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