Re: [SI-LIST] : Cavity Resonant frequecy and radiated EMI

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From: Doug (dmckean@gte.net)
Date: Sun Sep 24 2000 - 20:51:41 PDT


There are several modes to resonance in a cavity.
Need to know height, width, and length to accurately
predict anything.

Regards, Doug

"KHOO,KENG-KOK (HP-Singapore,ex4)" wrote:
>
> Hi all,
>
> I have somne questions on the cavity resonant frequencies. I found a formula
> in Noise Reductions Techniques in Electronics System by Henry Ott Pg.199
> that says
>
> f = 212/L,
>
> where:
> L is the largest dimension of the cavity in meters
> f the lowest resonant frequency in MHz
>
> I have a rectangular box with L=140mm so f = 1.514GHz.
>
> There is a cable running out of the enclosure and the cable lenght inside
> the enclosure is 210mm and 430mm on the outdside of the enclosure. Radiated
> EMI measurement shows cable emission around 100-150MHz.
>
> Enclosure
> 210mm | 430mm
> --------------|----------------------------------
> Inside | Outside
>
> Ferrite core works well in reducing the 100-150MHz emission when it is put
> on the outside of the enclosure but did NOT have any effect when put on the
> inside portion of the cable.
>
> Questions:
> How do I relate the 1.5GHz resonant frequency to the 100-150MHz emission
> from the cable?
> Have I interpreted the formula wrongly?
> Where did the 212/L formula came from?
>
> Thanks a lot.

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