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[aprssig] diplexer/duplexers for APRS

Jim Lux jimlux at earthlink.net
Wed Sep 7 00:18:22 UTC 2005

It occurs to me that since APRS is a single frequency kind of operation, 
you might be able to get a SAW filter made that just passes 25kHz around 
144.39, and another that rejects the 25kHz around that same frequency 
(actually, it might be the same filter).

These days, a typical attenuation through the filter might be 4 or 5 dB, 
and the isolation some 50dB.  Transmitting, say 10W (+40dBm) into a 
reasonably well matched antenna (say, 10dB return loss), you'd get +30dBm 
into your filter.  That's a bit much for a SAW, but perhaps not, and after 
the filter, it would be down to -20dBm, which might be within the rejection 
range of your receiver's front end.

You really only care about this on receive anyway (there's all kinds of 
ways to combine the transmitters)

A cleverer way would be to manage the voice/data intermixing 
appropriately.  The problem is that the voice Tx would block the APRS 
coming in, but presumably, the APRS data will repeat sometime in the 
future. The APRS Tx, since it's only your posit, and relatively infrequent, 
could be deferred until some number of seconds after your last Tx, to allow 
for a response to your voice Tx.


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