"Frank" wrote in message
news:VPqbe.902$0X6.797@edtnps90...
Thanks again Ed. From everyone of your posts I learn something new.

The MIL-STD-461E requirement for absorbed is a 10 dB return loss at 250
MHz.

I have 24" tall pyramidal foam, and that meets the requirement. As
frequency decreases, the foam essentially disappears. By 10 MHz, it has
almost no effect.

I think we were using 12" pyramdal foam, even on the floor, with inverted
foam to provide a walking area.

The pyramidal foam is expensive, about $50 / sq ft. If you want more
return loss, you need taller pyramids; those mythical governmental labs
have had foam up to 72" tall (and the wall absorbers tend to droop a bit
g).

With a 3m chamber, anything greater than 12" is not really practical.

Well, that's kind of what I was trying to say. The right way to do the work
is to start with the size of the device you need to test, then consider the
test standard that you need to apply, and that will tell you how much
"working volume" you need inside the chamber. Then, you can decide on
anechoic treatments, and that then defines the size of the shielded chamber.
This "working outward" approach is the right way, but if you find that you
have now specified a 30' high by 50' wide by 100' long chamber, likely you
can't afford that much "goodness." g

Most people find themselves in a situation where they have a chamber of some
kind, and then they are challenged to do good work on a product inside that
volume. Sometimes you can do "good enough" work, with a lot of effort and
some known limitations. Sometimes what you do is pretty decent, and good
enough for "pre-compliance" requirements. If your product line is rather
consistent (size, peripherals, external cabling), you can often use data
from a fancy, fully capable lab and compare that with data generated in your
own limited facility. When you find the deviations, you can use those as
future "correction factors."


--
Ed
WB6WSN
El Cajon, CA USA