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Re: Technical question about RF absorbtion



If the Chamber is near resonance at the frequency of the Satellite antenna,
a second antenna with a dummy load could be used to absorb the energy of the
transmitting antenna. A dummy load could be used and there are available,
hermetically sealed N connectors for this purpose.
The energy in an antenna builds until the energy radiated + the energy
dissipated in the antenna is equal to the power being applied to the
antenna. If the tip of the antenna has extra capacity to the chamber it will
lower the resonant frequency and increase the VSWR In air the radiation
resistance is 377 Ohms. Once in a metal chamber that figure completely
different. You are radiating energy in this chamber and some how it must get
out through connections, resistance in the metal, radiated through window
ports, or by reflecting stored energy back to your transmitter ( VSWR).
Those conditions are not typical of space, which is what you are trying to
simulate.

Art ,KC6UQH
----- Original Message -----
From: "Graham Shirville" <g.shirville@btinternet.com>
To: <amsat-bb@amsat.org>
Sent: Monday, April 04, 2005 5:32 PM
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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----
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