Re: Microwave doors, electromagnetic waves


<jimp@xxxxxxxxxxxxxxxxxxxx> wrote in message
news:k28n34-rto.ln1@xxxxxxxxxxxxxxxxxxx
Adam Chapman <adam.chapman@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:
Hello,
I am trying to design a stealthy aircraft for a university project. My
team are currently working on the air intakes for the jet engine. We do
not want the electromagnetic radar waves to enter the intake ducts.
I have read that a grill can be put in front of the intake, and this
reflects the incoming waves, like a microwave oven's door does. I also
read that this works because the spacing of holes in the grill is
smaller than the wavelength of the electromagnetic waves. However,
because most of my education has been aimed at the machanical side of
physics, this electromagnetic stuff i all new to me.

Why can the waves not pass through? I would think thta the amplitude of
the waves is a factor rather than the wavelength, which is measured
parallel to the waves direction.

Is there an optimal grill spacing? Perhaps this is found with funtion
of the wavelength? Is there something called Lifshitz equation and is
this useful to my application?

Also, Radar waves are polarised. The radar transmitter can send a pulse
of vertically polarised waves, then anther pulse of horizontally
polarised waves. Would polarisation affect the usefulness of the grill?

Any help would really be appreciated.

Thanks,
Adam

To simplify greatly, the smaller a thing is relative to the wavelength
of the radiation, the less effect it has on the radiation.

As a rule of thumb, something smaller than .1 wavelengths is for
practical purposes invisible to the radiation. That is, at .1 wavelengths
or smaller, the effect is small enough to ignore for a real world
application.

true 1/10


In the case of a grill or screen, once the holes in the thing are
smaller than .1 wavelengths, the vast majority of the radiation is
reflected instead of going through the holes.

If the holes are round or square, polarization won't mattter.

If you need to put numbers on things, the equations that give the
efficiency of a dipole in terms of size are probably close enough.


The OP is missing the "what is wrong in letting the radar hit the blades?",
which is, they modulate the radar pulses and can ID the airplane also.



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