Re: gEDA-user: basic anti-EMI design q

[John Doty <jpd@xxxxxxxxxxxxx>]

On Mar 25, 2006, at 3:18 PM, Karel Kulhavy wrote:

> What is attenuation in decibels of an infinite copper plane 0.2mm  
> thick
> @100MHz (driven by a plane space wave perpendicular to the plane)?

Well, the general way to do this involves wave optics with complex  
refractive indices, but here (normal incidence, thick medium relative  
to skin depth, extreme index mismatch) we can simplify.

Using handbook formulas, I get skin depth of 7 microns and sheet  
resistance of 3 milliohms. Empty space acts like it has a sheet  
resistance of 377 ohms, so you've got high Z driving low Z, loss  
roughly .003/377 or ~10^-5 in amplitude. 200 microns is ~30 skin  
depths, amplitude loss is ~exp(-30), or about 10^-12. On the far side  
of the sheet, low Z drives high Z so there's negligible amplitude  
loss. Overall we have an amplitude loss of ~10^-17 or 340 dB.

Even very thin metal foils are extremely good RF shields where  
contiguous. It's leakage though holes, joints, and especially wires  
going through that makes trouble.

John Doty              Noqsi Aerospace, Ltd.
jpd@xxxxxxxxxxxxx