Mechanical EMI shielding - 168 Mhz

[tfr]

I need to filter ~180 Mhz and lower emissions from an infrared transmitter while minimizing the amount of IR loss. What is the formula for determining the maximum aperture size that will still block a given wavelength or frequency? Is this formula usable to determine mesh size? Our short term solution is a fine wire mesh, but I need to determine the mesh size. Thanks in advance.

 

[nbucska]

If you can afford the size, you may use a resonant cavity.
You may reduce the size of the cavity by using an -- IR xparent -- dielectric with high "e".
<nbucska@pcperipherals.com>

 

[ClintBrown]

Per AFDH-1 the cutoff frequency for a rectangualr waveguide is fc=5900/W,
W largest dim of port in inches and fc cutoff in Mhz. Approximate attenuation
at 10*fc will be A=27.3*T/W, T in inches (port thickness), A in dB.

 

[curious]

The formulas in the preceding post are ok - but the waveguide is in cutoff for frequencies below the cutoff frequency and the attenuation formula applies to frequencies below the cutoff frequency, fc. There is little attenuation for frequencies above the cutoff. Consequently the attenuation at 10*fc is not A=27.3*T/W; however, this formula does give the attenuation for 0.1*fc.