Hi Dan, there are a couple of data points on this:
1> the old Bill Orr Radio Handbook has a chart showing rf attenuation vs
hole size in the section dealing with rfi and shielding considerations.
2> every commercial amplifier I have ever seen uses perforated holes for
ventilation; not screen. Any rf leakage is apparently very small or we would
have heard of problems.
A side note, the input air supply opening also needs screening as rf can
leak out at that point as well.
I once purchased a homebrew 432MHz amplifier that had no screen or shield
on the cooling air exhaust right above the tube anode. The exhaust hole was
about three inches diameter. There was so much rf leaking out that the
shack computer shut down. When I asked the seller about the problem he told me
he had operated like that for years with no problems. Sounds like a good
way to become an SK way to soon!
Yes, I did put a piece of perforated aluminum over the exhaust immediately.
73,
Gerald K5GW
In a message dated 10/5/2010 10:25:13 P.M. Central Daylight Time,
dan@andlev.com writes:
Am building an HF amp, and will need several large (~3" diameter)
openings in the chassis for ventilation. I would like the chassis to
be as RF tight as is reasonably possible, so trying to figure out the
best strategy for shielding the openings.
The two obvious options are to use a pattern of (small, say 1/4")
holes, or to cut a big hole and then cover it with (presumably)
aluminum window screen mesh or similar.
I've searched for answers, but all I found was some general stuff that
suggested that smaller holes were better, and to keep holes under
lambda/30 if possible. My guess is that fine grid aluminum mesh (as
in window mesh) would do better than holes - but since I am having
Protocase build my chassis, patterns of small holes are actually
easier for me to implement (since they can cut those, but I have to
deal with the screen myself).
Anyone have either a) answers or b) pointers to good sources?
Thanks!
***dan
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