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

From: D. C. Sessions ([email protected])
Date: Tue Jan 04 2000 - 14:56:39 PST

"Chan, Michael" wrote:
>
> D.C.:
> What is the variance "jt" respresent? I can understand "t"
> for time but not "jt"

Square root of minus one times time. Also, I goofed and didn't
include the speed of light. That way distance comes out to

x^2 + y^2 + z^2 - (ct)^2

which works rather nicely.

> -----Original Message-----
> From: D. C. Sessions [mailto:[email protected]]
> Sent: Tuesday, January 04, 2000 3:51 PM
> To: [email protected]
> Subject: Re: [SI-LIST] : Chassis hole opening and frequencies
>
> Jay Chesavage wrote:
> >
> > Last time I checked, chaos theory had to do with temporally varying
> > phenomenon, rather than spatially fixed phenomenon. Chaos theory, for
> > example, may be used to study weather, but doesn't work to well to figure
> > out where to put telephone poles. Unless the holes in your sheet metal
> > chase each other around a lot more than mine do, or you can convince
> > someone that EMI measurements need to be made while the antenna moves
> > randomly, I can't imagine chaos theory being applicable here.
> >
> > Is there a class of Chaos theory which has to do with optimizing fixed,
> > static structures? Sounds more like linear programming to me (explore
> > state space for minimum radiation at frequency f, where the variable is
> > hole placement), and then go repeat the process for each and every f, and
> > each and every hole placement(!).
> >
> > This seems on the surface to pay far less dividends for much more effort
> > than does, for example, quadrupling the number of holes, while quartering
> > the area of each one (assuming the aspect ratio has already been reduced
> > to 1 wherever possible).
> >
> > Am I missing something?
>
> You are. Chaos theory has to do with any complex nonlinear progression.
> Fractals (think Mandelbrot sets) are classic examples of chaos. Looking
> at it another way, time-variance is generally treated in physics as
> movement through the (x,y,z,jt) space.
>
> > On Tue, 4 Jan 2000, Adrian Shiner wrote:
> >
> > > Do Douglas' results provide yet another elegant demonstration of the
> > > interference effect of transmission of electromagnetic energy through
> (in
> > > this case imperfect) parallel slots?
> > > If so, then surely room for further development...chaotic hole spacing
> or
> > > narrow short slots at chatotic angular orientations and lengths. Read up
> on
> > > Chaos Theory for the use of chaotic in the sentence above.
> > > Bet wishes for the new year
> > >
> > > ----- Original Message -----
> > > From: Douglas McKean <[email protected]>
> > > To: <[email protected]>
> > > Sent: 04 January 2000 18:02
> > > Subject: Re: [SI-LIST] : Chassis hole opening and frequencies
> > >
> > >
> > > > 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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> --
> D. C. Sessions
> [email protected]
>
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```--
D. C. Sessions
[email protected]
**** To unsubscribe from si-list: send e-mail to [email protected] In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP.
si-list archives are accessible at  http://www.qsl.net/wb6tpu/si-list
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