Thanks, Jim. I wondered whether there was anything other than the
'near-field thicket' involved. Measurement results in the near field can
be reliably reproduced only in absolutely identical test set-ups. This
is not compatible with 'standardization'.
On 2024-10-11 16:48, Jim Bacher wrote:
John, you ask why the difference in levels measured between test
distances of 3 meters and 10 meters. It’s fairly common for a device
to fail at frequencies below 125 MHz at 3 Meter test distance and then
pass at a 10 Meter test distance. Besides all the other possible
factors (such as was a different chamber and test equipment used), the
question becomes, was it a Near Field or Far Field RF signal that was
being measured? Near Field RF levels drop faster than Far Field RF
Levels. The problem with a 3 Meter test distance is the frequency
being measured might be impacted by Near Field, verses Far Field only
measurement at 10 Meters.
I have read a number of papers that claim different wave lengths for
the Near Field effect. The values I have seen are between 1 and 3 wave
lengths (with RF think wave lengths). I suspect it is system dependent
and typically 1 to 2 wavelengths and I suspect the primary reason for
the effect between the two measurement distances.
Here are the approximate possible frequency ranges impacted by Near
Field at a test distance of 3 Meters:
Three wavelength signal: RF levels up to 280 MHz
Two wavelength signal: RF levels up to 140 MHz
One wavelength signal: RF levels up to 70 MHz
As far as I am concerned 10 meters is the better test distance as it
is in the Far Field for the frequencies between 30 MHz and 1 GHz.
Although 30 Mhz is close to one wavelength at 10 Meters.
Jim Bacher, WB8VSU
[email protected] or [email protected]
*From:*John Woodgate <[email protected]>
*Sent:* Wednesday, October 09, 2024 4:18 PM
*To:* [email protected]
*Subject:* [PSES] Technical musings
Reply to Derek @ LF Research, because his post is labelled as SPAM.
Yes, adding OATS is always healthy.😉
Is there an accepted explanation for the '3 m excess'? The published
results are consistent with the field being diffuse (that term is from
acoustics: I'm not sure how widely it's used in EMC circles), i.e the
resultant of a large number of direct, reflected and diffracted rays.
It is hardly surprising: a cuboid space is 'ideal' for producing a
diffuse field above 'eigentone' wavelengths. This might create at
least a 3 dB increase over 'inverse square' and maybe more. I suppose
things get complicated at wavelengths that cannot be called 'short'.
Has anyone tried a spherical chamber? If that's too difficult, a
'quartic sphere [(x,y,z)^4 = r^4, like a Swedish traffic circle] has
noticeably rounded corners and edges, so might be close enough for a
useful improvement.
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John Woodgate
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