Re: [SI-LIST] : Chassis hole opening and frequencies

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From: Michael Vrbanac (vrbanacm@swbell.net)
Date: Fri Dec 31 1999 - 17:44:08 PST


Doug,

There have been some good responses to your question so I
won't add to those. What I will contribute is that while
the theory is fine and good (and I do agree that much of
what has been offered as responses is correct), there are
a few interesting things one has to consider in addition
to those.

The first is: None of the compliance standards ever
requires that any radiating element be "efficient" or have
a standard fractional wavelength. The requirement is that
emissions are equal to or below the limits imposed.

The second is: The theories presented are very good for
a first approximation of the problem. You need to know
that there are radiating elements which may not appear
to be standard fractional wavelengths but an inadvertant
impedance matching structure to that radiating element
can transfer energy into it effectively enough for the
whole thing to be a problem.

The third is: The specific radiating element structure
may be such that additional directivity can be attained
which might not be obvious. This additional directivity
can make the "apparent efficiency" of the radiating
structure seem much higher than it is and a higher
emissions reading can result when it wouldn't seem like
it ought to be that way.

I have personally seen these conditions many times (no,
I won't cite any <grin>) over the years and have
investigated them to the point I could determine that
these situations were "non-standard" as far as the basic
theory goes. Actually, none of these are particularly
surprising for those who have done some work with
practical antenna design and theory and have had to work
with "sub-optimal antenna design" situations.

Hope that helps! Have a happy new year!!

Regards,

Michael E. Vrbanac

Doug Brooks wrote:

> As I recall, there is a relationship between a hole in a chassis and the
> frequencies that can pass through that opening. I recall that the longest
> dimension of the hole defines the wavelength, or quarter wavelength, or
> something, of the lowest frequency than can conveniently enter or escape
> through the opening.
>
> Can anyone give me the correct relationship?
>
> Thanks.
>
> And Happy New Year to All...........
>
> Doug Brooks
> and all of us here at UltraCAD
>
> .
> ************************************************************
> See our updated message re in-house seminars on our web page
> .
> Doug Brooks, President doug@eskimo.com
> UltraCAD Design, Inc. http://www.ultracad.com
>
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