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? :)