[aprssig] Catching a Jammer with APRS!

Stephen H. Smith wa8lmf2 at aol.com
Mon Oct 1 19:29:37 CDT 2007

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 

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
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