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Re: Technical question about RF absorbtion



I suggest the use of 141 hardline, you should be able to scrubb it and soak
it at 200 degrees before installing it. Thah should help the outgassing
problem.

Art
----- Original Message -----
From: "Tom Clark" <w3iwi@toad.net>
To: "Graham Shirville" <g.shirville@btinternet.com>; "AMSAT BB"
<amsat-bb@amsat.org>
Sent: Monday, April 04, 2005 8:47 PM
Subject: Re: [amsat-bb] Technical question about RF absorbtion


> We have always used a real coax dummy load on every spacecraft I've been
> involved with. The other option has been to pipe the RF to a bulkhead
> connector exiting the chamber and using an external load (this has the
> advantage that you can use a real wattmeter to calibrate the xmtr's power
> sensor).
>
> To be an effective RF "blanket", you would need at least a
> quarter-wavelength and preferably a half-wavelength thick of material to
be
> an effective absorber. That's a pretty big lump of stuff! A typical RF
> absorber material would involve black carbon impregnating some sort of
> sponge (rubberized horsehair has been a common material used at low
> frequencies. You might be able to get away with several layers of "377
ohm"
> cloth -- which is like cheese cloth impregnated with black carbon [the
> resistance of any square piece of this stuff is 377 ohms, the
characteristic
> impedance of free space]. The really state-of-the-art absorbent is made by
> Emerson&Cumings under the name Eccosorb (http://www.eccosorb.com/). If you
> look thru their products, you will only find a few that are spec'd to
> frequencies under 1 GHz; one is AN-W
> (http://www.eccosorb.com/catalog/eccosorb/ANW.pdf) and to be effective at
> 600 MHz, ANW-79 is 11.5 cm thick.
>
> As you note in your posting, you must remember that anything you place in
a
> vacuum chamber will outgas, and that will make you VERY unpopular with the
> owners of the chamber, as well as invalidating the vacuum chamber data on
> outgassing from the payload (usually gathered from a cryogenic "cold
> finger").
>
> 73, Tom
> ----
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