[sci.electronics] Radar detector detectors

wiz@xroads.UUCP (Mike Carter) (07/27/89)

 
Hmm. The Ontario police department must be feeling a budget crunch.
While I'm skeptical that a resonant cavity "Grid-dip" scan would
come anywhere near being effective (lookit all those nice cavities on
radiator grilles) the spectrum search for local oscillators
operating near the typical radar detector frequency would.
I wonder if there's a method to fake those out by jamming the spectrum
full of high energy radio transmission...say in the 800MHZ range?
Would a cellular phone transmitter start the lights on these detectors?
My Mobile 2M FM Xmtr sets off my Radar Detector if it's on high power.
 
In Australia during the 60's, the local government required a license
for T.V. reception. They would be seen driving up and down streets in
a white van with a loop antenna searching for houses with receivers
using the same principles and confiscate the T.V is the owner(s) refused
to cough up the price of a fine + a license. 
[ We were visited...my recently arrived from the States parents had no idea
  about the license bit ]

Has anyone devised an effective method to "jam" the radar detector detectors?
 
-- 
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= Mike Carter  N7GYX, Phoenix AZ| Q: Why did the Chicken cross the road  ?  =
= hplabs!hp-sdd!crash!xroads!wiz| A: To ESCape the Main Menu .              =
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karn@epic.bellcore.com (Phil R. Karn) (02/21/91)

It seems to me that there are some simple counter-counter-measures for
the radar detector detector problem.

Radar detector detectors work by sensing the stray radiation from the
first local oscillator in the radar detector. The stray radiation
leaks through the first mixer and comes out the antenna.

1. Use an RF amplifier. Amplifiers tend to be one-way devices; not
only would a good one suppress LO energy, but it would improve
receiver sensitivity.

2. Use a balanced mixer. A balanced mixer, by design, suppresses LO
energy from going out the input port. Cheap unbalanced mixers simply
combine the RF and LO signals and feed them to a nonlinear amplifier,
and there's nothing to keep the LO energy from going right out the
antenna.

3. Spread the LO before feeding it to the first mixer.  Then despread
the IF signal. Any stray LO radiation will also be spread, thus making
it much more difficult (if not impossible) to detect.

Spreading should be easy to do - use a common PN sequence generator
for both the spreading and despreading operations. This will also
require two additional balanced mixers, one each for spreading and
despreading.

Phil