Re: Radar detector/jammers (fwd)

[Thomas Tornblom <Thomas.Tornblom@nexus.comm.se>]

Now I'm no great believer in "too good to be true" gadgets. Thet
normally are just that ;-). I'm also not a radar scientist, but have
some basic knowledge of microwaves and physics.

   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.

But wouldn't normally only a tiny fraction of the emitted energy be
reflected back? The random shapes of the body would reflect it in
other directions than back to the receiver. If you now have an antenna
that actually reflects most of the received energy back in the same
direction, then it wouldn't be too hard to jam the receiver.


   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.

Directing the signal back in the same direction is actually pretty
simple. This is just optics. Gadgets like this is used on many small
boats as radar reflectors to allow larger ships to "see" them.

Thomas