Re: High-tech tracking by police raises legal outcry

[jim bell <jimbell@pacifier.com>]

At 06:39 AM 1/6/97 -0600, Mike McNally wrote:
>Secret Squirrel wrote:
>> 
>> The technology, marketed by a company called Teletrac, is simple: 
>> A tramsmitter sends a radio signal to a computer ...
>
>Anybody know the frequencies used?
>
>(Anybody willing to guess whether the FCC might quietly introduce
>prohibitions against scanners that can receive those frequencies?)
>
>(Gee, that looks paranoid.)


This wouldn't do a great deal of good.  The systems were already described 
as frequency-hopping.  The hops are probably fast enough to outwit a 
scanner's receiver.

However,  it turns out that there is a comparatively simple way to detect 
such transmitters:  An old-fashioned diode detector using modern components. 
 Take a loop of wire (about a foot in diameter), add a microwave-capable 
signal diode to rectify the signal and send it to a small capacitor and then 
go to a DVM.  (digital voltmeter.)   (Sensitivity can be dramatically 
increased by also inserting a blocking capacitor in the loop and adding 
enough DC voltage to barely forward-bias the diode.)   A momentary (or 
continuous) increase in output voltage indicates that the diode is 
rectifying AC, which indicates a transmitter.  Buffering the DC-level signal 
and sending it to a set of earphones will indicate a pulsed transmitter.

Interestingly, this is probably more or less the circuit that was originally 
used in 1970's Fuzzbuster-type radar receivers before the heterodyne systems 
were developed.  The "disadvantages" of that circuit, in microwave-radar 
detection, are either not disadvantages or are in fact advantages in 
hidden-transmitter hunting.  The first "disadvantage" was that this receiver 
was EXTREMELY broadband, practically "DC-to-daylight," or at least up to the 
rectification capability of the diode chosen.   (I've implemented systems 
that will do 20 gigahertz easy, and I was using old diodes!)   False 
triggers were a common result, due to hard-to-avoid rectification of CB and 
ham radio transmissions for example.   In the case of transmitter hunting, 
that disadvantage is a solid advantage, because you don't need to know what 
frequency the transmitter is at.

The second disadvantage, comparatively low sensitivity, was a problem if 
you're trying to detect a 100 milliwatt radar transmitter 300 meters away, 
but if you know you only have to search a car from a meter away, the 
inverse-square law indicates that you're going to see a signal with around 
100,000 times the power level all other things being equal.


Jim Bell
jimbell@pacifier.com