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

From: Ron Miller ([email protected])
Date: Tue Jan 04 2000 - 12:22:14 PST

Hi Doug

The relationship between the size of the hole and the radiated power
is not linear. As the aperature is reduced well below a 1/4 wavelength
the losses are pronounced.

Several years ago I had to design an aperature(hole) coupling structure
and researched the MTT journals for relationships for waveguide aperature
coupling. Also, the Book by Mathias and 4 others on waveguide structures.

So, after getting all the formulas and designing the structure my measurements
showed about 20 db more loss than I expected. So, I had to put in a coaxial probe
to couple the power between guides.

The bottom line is that the losses are not linear but perhaps an inverse square function
as the size of the holes are reduced well below 1/4 wavelength.

Ron Miller

Douglas McKean wrote:

> 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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```--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
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