cjh@csin.UUCP (06/03/83)
If the foil were fluttering significantly, it wouldn't stay on for long.
A much more likely jammer would be a corner. (Visualize a regular octahedron
as a skeleton, and make the 3 internal planes (described by any 4 coplanar
vertices) of a reflective material; increases reception at the other end from
inverse 4th to inverse 2nd. This would also be very difficult to mount
permanently on a car.
It's worth noting that where anti-detector laws have been tried in court,
they have not come out favoring the state. On the other hand, most of the
people I know who were willing to drop $200-300 on a detector probably
shouldn't be on the road at all. . . .
CHip
(Chip Hitchcock)
ARPA: CJH@CCA-UNIX
usenet: ...{!ucbvax,!decvax}!cca!csin!cjhkarn@eagle.UUCP (06/06/83)
I've had a (purely academic) interest in the topic of police radar
jamming for some time. I would refer interested readers to the May 1978
issue of 73 Magazine (an amateur radio rag put out by Wayne Green).
It contains an article on page 80 entitled "Can Hams Counter Police
Radar?" and subtitled "Electronic warfare: another step."
The author lists four basic ways to jam police radar:
1. CW jamming - put a transmitter directly on the radar's frequency
2. Noise jamming - covering the radar with broadband noise
3. Baseband jamming - modulate a nearby 10 Ghz carrier with the correct
audio beat note.
4. Passive jamming - modulate the radar's return echo in such a way
as to mimic a different doppler beat tone.
Some explanation of these techniques is in order. First of all, ALL
police radars function as CW (continuous wave) Doppler radars. Even the
so-called "pulsed" radars work in the same fashion. Police radars do
NOT send a pulse and time its return; rather, they put out a continuous
signal, albeit for a short time, and while it is transmitting a sample
of the outgoing signal is fed to a receiver mixer diode. The return echo
from the vehicle is also fed to this mixer. Since the return echo is
doppler shifted, it produces a beat note (in the audio range) which is
then fed to a frequency counter calibrated in MPH.
This kind of radar has many problems. Multiple returns (e.g., the
leaves in a tree being rustled by the wind) or a return that is otherwise
modulated in some fashion (e.g., by a fan blade) can easily confuse the
frequency counter, since its input would be nonsinusoidal. Since most
frequency counters count zero crossings, and complex waveforms often
have many zero crossings per "cycle", spuriously high readings often
result. This is probably the basis for the "rustling aluminum foil"
trick, but I suspect that it would cause HIGHER readings, if any
readings at all were taken as valid.
Baseband jamming, #3 above, is the most practical. It works on the
following principle. The X-band police radar frequency is 10.525 Ghz.
There just so happens to be an amateur band from 10-10.5 Ghz. The
waveguides and diodes in virtually all police radar guns are very
broad banded. If you transmit even a small amount of power (50 mw,
say) in the amateur band, it will swamp the police radar receiver
and it will see this signal instead of the echo return from the vehicle.
If you amplitude modulate your outgoing signal, you create an audio note
in the receiver, but this will NOT vary with vehicle motion! (Unless
you are going an appreciable fraction of the speed of light.) You
can construct a box which produces a synthesized audio note at any pitch
you desire, allowing you to produce a jammer who will cause the radar
gun to read any speed you desire! The readout will be constant
regardless of your speed - coming, going or standing still. If you
simply transmit an unmodulated carrier, the radar would get no reading
(or 0 MPH.) Legend has it that some particularly sadistic individuals
constructed such a jammer, hid in the bushes, and fed high readings
into the radars being used at a police speed trap, causing some
innocent motorists to get tickets.
Passive jamming works on a similar principle, except that it audio
modulates the radar's own signal. A tuned horn is electrically shorted
and unshorted at an audio rate; at microwave, this has the effect of
making the horn appear to grow and shrink at an audio rate, thereby
modulating the return echo. Unfortunately, this method is not too
practical, as the antenna has to intercept a majority of the microwave
energy hitting your car (i.e., the antenna area has to be on the order
of the frontal area of your car!)
I have NEVER tried out any of these techniques. While you could claim
that you are within your rights as an amateur to transmit legal amounts
of power within an amateur band, the fact that your INTENT is to
interfere with police radar violates the Communications Act ("no
transmission shall intentionally interfere...") On the other hand,
there could be a real challenge in constructing an experimental
collision avoidance radar system on 10 Ghz that would JUST SO HAPPENS to
interfere with poorly designed police radars...
On a related topic, my favorite radar story has to do with the Air
Force. To keep skills up, the pilots periodically hold mock dogfights,
with the pilots of one type of fighter going after those of another.
Scores had been consistently high for some time, as the chased plane had
inferior maneuverability - they always got "hit" in the mock battles.
One day, however, the hit rates went way down. Every time the chasing
plane would get a radar lock on the target in preparation for firing its
"weapons", the target would suddenly drop away.
The mystery was solved when the pilots of the target planes revealed
that they had gone to the local PX and bought a number of cheap X-band
police radar detectors. They had mounted them in their cockpits, with
the horns facing rearward. Each time the chase plane would lock in
on radar, the detector would go off and the pilot would take evasive
action.
Sort of reminds you of the Ewoks vs the Storm Troopers, doesn't it?
Academically yours,
Phil