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

About this list Date view Thread view Subject view Author view

From: Michael Vrbanac (vrbanacm@swbell.net)
Date: Mon Jan 10 2000 - 17:16:06 PST


Jeff,

I also agree. If someone hasn't already mentioned it, the electrical distance
between the source and the "radiating" aperture needs to be known. Many
of the "equations" out there are for sources located in the "far field" so care
needs to be taken. That is true even if you add some depth to the aperture
but go ahead add depth if you can. Its a good thing.

Also, from my work, the 1/10th wavelength to the very highest significant
frequency expected to impinge on the aperture should be fine provided the
source is not too "electrically close" although its probably about as large an
aperture as I would use in any case. I would think twice about using that as a
default, however. When you get to emissions in the Ghz range, you might
consider being a bit more conservative. I have gotten some very interesting
cumulative effects with "every other aperture and feature of the box" up
there.

Michael E. Vrbanac

"LATOURRETTE,JEFF (HP-SanJose,ex1)" wrote:

> Doug & Chris:
>
> I agree with Ray's 1/10th wavelength and have even heard 1/20th recommended.
> Remember that on your 1/4 wave example that the 1/4 wave frequency is the
> most efficient which will radiate, but certainly other frequencies both
> higher and lower can and will radiate from that line. The same is true for
> apertures sizes in a chassis. It is true, however that you can achieve
> cut-off by having sigificant depth in your aperture so that it resembles a
> waveguide. The amount of attenuation to signals below the cutoff frequency
> is directly proportional to the length of the waveguide structure. I've
> captured some of this in an Application note, AN 1166: [18pp/195KB (Pub.
> 3/99)] "Minimizing Radiated Emissions of High-Speed Data Communications
> Systems" which can be downloaded at:
>
> http://www.semiconductor.agilent.com/fiber/fiberapps.html
>
> Also, Make sure meshes used actually make conductive contact at its own
> cross points, as well as on its edges to the chassis. Some cheaper screens
> are weaved wires without electrical contact. Some use two mesh screens and
> for extra high performance use an extruded mesh structure with depth (like a
> honeycomb) in place of a screen.
>
> Regards,
>
> Jeff La Tourrette
> SJ Applications Manager
> Agilent Technologies Fiber-Optics Components
> 370 West Trimble Rd. M/S 90TH
> San Jose, CA 95131
> Voice: 408 435-4083 FAX: 408 435-4915
>
> -----Original Message-----
> From: Ray Waugh [mailto:ray_waugh@agilent.com]
> Sent: Monday, January 10, 2000 11:09 AM
> To: si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : Chassis hole opening and frequencies
>
> Doug...
>
> Have you ever seen a parabolic dish antenna which uses a mesh (instead
> of a solid metal surface)? It lets the wind and rain through. As a
> rule of thumb, keep the holes in the mesh under a tenth wavelength in
> dimension.
>
> I'd suggest doing as the RF instrument people do -- cut a hole in your
> chassis large enough for your fan, and mount a mesh over it. To avoid
> having the mesh act as a RF radiator, make certain that it is
> continuously attached to the rim of the chassis.
>
> You could probably build and test a dummy chassis quicker than you could
> do a proper analysis.
>
> Ray
> ------------------------------------------------------
> Raymond W. Waugh - WSD Diode Applications
> E-mail: ray_waugh@agilent.com
>
> USPS : Agilent Technologies
> Wireless Semiconductor Division
> 39201 Cherry Street, MS NK20
> Newark, California 94560
> ------------------------------------------------------
>
> -----Original Message-----
> From: Chris Padilla [mailto:cpad@cisco.com]
> Sent: Thursday, December 30, 1999 1:39 PM
> To: si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Chassis hole opening and frequencies
>
> Doug,
>
> In antenna theory, a microstrip (for example) that is fed (by an ideal
> infinite rise-time source) in one end and
>
> (a) shorted at the other end will radiate at the frequency that
> corresponds
> to a quarter wavelength
>
> (b) open at the other end will radiate at the frequency that corresponds
> to a half wavelength
>
> So if our microstrip is 12 cm long and it is shorted at one end, we
> should
> be able to radiate 625 MHz pretty well. If you imagine your ideal
> source
> is in the hole of the chassis and connected across the longest dimension
> of the hole, we have the same thing set up in (a) above. Our source is
> referenced to ground and the short is, effectively, ground, too. Does
> this
> make any sense???
>
> So my long-winded answer is a "quarter wavelength of the longest
> dimension
> that defines the hole."
>
> This should get you very much in the ballpark.
>
> WARNING::DEVIATION FROM QUESTION::WARNING---------------**************
>
> Now the next question is: What if I have 2 or 3 or a whole matrix of
> holes
> (air holes for cooling for instance) in my chassis. How will THAT
> radiate?
>
> I can indirectly answer that question as follows:
>
> We can get SE (shield effectiveness) from:
> 20*log(wavelength/2*max_length)
>
> where max_length is the maximum dimension of the slot or aperture.
>
> So the SE of a 100 mil opening at 5 GHz is 21.45 dB-->watch units!
>
> Mulitple apertures reduce the shielding effectiveness. Lets call it
> MA_n.
>
> The amount of reduction depends on (1) the spacing between the
> apertures,
> (2) the frequency, and (3) the number of apertures. When apertures of
> equal
> size are placed close together (less than a half wavelength), the
> reduction
> in shielding effectiveness is approximately proportional to the square
> root
> of the number of apertures n:
>
> MA_n= -10*log(n)
>
> So now our total shielding effectiveness, SE_tot = SE + MA_n
>
> Example: 4 100 mils holes at 5 GHz provides a reduction in shielding of
> 6 dB
> if the holes are less than a half wavelength or 1.18 inches apart.
>
> So now our SE = 21.45 + (-6) = 15.45 dB at 5 GHz.
>
> I know, I know, it is more than you wanted to know.
>
> ----->Chris
>
> At 09:45 AM, you 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
> >
> >
> >**** To unsubscribe from si-list: send e-mail to
> >majordomo@silab.eng.sun.com. 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
> >****
> >
> >
>
> **** To unsubscribe from si-list: send e-mail to
> majordomo@silab.eng.sun.com. 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
> ****
>
> **** To unsubscribe from si-list: send e-mail to
> majordomo@silab.eng.sun.com. 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
> ****
>
> **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. 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
> ****

**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. 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
****


About this list Date view Thread view Subject view Author view

This archive was generated by hypermail 2b29 : Thu Apr 20 2000 - 11:34:33 PDT