Hz or +/- a few kHz ) for all of the 
AX.25 Bell 202 packets. You're limited only by the
speed of your processor.

Hasn't someone already written the "search-for-and-receive- 
all-on-frequency-and-off-frequency-HF-packets" soundcard 
demodulator program?

Douglas KA2UPW/5
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