>Even now, correlation between 3 meters and 10 meters
>is not guaranteed. And further, 3 meter to 10 meter
>correlation is at least "better" (define better anyway
>you wish) in the horizontal. Vertically it's terrible
>(define terrible anyway you wish). At least in my
>experience.
Doug,
In our 3m chamber we use a "Vertical Correction Factor" (VCF) to account for the
worst-case difference between 3m and 10m vertical Radiated Emissions
measurements. We usually have +/-1dB correlation, occasionally out to +/-2dB,
between measurements taken in the chamber at 3m including the VCF against
official measurements taken on our 10m Open Air Test Site (OATS). We also
sometimes have some measurements taken at 10m that are much better than what we
predicted at 3m with VCF, meaning that we have over-engineered the product in
trying to ensure that we will pass the 10m tests...
The VCF is more pessimistic than the 10m CISPR limits by:
* About 1dB at 30MHz, increasing to
* About 7.5dB at 230MHz, dropping to
* About 0dB at 450MHz and above.
As I understand it, the VCF was calculated by modelling:
* A transmitting dipole antenna 1m above the groundplane with a receiving
antenna 3m away, at 1m to 1.7m above the groundplane (the
range of available heights in our 3m chamber)
VERSUS
* A transmitting dipole antenna 1m above the groundplane with a receiving
antenna 10m away, at 1m to 4m above the groundplane (the
range of heights required by CISPR testing).
maximum received signal at 10m at f MHz 10^2
VCF (f MHz) = ----------------------------------------------------- * ----------
maximum received signal at 3m at f MHz 3^2
In the far field we expect the signal to drop off at 1/r^2. But for
vertically-polarized signals the receiving antenna sees not only a direct-path
signal but one bounced off the groundplane. These two can add to double the
voltage at the receiving antenna, or subtract to nearly zero, depending on the
phase difference between the two paths. Running the antenna up and down helps
get away from the worst nulls, but
doesn't compensate for them completely. Considering that the actual source of
the radiation may be at various heights or angles, not centered on the table,
and the signal may reflect off other metal pieces, the Vertical Correction
Factor is a huge help to us in trying to meet the Radiated Emissions limits.
John Barnes Advisory Engineer
Lexmark International
dmckean%[email protected] on 06/17/99 04:39:53 PM
Please respond to dmckean%[email protected]
To: emc-pstc%[email protected]
cc: (bcc: John Barnes/Lex/Lexmark)
Subject: Re: NEAR/ FAR FIELD CORRELATION ISSUES
At 11:18 AM 6/17/99 +1000, you wrote:
>
>Greetings and Salutations! I was wondering if this could be mailed out via
>the epc-pstc channels.
>
>I want to know if anyone is doing any work in "near/ far field correlation
>to commercial EMC standard limits" area and possibly correspond with them
>with a view to exchanging notes.
Hi Arun,
At a former company I spent a very large amount of time
trying to correlate near field probe measurements of
the surface currents and voltages of a product to far
field (10 meter) measurements.
In brief - it didn't happen.
And a product could be analyzed as being constructed of
a variety of antennas - slots, corner reflectors, tuned
cavity, tuned arrays, and either electric or magnetic
dipoles ... each reacting it's own way in the far field.
Now I'm not going to say it's impossible, but it seems to
me that one must assume something to begin with instead of
being able to blindly take a surface current measurement
or near field measurement of X and state confidently that
it WILL be Y in the far field under all circumstances.
That's ultimately what one would have to be able to do
without regard to the product. After a few rounds with
a particular product, I've done this. I'm sure everyone
at some point has done this. But with NO prior history
of the product, I don't see how it's done.
Regards, Doug McKean
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