Re: [ST] Radar Blocker???

[enrico@xxxxxx]

> 
> Police RADAR determines a vehicle speed by the Doppler effect.  It uses
> electromagnetic radiation (i.e. X, K, Ka band) or microwaves, and relies
> on
> the wavelength shift as the initial (known wavelength) is reflected from
> the
> target.  The reflected signal shifts in wavelength vary with the speed.  I
> think, but don't quote me on it, that the faster you go the shorter the
> reflected wavelength gets.  Then, by working mathematical magic with the
> known wavelength and the reflected wavelength they calculate your speed.

This effect depends on the direction of the measured object. If the object
comes nearer your theory is ok - if the object is removing from the
measurement equipment the frequency shift is negative (means lower than the sending
frequency).
You can watch this simply on the street - if a car will come nearer the
engine sounds higher, after passing you the engine sounds deeper. Simply the
Doppler effect (btw. an Austrian physician)...
> 
> Now this Phantom II operates by taking the radar wavelength, shot at it,
> and
> adds an FM chirp component to the reflected wavelength to confuse the
> radar
> gun detector.  This hiccup in the reflected component will change the
> calculation.  Add the right hiccup (i.e. frequency) and you can make the
> radar gun display 0 mph or 55 mph, or whatever you want.

Possibly the signal from the Phantom is more powerful than the reflected
signal of the radar gun. So the gun will show the detected difference frequency
of the Phantom because the radar gun believes that the received signal is the
echo of the gun itself.

> Now not knowing exactly what the insides of a radar gun are, common sense
> would tell me that there's got to be some sort of filtration on the
> receiving end to eliminate noise so the gun can pick the reflected
> wavelength correctly.  Whether or not this FM chirp the Phantom II mixes
> in
> is filtered by the radar gun?, I don't know.  If it's not today, you can
> bet
> next generation radar guns will.

Maybe you are right. Modern generation radar will send an (always) changing
code on the radar impules. So they can detect if the echo is from the own
radar gun or an foreign echo.
The same principles (coding the own pulses) are used on laser guns.

Enrico
Vienna, Austria

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