Tom,

Thanks for the comment, and I need to clarify mine. You correctly
state that an intact metallic enclosure will shield the interior
from RF of any wavelength. And the degree of attenuation is a factor
of the shielding conductivity and/or the number of layers. As you 
state, such complete enclosures are "Faraday cages" even when 
perforated with openings much smaller than the wavelength of concern,
like the screen in the door of a microwave oven.

My comment about the utility of shielding against long wavelengths
was directed toward attempts with materials like aluminum foil that
do not provide a complete conductive path around the object. A foil
patch will shield the chicken drumsticks in the microwave because they
are (duh) microwaves. Long wavelengths require a complete shield
with no breaks in the conductivity at the joints.

When I was building the cyclotron, we had a linear amp to provide 12 kw 
of 10.033 mhz RF (sine wave) to drive the accelerating electrodes. 
Because of the resulting high RF voltages, most of this ended up
as RF everywhere or heating of the high-Q elements (only 300 watts
ended up in the particle beam), so all our experimental enclosures
had linear phosphor-bronze contact fingers around all the door seals,
and we even needed nested enclosures for some detectors.
(for the hams on the list, I used a 4CX1000 tube in the amplifier,
operating off 7.5 kilovolts at 2 amps (a "Texas kilowatt" ?!)).

But the first line of defense against RF interference in seismic
amplifiers is in good amplifier and power supply design. 

Regards,
Sean-Thomas
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