cult by the fact that very narrow beams are
used, making for a small ‘detection area’.
Some users of radar detectors have noticed
that the beam can also be detected when it
is reflected off other cars ahead and have
gladly made use of this property.
And now for the final problem: most radar
equipment can take measurements of
approaching (from the front) or receding (from
the rear) vehicles. But the sensitivity of most
detectors is limited to just one direction. To
be prepared for any eventuality the vehicle
should therefore have a detector at both the
front and rear!
And finally...
As the saying goes, if you play with fire you
may get burned. As we have seen in this arti-
cle, speed cameras have become reliable
instruments that are difficult to locate or
interfere with.
Knowing how to detect a SHF beam is one
thing; to disrupt a speed measurement is
another story. In short, although most in-car
radar detectors sold at the moment do work
and show the presence of speed cameras,
they are of little use because the speed cam-
era is likely to be triggered
before
the driver
can slow down.
Even though speed cameras are some-
times situated in places where they aren’t
justified (which has given rise to the name
‘euro-pump` on the continent), in our opin-
ion it shows more sense to respect the speed
limits (also out of respect for other road
users), rather than attempt to get round the
law at all costs.
(020165-1)
it. Speed cameras are no exception
to this, and numerous boffins
(whether competent or not) have
contributed to the development of
counter measures.
Before we continue we would like
to make one thing perfectly clear: in
the majority of European countries
the possession, and even more so,
the
use
of equipment intended to
disrupt the operation of speed cam-
eras is illegal. Indifference in this
area is not recommended, since the
consequences can be dire (handing
in of the driver’s licence, confiscation
of the vehicle, prosecutions, etc.).
In spite of this, some drivers think
it is worth the risk and don’t shrink
back from using these devices.
Loosely speaking, there are two
types of ‘anti-radars`: jamming
devices and detectors.
The jamming devices are simply
small SHF oscillators, which are
used to send a ‘fake’ signal back to
the speed camera, causing the mea-
surement to fail and preventing the
logical analysis of the frequency.
Besides the fact that these devices
are relatively ineffective (in most
radars the circuits are less sensitive
to interference signals: the frequency
of the jamming signal therefore has
to be as close as possible to that of
the speed camera, and every device
has its own frequency), the elec-
tronic circuits in the radar can detect
such jamming signals and notify the
police. A jamming device is therefore
a sure-fire way to get caught!
A detector on the other hand con-
sists of a simple SHF receiver, and
by definition these can’t be
detected. In the USA (where their
use is permitted) they are sold in
large quantities. On the Internet
they are readily available and in
some European countries they are
also freely available (their sale is
permitted but their use isn’t!). These
are usually relatively simple circuits
containing a microwave detector
and a comparator that drives an
alarm. In short, simplicity itself
(although their price seems to sug-
gest the opposite is true!).
It isn’t difficult to design a broad-
band detector that reacts to fre-
quencies between 2 and 10 GHz,
which is the range where most mod-
ern devices operate. However, if the
oscillator of a speed camera is set to
a frequency that is outside the range
covered by the detector, or it uses an
optical laser, then you’re bound to
get caught.
The second problem is that in
order to detect something, there first
has to be something to detect (obvi-
ous, isn’t it?). Older radar equipment
transmitted continuously, which
made the task simpler, but newer
models only transmit intermittently,
either randomly or in short bursts,
reducing the chance of detecting
these devices. Some models (such as
the Mesta 208 sold in France where
the author lives) are even more cun-
ning and only come into action when
a car comes within their range.
These ‘green bullets’, as they are
known because of their shape and
colour, have an optical detector on
top that literally sees the vehicles
coming.
As soon as there is some move-
ment in front of the device it springs
into action.
This brings us to the third prob-
lem: a radar detector will sense the
beam at that instant. But at the
same time the speed camera is
already doing its work. From this it
follows that in the time taken by the
driver (a typical reaction time for
people is about half a second) to
take appropriate action (to brake or
disrupt the measurement), the radar
will already have taken four or five
measurements.
The detection is made more diffi-
GENERAL
INTEREST
21
11/2002
Elektor Electronics
020165 - 13
SHF
oscillator
(2 – 10 GHz)
command
processor
amplifier/
shaper
analyser
display
TX/RX
parabolic antenna
Receiver/mixer
Figure 3: Basic principle of a high-frequency speed camera.
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