Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!unmvax!bbx!yenta!dt
From: dt@yenta.alb.nm.us (David B. Thomas)
Newsgroups: sci.electronics
Subject: Re: Detecting Radar Detectors
Message-ID: <1695@yenta.alb.nm.us>
Date: 1 Aug 90 08:08:30 GMT
References: <324@bally.Bally.COM>
Distribution: na
Organization: yenta unix pc, Rio Rancho, NM
Lines: 32

jkimble@bally.Bally.COM (The Programmer Guy) writes:

>I saw a story on the world news a few weeks ago regarding drivers
>(primarily 18-wheelers) using radar detectors.  During the news
>segment, the reporter was talking to a highway patrol officer who was
>using a device to detect if the oncomming vehicle was using a radar
>detector.

Radar detectors are superheterodyne receivers -- a long scary word for
a very simple thing, as usual.  Inside the radio, the incoming signal
is mixed (heterodyned) with a locally-generated signal (the local oscillator),
which differs from the incoming signal by a fixed amount (the difference is
the Intermediate Frequency or IF).

When signals are mixed, the sum and the difference automatically come into
being.  The sum is ignored and the difference (the IF) is amplified and
dealt with as the received signal.  Superhet receivers are better in many
ways I won't go into here, but the advantages are so great that almost any
radio receiver of any kind you can buy is a superhet.

The upshot of all this is that the local oscillator does generate a
signal close to the signal being received.  This can set off other
detectors, as you have observed, and can be picked up by anyone
scanning for it.

It is possible to shield the local oscillator so it will emit almost no
radiation.  Broadcast receivers used to interfere profusely with each other,
too, until this type of shielding was required by the FCC.  Soon, perhaps,
radar detectors will follow suit.  Until then, you are legally broadcasting
the fact that you have a radar detector.

						David