From: dave@eram.esi.COM.AU (Dave Horsfall)
Newsgroups: aus.radio
Subject: Re: Radar
Date: 11 Nov 92 22:51:05 GMT
Organization: Pacific ESI, Sydney, AUSTRALIA

In article <1992Nov7.120228.8237@ucc.su.OZ.AU>,
    andy@extro.ucc.su.OZ.AU (Andrew Miehs) writes:

| I am after information concerning RADAR Detector units as used by the police..
| IE: How they work, etc... frequencies and the like. (In Australia)

K-band, mostly (24.150 GHz +/- 60 MHz).  Happily within the Amateur band,
and not far from the primary simplex frequency of 24.125 GHz...  X-band
(10.525 GHz) is no longer used (but is popular in USA), and isn't an
Amateur frequency anyway, so no loss.  There are supposed to be some units
on Ka-band (33 GHz +/- 38 MHz), also unfortunately outside the Amateur
allocation (I think the speed-cameras use this band).  Finally, they are
experimenting with lasers instead (undetectable, unless you happen to
have a military-surplus laser-illumination detector - they are somewhat
hard to come by...).

Radar works on the doppler principle - reflected waves are up-shifted
(or down-shifted, if receding) by an amount proportional to the vehicle
speed (more accurately, modified by the cosine of the angle of approach)
and the two are mixed together in the radar receiver, producing an audio
tone.  The higher the tone, the faster the vehicle.  Reflections from
other things in the vicinity tend to confuse them, thus the practice
of attaching tuned al-foil strips to one's radiator fan...  Note that
the police take a dim view of ECM, by the way.

Lasers work on the time-and-distance method - a pulse is sent and the
time for the reflection to appear is measured with an accurate clock.
Undetectable, unless you have the aforesaid illumination detector.

| One other thing that I could never work out was their radar detector detectors...? It doesnt seem posible to pick up a reciever.... ( Not with any great accurAcy anyway.. cause wouldnt it just pick up your car radio?)

You'll want to do something about your RETURN key - I think it just failed.
Anyway, if the receiver is a super-het, you merely detect what is called
the local oscillator (LO).  They work by mixing the incoming waves with
another frequency, producing (amongst other things) a third (and usually
lower) frequency that is more readily handled.  I won't go further into
super-hets, but suffice to say that a receiver (or in this case, a radar
detector) is also a miniature transmitter, and usually steps are taken
to prevent leakage of LO radiation.  However, since most detectors are
cheap & nasty devices produced for the mass market, this is not normally
a consideration, as it adds to the cost.  Indeed, the usual design of
a detector puts the oscillator in the same cavity as the mixer (they may
even be the same diode) and thus they make very efficient transmitters.
All the detector-detectors do is look for this leakage, and before you ask,
yes, it is possible to build a detector-detector-detector.  It's called
a detector :-)

| Please email me at andy@extro.ucc.su.oz.au

It's considered bad form to say "please e-mail me" unless you promise to
summarise to the group.

-- 
Dave Horsfall (VK2KFU)         VK2KFU @ VK2RWI.NSW.AUS.OC
dave@esi.COM.AU                ...munnari!esi.COM.AU!dave
           Disclaimer: "Me, speak for us?"