10 meter chambers can and will become resonate somewhere below 80MHz if
they are not large enough. In order to make them large enough to avoid
this problem you must build a building taller than most buildings to contain it!
I previously worked at a company where we did not trust measurements in that
range.
They had a quiet open site like yours which was accurate.
> From: "Steve Kuiper" <aegisl...@email.msn.com>
> To: "EMC-PSTC" <emc-p...@majordomo.ieee.org>
> Subject: Indoor vs. Outdoor emissions testing
> Date: Tue, 27 Oct 1998 12:31:59 -0800
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> Joe,
>
> Our laboratory was used to compare readings in the low-end, specifically 30
> MHz. to 90 MHz. on an OEM product In particular the OEM customer who
> maintains 10m indoor anechoic facilities claimed that our customer was
> failing the CISPR22B limits by 5-6dB in the above freq. range. After taking
> our own readings and comparisions with 3 different OATS in a 1.5 day
> turnaround we showed a 1.8 - 2.5 dB correlation between the OATS!!!!!! The
> meter reading at all three sites indicated a passing margin of 3-4dB using
> the identical product and configuration previously tested in the Anehoic
> Chamber. All OATS test sites used contained different test, equipment,
> ground structure, ambients. etc. Of course only 3 or 4 frequencies were
> observed for this comparison but we found our OATS correllation from site to
> site to be extremely good in this case.
>
> In my opinion, for engineering purposes only the 10m chamber is ideally
> suited for accomplishing the task of high volume testing. Zero ambients,
> and automated testing is the only way to fly. However, with all the
> improving absorber technology as such I still believe for correlation and
> repeatability there is no equal to the 10m OATS, especially for those
> located in a low ambient environment. Our faclity is located deep within a
> canyon at the base of a 4000 foot mountain in So. Cal near a creek bed. The
> floor noise is excellent and the ambients are low. Line noise is
> non-existent. Our customers see many advantages.
>
> Hope this helps.
>
> Regards,
> SJK
>
>
>
>
>
>
>
>
>
>
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_/_/_/ _/ _/ _/ _/ Gordon Andrews
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