Re: Radar detector/jammers (fwd)

[John Greenstreet <jgreenst@motown.ge.com>]

> From: glen.powell@ccmailny.smc.com
> 
>   Well, just gottah take issue with the statement that "passive jammer are
>   *totally ineffective*.
>   Have you ever used or tested one? I have. They are *very effective*. I
>   also wondered about
>   the small reflector size, relative to the size of the vehicle. The way it
>   was explained to me, by
>   an expert in the area of things RF, the reflector is "tuned", just the
>   same as an antenna is tuned
>   for a frequency range, and is therefore *far* more efficient at
>   reflecting the signal Vs the random
>   shapes of the vehicle surface. Me? I just know *it works*! The unit I
>   have is called the _Eclipse_.
>   It looks and mounts just like a detector and works great in parallel with
>   a detector.
> 

Wow. If the jammer's antenna is "*far* more efficient at reflecting the
signal, then your car should be _easier_ to detect by radar!  Also, if
the antenna is tuned, at what frequency is it optimized?  The X, K, and
Ka are all quite different as are the frequencies in use within those
bands.  Even with separate antannae, they can't be "tuned" to all
frequencies in the band at once, just as a flute can produce only one
note at a time.

Having worked in the radar field for 17 years, I am very skeptical of
these claims: if it were really possible to passively return more
signal that that even returned by a curved surface and at the same time
change its frequency, it would be a cinch to design objects that were
be invisible to radar, and these would constitute a major threat to
modern radars.  However, thanks to the laws of physics, we don't have
to worry about them.

Of the many faults with the concept of passive jammers, perhaps the
most glaring is the requirement that the passive device accept signals
with a given angle of incidence and return with the same angle.  Since
the device is passive, the only way it can "amplify" the signal is by
concentrating it--making it directional--, and indeed this is how radar
antennae work.  However, now that it's concentrated, how does one focus
it upon the source and return it?  Unlike management, electromagnetic
waves cannot focus everywhere all at once.  And the problem of changing
the signal's frequency without signficant loss remains, and that in
itself is a very "lossy" process.

I would have to say that passive jammers should be put into the same
category as perpetual motion machines.  Really, the only thing these
jammers prove is the placebo effect (which has been shown to be
quite significant).

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John Greenstreet, Senior Engineer           (jgreenst@motown.ge.com)
Martin Marietta Government Electronic Systems    Moorestown NJ 08057
WPI Class of '75, Temple Class of '94

My new car history:
  1975    1978    1982       1986        1989      1992      1995
   VW ->  Audi -> Audi  -> Mercedes -> Mercedes -> Audi -> Mercedes
Scirocco Fox GTI  4000S    190E 2.3    190E 2.6    100CS     S320

POSSLQ's* new car history:
         1978       1981       1985      1988        1990     1993
       Triumph ->  Toyota ->  Toyota  ->  VW    ->   Audi  -> Audi
       Spitfire    Tercel     Corolla   Jetta GL      80      90S

*POSSLQ = Person of Opposite Sex Sharing Living Quarters
Note: All Audis and Mercedes above were sold to friends or family.
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