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] 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.
--
OOO - Own Opinions Only
Best Wishes
John Woodgate
Keep trying
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Best Wishes
John Woodgate
Keep trying
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