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Mr.RC-Cam

Small/affordable APRS transmitter kit

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The Oct 2004 QST magazine had an article on a small APRS transmitter project. It operates on 144Mhz using your ham license. Power is 250mw, so range is good for a couple miles (or tens of miles if a digi-peater is nearby). It operates on a 9V battery for several days and everything fits in a Altsoid tin mint box (including the TinyTrak3 interface). It would make an excellent stand-alone backup transponder for downed UAV's.

The kit is sold by these folks: http://www.byonics.com/pockettracker/

pockettracker8.jpg

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I just took delivery of the new SMD tinytrak3 and this thing is shockingly small.

I'm integrating it to my 600mw transmitter... has anyone else done this yet (with this particular tx)?

I'm finding that the audio level is really really low on the output pin even with the POT turned all the way up. I ended up circumventing the pot and and one resistor on the output, but now I'm overdriving the audio a little bit, and will probably just short out the resistor and go back to the pot output.

Any other feedback would be appreciated.

Did I mention this thing is shockingly small? :)

Regards,

Bill

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I'm overdriving the audio a little bit, and will probably just short out the resistor and go back to the pot output.

I think you need a 1V audio level. That would imply that the fixed series resistor should be about 50K rather than what I believe is the stock 220K (that is what the TT2 used). The resistor's idealized value will be affected by the input impedance of the video Tx's audio path, but the 50K is a good place to start.

If the TT3 is like the TT2, then the output of the descete A/D is not buffered. So, radical changes in that series resistor might dork FSK fidelity. If you have a scope then verify that the stepped sinewave is not a bit squished or mangled after the fix.

RC-CAM

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Good point, I will fire up the test-tone on the TinyTrak and see what the signal looks like at the output pin. The test tone looked pretty good when I ran it through the frequency analyzer included with AGWPE.

I shorted the resistor last night and at least the tone can be adjusted over a reasonable sounding level now. I'll probably end up doing as you suggest and simply replacing it with a lower value to help maintain the integrity of the design. I even managed to get the AGWPE TNC to recognize a packet after fiddling with the frequency adjustment in the tinytrak configuration software.

Regards,

Bill

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