🚀 go-pugleaf

In article <414@ctycal.UUCP>, ingoldsb@ctycal.COM (Terry Ingoldsby) writes:

|  While we're on the subject of automotive electronics, 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.  ...  Would not the metal cage
|  of the car shield out virtually all of the radiation emitted ...
|  the signal emitted from the detector must be quite small.

When you have *any* kind of steady narrowband emitter, even a very small
one like the local oscillator (LO) in a superhet receiver, a great deal
of physics and mathematics aids the cause of anyone who wants to detect
it.  Approached from the proper signal-processing perspective, such
emitters stick out in the electromagnetic environment like a magnesium
flare in a darkened room, and can be detected at almost arbitrarily low
amplitudes with processing-gain techniques.

The problem of detecting continuous LOs is infinitely easier for example
than the (still manageable) military problem of detecting
frequency-agile sources or those that are inherently wideband or
noiselike.  (And less urgent besides, since a consumer radar detector is
not normally a device trying its damnedest to kill you.)

If you've been in metropolitan England you may well have noticed the
receiver-detecting crews going around looking for unlicensed (and
therefore untaxed) TV receivers (yes, receivers).  They used the same
principle, detecting the LO, in what is surely a much more cluttered
electromagnetic environment than the highway police encounter.  (I don't
know if they still do this, but in 1972 the English TV-detector crews
dressed in exotic uniforms and helmets like alien invaders from some
low-budget Hammer-Studios film, or an episode of _The Avengers_).

From past experience as an electronic-countermeasures engineer for the
US government, I believe that detecting continuous narrowband emitters
is not even considered an interesting, let alone a challenging, technical
problem.  If you wanted to build a car radar detector that was hard to
find -- I stress that this is a hypothetical, purely technical discussion
-- you'd make it completely passive, or at the very least, hop the IF
frequency.  Such measures are expensive (and imagine trying to sell such
a product and describe it intelligently in a consumer advertisement).
I have heard one account of custom microwave ICs actually designed to
actively confuse police radar -- a more daring, not to mention
sophisticated, approach -- by employees at a well-known US instruments
firm, but I don't know about it firsthand, it may have been just a story.

Copyright (c) 1989 by Max W. Hauser.  All rights reserved.

--
"As a ... dramatic illustration of the effect of larger specific-impulse
variations, consider four identical, single-stage rockets each having a
mass ratio of ten (initial to final weight) loaded with hypothetical
propellants having specific impulses of 200, 300, 400 and 500 lb-sec/lb,
respectively.  The first can attain a maximum horizontal range of about
1300 nautical miles (theoretical, drag-free), the second about 4100 
miles, the third can become an earth satellite, and the fourth can
escape from the earth entirely."

-- Holzmann, _Chemical Rockets and Flame and Explosives Technology_,
Marcel Dekker 1969.  (Quoted with respects to Norm Strong.)