[Amps] Holes vs. screen for chassis ventilation?

[Carl]

Hardware cloth is fine for a HF amp, use screen for VHF and up. I used a UHF 
GDO to sniff the RF from the 1500W 2M amp. Using hardware cloth I could see 
some 144 energy and a LOT of the 3rd harmonic. Screen eliminated it all.

Carl
KM1H


----- Original Message ----- 
From: "Dan Levin" <dan at andlev.com>
To: <amps at contesting.com>
Sent: Tuesday, October 05, 2010 11:24 PM
Subject: [Amps] Holes vs. screen for chassis ventilation?


> 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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