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

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From: Douglas McKean (dmckean@corp.auspex.com)
Date: Tue Jan 04 2000 - 10:02:14 PST


Hi Doug,

Henry Ott has a bunch of relationships regarding
holes. Namely circular, rectangular, an array of
circular holes, an array of rectangular holes.

He begins discussing cutoff frequencies for
individual types of holes. Circular hole cut
off frequency is based on the diameter.
Rectangular hole cut off frequency is based on
longest side. I have an Excel spreadsheet where
I translated these equations for ease of use. The
actual relationships I can look up for you.

The following results are linear so I'll use
1 inch and the result for 1/10 of an inch is
simply 1/10 the result for the 1 inch and so on ...

 1 inch Circular Hole: cut off freq = 6.90E+09
 1 inch Rectangular Hole: cut off freq = 5.90E+09

Mr. Ott continues the discussion with Shielding
Effectiveness (SE) for the geometry of a particular
hole, i.e. circular and rectangular and the
thickness to diameter ratio. Again, the
relationships are linear so I'll normalize them
for you at 1:1 for thickness:diameter

 SE for circular hole 1:1 (thick:dia) = -32dB
   (Thus, a ratio of 1:10 = -3.2 dB)
 SE for rectangular hole 1:1 (thick:dia) = -27.2dB
   (Thus, a ratio of 1:10 = -2.7 dB)

Intuitively, it should become obvious that the
length of the hole forces the "hole" whatever
geometry it is the deciding as to how much the
of a cavity effect begins to dominate.

Mr. Ott also discusses the "pattern effectiveness"
of an array of holes (circular or rectangular IIRC).
I'll give some results from my little spreadsheet.

A 4x4 inch array of 1/4 inch diameter holes with
a 1 inch center to center separation in 18 gage
sheet metal (thickness =0.0478 in)

 SE = -52.1 dB

Same array of holes as above changing only
the separation to 1/2 inch,

 SE = -40 dB

It works out with this relationship that halving
or doubling the separation of holes results in
changing the SE by about 12 dB. IOW, a 2 inch
separation of the above array gives an SE = -64 dB
or -12 dB added to the -52 dB for the 1 inch.

Keep in mind that there are many assumptions
made with these results. And the rules of thumb
regarding linearity or changing results by 12 dB
are merely theoretical.

One further note, Dr. Hubing at an EMC presentation
here in Santa Clara two summers ago, discussed
results from mucho research on his behalf about
holes in covers. The bottom line is that slots
are the thing to worry about and not holes. And
with that conclusion I wholeheartedly agree.

Regards, Doug McKean

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