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Re: 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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