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[aprssig] Catching a Jammer with APRS!

Scott Miller scott at opentrac.org
Tue Oct 2 05:14:15 UTC 2007

And there's a firmware version for the OT1+ that reports one of the 
external ADC inputs as an omni DF signal level.  I sent it off to 
someone who was going to try it out, haven't heard back yet.  I've only 
tested it myself with a variable power supply input.


Stephen H. Smith wrote:
> Bob Bruninga wrote:
>> Yes, this is a kludge, using only existing hardware, and no special 
>> firmware and no special software and not even APRS.  THis is just a 
>> simple way to do the original APRS [plan for automatic DFing networks 
>> which we never could implement because only APRSdos had it built in 
>> and without other players, it is one hand clapping.
>> But it is easy to see, that Opentrack, Tiger track or Byonics or 
>> anyone that is making pic trackers could easily modify an input pin on 
>> their devices so that not just the presence of the squelch on the 
>> moiitor can be transmitted, but now also the SIGNAL STRENGTH.  Doing 
>> this, gives us the automatied DF signal strength network APRS was 
>> originaly supposed to do...
> While the TinyTrak and TigerTrack don't have an analog input for RSSI 
> (Received Signal Strength Indication), the OpenTracker DOES have such an 
> input now.     Usually it's used for temperature or battery voltage 
> reporting.
> The real problem is trying to get a point in the radio receiver that 
> varies with signal strength. On most FM rigs, the IF tends to saturate 
> quite abruptly somewhere between half a microvolt or so and about 1-2 
> microvolts (i.e. typical FM limiting effect - note how the S-meter on 
> most FM rigs tend toward an "all-or-nothing" reading).  
> I've struggled with this issue for over a decade now building setups for 
> radio signal strength coverage mapping systems.    I've wound up tapping 
> the FM rig's IF somewhere before the limiting saturated stages, and 
> routing it to an external spectrum analyzer or an HF-SSB rig turned to 
> the FM rig's IF freq.      The non-saturating IF of the SSB/AM rig 
> provides a useful logarithmic S-meter response over a wide range of 
> signal levels (at least 4 decades).     My best measuring setup uses an 
> IF-1500 service monitor.  This unit has an FM receiver and an AM 
> receiver driven off the same IF system.  The receivers demodulate 
> whatever  is at the center of the spectrum analyzer display.   The AM 
> receiver drives a S meter with a useable more-or-less logarithmic 
> response between 0 and full scale (yes, it's an analog meter!) of over 
> 60 dB. 
> I can tune the IFR's receiver to any freq between about .5 and 1000 MHz 
> so I can tap the IF of just about anything.   One could connect an 
> antenna directly to the IFR  RX-in, but it's not very sensitve, and it 
> has NO front-end selectivity. The transceiver being tapped provides 
> 10-20 dB gain and the selectivity of it's front-end and 1st IF filters 
> which makes the IF VASTLY more usable for off-air monitoring.
> -- 
> Stephen H. Smith    wa8lmf (at) aol.com
> EchoLink Node:      14400    [Think bottom of the 2M band]
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>  http://wa8lmf.net/APRSmaps
> JavAPRS Filter Port 14580 Guide
>  http://wa8lmf.net/aprs/JAVaprsFilters.htm
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>  http://wa8lmf.net/DigiPaths
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