[sci.electronics] Homemade radar detector^2 ?

commgrp@silver.bacs.indiana.edu (12/09/89)

I decided to post this here, where the smart people hang out. :-)

There has been a long discussion on rec.autos about the Canadian radar 
detector-detector which the Dudley Do-rights use for sniffing out 
illegal radar detectors in peoples' cars (followed by fines and
confiscations).

It works by detecting 11.5 GHz local-oscillator radiation.  Most 
superhet radar detectors apparently copy Cincinatti Microwave's basic 
design with 1-GHz first i.f.  A harmonic of the local oscillator is 
used to detect K-band radar.

Has anyone actually seen the infernal Canadian device?  What does it 
look like?  What is its range?

Many brands of radar detectors mutually interfere because of local-
oscillator radiation.  Some of the Gunn-diode oscillators can be 
mechanically retuned down to 10.5 GHz.  Has anyone tried to make a 
detector-detector by retuning an ordinary detector's local oscillator 
to 12.5 GHz and removing the series of screws in the horn antenna 
which form a trap to suppress 11.5 GHz transmission?  It could be a 
fun project, useful for an *sshole early-warning indicator and other 
ECCM experiments.  Unfortunately, you could not use a conventional 
radar detector simultaneously.

Local oscillators in radar detectors include a varactor diode which 
sweeps their frequency, searching a few hundred MHz either side of the 
10.525 GHz police radar frequency.  Radar transmitters drift with age, 
temperature and voltage, and sometimes the cops (illegally) retune 
them, attempting to evade detectors.

I don't know if detectors sweep continuously, or "lock on" when they 
detect a signal.  There might be problems trying to use a sweeping 
detector to detect another swept signal.

An extremely useful tool for such work is an X-band wavemeter.  It's 
an adjustable resonant cavity with a tuning-screw calibrated in GHz.  
It has two waveguide flanges, one for input and the other for a diode 
detector.  Tune for maximum output, read the frequency dial.  I got 
one at a hamfest for $35.

--

Frank Reid     W9MKV     reid@gold.bacs.indiana.edu