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RE: Technical question about RF absorbtion



I assume the walls of chambers ( or large parts close to it) are made of
metal.?
There is a question of whether you need
1.  the *antenna* to operate as it would in space... or
2. the AMp connected to antenna to 'see' something similar to the antenna.

If answer is (1), then unless you are prepared for > 50% change in response
(wrt to real thing) , I would give up now. The wavelength is too large. I
think the closest you would come is to encase it (at least > 30cm radius) in
a heavy foam, or other non-conductive stuff to slow and absorb the EM waves
down. I would still expect to see > 3-4 db change wrt in-space. ( It will
depend on how much metal is surrounding the surface-volume  of antenna.)

If the answer is (2) I recommend getting an network or similar analyser, and
measure S11 in close-to-free-space. Then simply Synthesise an 'antenna
emulator' using a one-port L-C-R network. Id bet that in the chamber, this
will come a lot closer to appear like the real antenna than the actual
antenna itself. 

If its getting close to actual VSWR that you're concerned about, why not
just use a, say,  25ohm resistor, instead of (50ohm) dummy load?

Oh - I just thought of another way.... if measurment is only over
operational BW, then its conceivable that a properly design 2-cavity
resonator could work. Place the 1st cavity over the antenna, with a
tuned-antenna & load in the 2nd cavity ( which is coupled to the first).
Again, I suspect you would be serevly changing the S11 that the antenna
presents to the amp, and I still think soln as (2) is the closest match to
space-functional environment.
David M.
VK5 DSM

-----Original Message-----
From: owner-AMSAT-BB@amsat.org [mailto:owner-AMSAT-BB@amsat.org]On
Behalf Of Graham Shirville
Sent: Tuesday, 5 April 2005 10:03
To: amsat-bb@amsat.org
Subject: [amsat-bb] Technical question about RF absorbtion


Hi All,

Imagine you have a satellite under test in a very small thermovac chamber
with a 70cms 1/4 wave whip sticking out of one end of the spacecraft.

The tip of the antenna is probably only a few inches from the wall of the
chamber but you want to test the transmitter to characterise thermal
properties etc etc.

You do not want to substitute a dummy load for the antenna cos it will cause
other problems.

What could you "wrap" the antenna with that would absorb most of the RF or
at least not present a terrible SWR and would not upset the owner of the
chamber due to excessive outgassing?

Any valid suggestions will be much appreciated and if used will attract a
prize from a certain satellite team!

many thanks

Graham

G3VZV
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