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I'd like to make a small anechoic box for RF measurements. I need roughly a 30x30x30cm cube of usable space, and I'd like the reflections from the walls to be attenuated by 10dB or more from 100MHz to 1GHz. If this can be made from cheap and readily available materials, that would be ideal.

My current idea about this is to take an old microwave as the shielded box, and use salt water as the absorbing material. It is a bit hard to get the water to go all the way around the device under test, but perhaps one plastic box nested inside another box with the in-between space filled with water would work to surround it from the sides and bottom; something else would have to be used for the top.

Would salt water work as absorbing material? What is the best salt concentration that gives a good impedance match to free space? (And how do I model this?)

Is there some other material that is better than salt water? (Perhaps bits of crushed salvaged ferrite choke cores...)

Even better: is there an instructable / blog post that describes how to DIY this? :)

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    \$\begingroup\$ Microwave oven cavities are not all that well shielded for broadband noise. \$\endgroup\$ – Spehro Pefhany Dec 31 '17 at 7:17
  • \$\begingroup\$ My best guess is Led usually works quite well, I don't know where you could get any though. \$\endgroup\$ – Flo Dec 31 '17 at 7:18
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    \$\begingroup\$ @Alex it isn't, and I wouldn't recommend taking advice from someone who doesn't know how to spell 'lead' \$\endgroup\$ – pericynthion Dec 31 '17 at 8:02
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    \$\begingroup\$ Can you describe more about what you're trying to accomplish? This might be an "X-Y problem." Do you really need anechoity or is shielding sufficient? Are you trying to make antenna beam pattern measurements? Near field or far field? Spehro is correct that microwave ovens have poor shielding away from 2.4 GHz. \$\endgroup\$ – pericynthion Dec 31 '17 at 8:05
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    \$\begingroup\$ I've seen conductive foam used. It isn't exactly cheap but it's easy to get. You can even get it from Amazon. \$\endgroup\$ – Robert Endl Dec 31 '17 at 9:01
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26 % Nacl solution has a maximum conductivity of 244000 mS/cm. The skin depth = sqrt (2/ (mu sigma omega)) = 1cm where attention of E field is 1/2.73 10 dB which implies attentiation of electric field by 1/10^0.25

Thus length required = skin depth * ln(10^0.25)) = 0.6 cm

Thus a saltwater layer which is 3 mm in thickness would provide a reflection isolation of 10 dB for an anechoic chamber.

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It might be difficult to make a quality 1 ft^3 anechoic chamber. You will waste a lot of time qualifying/characterizing your test rig before you can get any meaningful results for your own device. Better buy an industrial-grade "enclosure", like this line from Ramsey, or look for a thing called "TEM cell" and buy one. If outside your budget, you might try to DIY TEM cell

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