Measure at 2 or more distances and compare the differences in the readings.
There is about 18 dB difference when the distance is doubled or halved for a
3rd order roll off.
Try the formula -> diff = 20 * (order of rolloff) * log(dist1/dist2).
If my algebra is right...
roll off order = (level at dist1 - level at dist2) / 20 log (dist1/dist2)
The sign may come out wrong, but intuition can fix that.
Real world measurements often don't come out quite as expected, but close,
sort of.
-
Bill
--- On Thu, 7/15/10, rehel...@mmm.com <rehel...@mmm.com> wrote:
From: rehel...@mmm.com <rehel...@mmm.com>
Subject: Roll-off
To: emc-p...@ieee.org
Date: Thursday, July 15, 2010, 12:18 PM
How do you determine roll-off (extrapolation factor) for frequencies
under 30
MHz, in particular, for a test site? The "standard" roll-off is 20 dB/decade.
Thanks,
Bob Heller
3M EMC Laboratory, 76-1-01
St. Paul, MN 55107-1208
Tel: 651- 778-6336
Fax: 651-778-6252
-
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