Re: Radar detector/jammers (fwd)

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

> From quattro-owner@swiss.ans.net Tue Nov 15 10:39:40 1994
> From: glen.powell@ccmailny.smc.com

> 
>   Well, what can I say? I've own one and it works. Have you tested one?

Others have tested them and found that they do not work.  Personally, I
don't need to test a perpetual motion machine to determine that it
won't work; it's a physical impossibility.  It would however be very
easy for me to test one if one were available, as my office adjoins a
lab where radar front-end equipment is prototyped.

> 
>   The passive jammer does *NOT* change the frequency, it injects noise into
>   the reflected signal to confuse the receiver and delay/prevent lock on.
 
Wait a minute here.  If noise is being created and transmitted by the
jammer, it is NOT a passive device as you state.  In the Electronic
Countermeasures (EC) field, this is known as a barrage jammer.  These
work by transmitting a signal so strong that it overloads the front-end
of the radar being jammed.  However, this is old news in the EC
business, because it is so easy to detect this type of jammer and lock
onto it.  

Furthermore, with a barrage jammer, one needs a lot of signal because: 

(1) its energy has to be spread over a wide region, as the radar gun's
beam could come from any angle (but due to cosine error, +/- 60 degrees
is sufficient); and

(2) it takes a substantial amount of signal to overload the front-end
of any radar, since the signal returned from a given object varies 
with the fourth power of the distance, so the automatic gain control
circuits are necessarily quite robust.

>   Have you heard of a type of aircraft technology called "stealth"?
>

Yes, and the technology employed is quite the opposite of what you 
describe, as it involves minimizing the radar-cross-section of the
target and therefore the radar return, rather than sending back
even more signal for missiles to lock onto.

One does not need to be a radar expert do prove that passive jammers
cannot function as advertised; common sense and a rudimentary knowledge
of physics should suffice.   That people actually can be persuaded to
believe that such a device could work is a sad commentary on the general
ability of society to deal with technology; that people would actually
believe that it does work merely verifies that reality cannot compete
with persuasive advertising.  (I'm just pissed-off that I didn't
come up with it first :^)  ).

Incidentally, selling what amounts to high-tech snake oil is nowhere
more prevalent that in the audio field, where they sell devices that
claim to organize the electrons going into one's stereo equipment.
And many intelligent people buy them, so I don't mean the above
commentary as an affront on anyone's intelligence, just an observation
on our society in general.

BTW, it's perfectly possible to make an *active* jammer, and it's been
done, but to do it right and cover all the bands would be quite
expensive.

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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
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POSSLQ's* new car history:
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