🚀 go-pugleaf

>...my curiosity was whetted by an article in the paper the other 
>night.  There was a picture of a device used by the Ontario Highway 
>Police (Provincial Police) of a gadget said to detect radar detectors 
>in passing cars.

>How do these work?  I presume that they attempt to detect the
>oscillator in superhet receivers.  How do they know that all the
>oscillators run at the same frequency?  Would not the metal cage
>of the car shield out virtually all of the radiation emitted from
>radar detector (I realize that windows are large holes at microwave
>frequencies, but still, the signal emitted from the detector must
>be quite small)...
>-- 
>  Terry Ingoldsby                       ctycal!ingoldsb@calgary.UUCP


The Canadian radar-detector detectors are probably for real, and 
almost surely work by detecting local oscillators.  The same principle 
was used in WW2 when the Germans installed radar detectors on U-boats 
to warn of approaching radar-equipped antisubmarine aircraft.  (Nazi 
"experts" had previously assured their leaders that airborne radar was 
impossible.  Sound familiar?)  But the ASW people got wise:  They 
turned off the radars and looked instead for the local oscillators of 
the detectors.

A ham who works for Cincinatti Microwave, manufacturer of _Escort_ 
brand radar detectors, told me how they work:

The Escort is a little spectrum-analyser; its local oscillator runs at 
about 11.5 GHz and is swept over a few hundred MHz by a varactor 
diode.  The first i.f. is 1 GHz, followed by a second conversion (to 
100 MHz I think).  The remaining circuitry analyses the "video" signal 
for valid alarm conditions.  K-band is detected via a harmonic of the 
local oscillator.

The spectrum analyser method allows the receiver to be extremely 
sensitive with very narrow i.f. bandwidth, yet still able to detect 
radar signals over a considerable frequency range.  The microwave 
section of the Escort contains Gunn oscillator, varactor and detector 
diodes.

Other superhet radar detectors probably use the same principle, though 
may use different frequency-conversion schemes.  Some brands of radar 
detectors mutually interfere because of radiated local-oscillator 
signals.  The local oscillators emit several milliwatts, roughly 
equivalent to radar burglar-alarms and door openers (which will 
trigger an automotive radar detector a block away, conveniently 
letting you know that a liquor store is nearby :-).  The Escort's 
waveguide assembly has three screws protruding into the horn which act 
as a trap to limit local-oscillator radiation; I don't know how 
effective they are at doing so.

The Canadian ECCM device is probably a microwave spectrum-analyser, or 
some specialized and relatively inexpensive variant thereof.  It might 

even be a modified Escort.

The detector detector was discussed on rec.autos last year; someone 
claimed that it works even on detectors which are turned off.  That 
could be done, at least theoretically, by using the microwave 
equivalent of a grid-dip meter and searching for resonant cavities in 
the frequency range of interest.  Another method would be the 
principle used to locate passive bugging devices:  The target area is 
bombarded by microwaves and a receiver looks for harmonics generated 
of the exciting signal, generated by semiconductor junctions.  In the 
latter two cases, range would probably be only a few feet.

--

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