Thanks for the responses, everyone. I was probably misleading when I used the phrase 'characterize a screen room'. It sounds too much like 'calibrate'. I was (and still am) pessimistic that we can any get decent information from a screen room. I was hoping to at least figure out a way to mark frequencies where the room was unreliable. Enough said.
In light of comments made (especially the one below that mentions 20dB variations), it sounds like pre-scans to _discover_ problem spots are invalid as well. - If you saw a bump, it may be a reflection/resonance. - If you saw a quiet area of spectrum, it may be a null. It sounds like the process should be: 1) Discover frequencies by going to an OATS and doing a valid scan. 2) Put the same test setup in the screen room, and move it around until you saw the same relative amplitudes (ignore the absolute amplitudes). Mark the location of that exact test setup! 3) Work on the unit to reduce the relative amplitudes. BTW, I am going to try measurements in the parking lot. If a screen room isn't purchased, there should be lots of money available for lounge chairs and sun screen (I live in Southern California). On Wed, 28 Apr 1999 11:14:38 -0500, Patrick Lawler <plaw...@west.net> wrote: >At 04:21 PM 4/27/99 GMT, Robert Bonsen <rbon...@orionscientific.com> wrote: >>My company is planning to purchase a screen room for radiated emissions >>precompliance testing. >> >>I'm aware that reflections can cause resonances and drastically influence >>readings. What kind of testing could I do to characterize the room (aside >>from simple experience)? >> >The simple answer would be this: don't even try. You're much better off >using the company parking lot to do pre-compliance radiated emissions >testing. For conducted emissions/immunity, and to a certain extent radiated >immunity, a shielded room is great. But not for RE. > >The reason for this is the reflections/resonances you get from the walls >and the ceiling. You can get higher than 20 dB ripples on your measurements >in an untreated (no absorber materials on walls/ceiling) shielded room. And >these ripples are not very repeatable, they will change considerably with >position (eg, moving your antenna or EUT less than an inch may result in >field variations of much more than 10 dB). Because of these huge >variations, testing cannot help you characterize your room and take these >reflections into account in your emissions measurements. > >If you absolutely need to use a shielded room, try lining it with absorber >materials. Even a few absorbers are better than none at all. Or try using >another type of pre-compliance device like a GTEM or something similar. >Another alternative would be to turn the shielded room into a mode-stir >chamber. By rotating the properly designed mode stirrer, you will even out >the variations which will result in fairly usable, repeatable numbers. The >size of the room determines the usable frequency range. -- Patrick Lawler plaw...@west.net --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
