From: mshar@mentor.cc.purdue.edu (Mike Mshar)
Subject: Re: Quick radar question about traffic ticket
Organization: Purdue University Computing Center
Date: Thu, 5 Nov 1992 20:08:46 GMT

In article <1992Nov4.215533.2792@erg.sri.com> rat@erg.sri.com (Ray Trent) writes:
>In the referenced article, mshar@mentor.cc.purdue.edu (Mike Mshar) writes:
>>>return the signal from their radiators (the tilted radiator in a Corvette plu
>>
>>radiator, with its fairly small frontal area (I say small frontal area
>>because you have to consider all of those little holes that allow air to
>>pass through) is going to return a larger signal than the side fender, door
>>panel, and quarter panel of a car?? That is a very hard one for me to
>>grasp, with the possible exception of cars like the Saturn that have
>>plastic body parts. Perhaps if the car is coming towards the cop head on,
>>then maybe, but if the cop is at any angle to the car in question, I would
>>think that the large, flat metal pieces of the car would return a lot more
>
>Hmmm, by this reasoning, microwave oven doors don't reflect radar. :-)
>
>Seriously, though, the holes in a radiator aren't big enough to allow
>microwave radar through, anymore than the holes in a microwave oven
>door allow microwaves through. The engine block *would* return a big
>signal, but the radiator gets hit first.

OK, then, what is the amplitude of a radar gun`s emmited radiation? I
understand the microwave part, i.e. the holes in the screening are smaller
than the amplitude of the waves, therefore the waves make contact with the
metal screen as they pass through, and are therefore absorbed. But I was
under the impression that radar waves were not very powerful, and did not
have a large amplitude. So, my reasoning was that they would be able to
pass through a radiator. Besides, doesn`t the clear plastic door panel of a
microwave oven reduce the energy of the exiting waves to the point where
they are not harmful to humans? I know that when your car is parked in the
sun, UV waves go through the glass into the car. On their way through the
glass, some of their energy is lost, and thus they are reduced to infrared
waves. When this happens, the waves no longer have enough energy to pass
back through the glass, so they just bounce around and are absorbed inside
the car, thus producing the heat. Doesn`t a microwave oven door do the same
thing to waves coming out of it??

>In terms of side panels and other fairly flat surfaces, the old rule
>to remember is "angle of incidence == angle of reflection". Radar that
>hits side panels bounces off down the road instead of returning to the
>source (unless, of course, the radar hits them perpendicularly).

About the body panels, also, I had assumed that because the waves had a
very small amplitude that they would bounce off of the individual surface
irregularities on the bady panels. Therefore, the waves would be scattered
by the bumps and cracks in the paint of the panels. Also, the waves would
bounce off things that you mentioned, engine block, radiator, headlights,
bumbers, etc. 

So, in essence what I am asking is, what is the amplitude of police radar
waves?? Being an ME, I am clueless as to what it might be. 

Anybody have any info??

>To complicate things a little more, radiators are reasonably
>dispersive, because they *do* have varying densities and conductances,
>so the signal that returns is always weaker than would be seen if the
>radar hit a corner reflector, say. But it also works against you,
>because it insures that at least a fair amount of the signal gets
>reflected back to the radar even in the presence of reasonably
>significant deviations from a perpendicular angle of incidence.
>
>Headlights actually return a fair amount too, though they aren't that
>big, because the parabolic reflector relects in a pretty large range
>of angles.
>
>That reminds me of an amusing idea a friend of mine had (which is
>impractical because of the size of reflector needed to affect
>microwaves): Build two of those little anamometers (sp? the thing you
>measure wind speed with) with metal corner reflectors on them, and
>mount them on your car, one in front and one in back, such that the
>corner reflectors spin away from the front and back (respectively). In
>principle, the corner reflectors would return a huge signal relative
>to the car, but moving slower. This is hard to see without a hand
>drawn picture (I tried ASCII graphics, to no avail), but it's amusing.

Jeez, if you are going to go that far, just buy some radar absorbant
material from the government, and coat your car with it. Or just buy a good
radar detector, and try not to speed. :-)

MGM
-- 
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