[Amps] Shielding Effectiveness

[Roger (K8RI)]

It's important to remember that hardware cloth which comes in many 
sizes, or screen does not normally give a good contact around the edge. 
If fender washers are used they basically need to be edge to edge. 
Commercial equipment uses a bracket, or frame to hold the screen in 
tight contact with the cover/chassis around the entire periphery.  
Although screen can be a much better shield at VHF, that advantage is 
lost if that contact is not continuous around the edge.

73

Roger (K8RI)

On 10/6/2010 11:13 AM, Ward Silver wrote:
> Per Ott's "Electromagnetic Compatibility Engineering", section 6.10 - shielding effectiveness in dB is:
>
> S = 20 log (wavelength / 2xL) where L is the maximum linear dimension of the aperture.
>
> If you have multiple apertures, such as an array of holes, the general procedure is to calculate S for one hole and then reduce S as 10 log (n) where n is the maximum number of holes in a straight line in the array.
>
> For example, if S for one hole is 52 dB (a 2.5 mm hole at 2 meters) and there are at most 20 holes in a line, S would be reduced by 10 log (20) = 13 dB to S = 52 - 13 = 39 dB.
>
> This is an approximation, of course, plus there are seams and other things to worry about.  But it's a good way to at least compare types of apertures in shields.
>
> 73, Ward N0AX
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