[aprssig] 433 MHz operation

Scott Miller scott at opentrac.org
Wed Feb 15 12:12:14 CST 2012

Hi all,

I've been playing with some half-watt 70 cm data transceivers lately. 
I'm currently testing at 434 MHz, and with the standard crystal they 
should cover about 430 to 439 MHz.  I'm not really familiar with the 70 
cm band plan and figured I'd check here and see if people consider that 
a reasonable part of the band to be working in.

I've got another small batch of them coming soon and might start selling 
a board based around it for people who want to experiment.  Right now 
I'm using an OpenTracker USB variant.  It's currently running at 4800 
baud with some basic FEC - a (7,4) Hamming code with interleaving, not 
fancy but easy to implement.  The transceivers will go up to 115200 bps. 
  When I have time, I'll see if they can be made to inter-operate with 
G3RUH TNCs, but for now I'm going to assume that won't work.

The really attractive part is that they're cheap - retail price should 
be not more than 1/3 what we're selling the half-watt transmit-only 
2-meter modules from SRB for, and they're about the same size.

There are still some bugs to be worked out, but the OpenTracker USB (and 
by extension the Tracker3) will presently work transparently with these 
radios, doing APRS at 4800 baud with error correction.  I'll be working 
some more on them to improve mesh routing and channel access control to 
try to make them more suitable than vanilla APRS for short range, high 
rate tracking applications.


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