Yes, there's not only the intractable near-field issue, but all the EM
influences between the various pieces of equipment. This all adds to the
uncertainty and irrepeatable results.
On 2024-10-11 18:08, doug emcesd.com wrote:
A criteria I have seen and agree with is that the distance from the
EUT to the antenna be 10x the size of the EUT to insure the antenna is
seeing a uniform field so it’s calibration is valid. This is not the
same as being in the far field. This is a big issue at 3 meters.
I have significant issues with many, if not most standards I have
read. For instance, the people who wrote IEC 61000-4-4 did not
understand the way the "capacitive" clamp works. It is also an
"inductive" clamp and as a result it is directive and more energy is
sent to the auxiliary equipment than to the EUT, there is no excuse
for this. the clamp is positioned backwards in the standard!!!! I have
been pointing this out for 30 years now to my clients and others. Here
is a link to a paper I wrote on this almost 30 years ago:
https://emcesd.com/pdf/esd96-w.pdf
In my opinion, neither the clamp nor the standard accurately describe
actual EFT although in later years some progress has been made, not
nearly enough though.
I see problems like this in many standards I read.
Another problem that is much harder to control happens over in the ESD
side. My personal discharge at 4 kV holding a small piece of metal
with a measurement chain with 5 GHz bandwidth has a peak current twice
what the standard calls for but the follow-on "hump" is more of a
straight line down to the horizontal axis much faster than the
standard calls for containing a lot less energy. I think this is due
to the fact I have less capacitance (surface area, I am about two
meters tall but on the skinny side from running 3,000 miles a year)
that what was used for the standard which is probably closer to
average than me. I have no idea how to account for variability between
people and the actual environment they are in when an ESD event happens.
Doug Smith
Sent from my iPhone
IPhone: 408-858-4528
Office: 702-570-6108
Email: [email protected]
Website: http://dsmith.org
------------------------------------------------------------------------
*From:* John Woodgate <[email protected]>
*Sent:* Friday, October 11, 2024 8:58:14 AM
*To:* [email protected] <[email protected]>
*Subject:* Re: [PSES] Technical musings
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] <mailto:[email protected]> or
[email protected] <mailto:[email protected]>
*From:*John Woodgate <[email protected]> <mailto:[email protected]>
*Sent:* Wednesday, October 09, 2024 4:18 PM
*To:* [email protected] <mailto:[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.
--
OOO - Own Opinions Only
Best Wishes
John Woodgate
Keep trying
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