Ballpark figure for "significant" hole sizes is perimeter >1/2 wavelength,
so, at 14 MHz, a 10 m perimeter hole, or about 10 ft in diameter... The hole
acts as a dipole antenna (Babinet's theorem says that a slot in a infinite
plane is electrically the same as a conductor of the same shape as a slot in
infinite space)..
At low frequencies, magnetic fields are more of an issue than electric
fields. Field "outside the box" induces a current in the wall of the box,
which, if skin depth is big enough, appears at the inside of the box, where
it radiates. Since skin depth is where current is 1/e (i.e. about 1/3) that
at the surface, to get, say, 30-40 dB of isolation requires many skin depths
thickness (more precisely, ln(1000) for 30 dB). For copper where skin depth
is quite small at RF, this isn't a big deal. For iron or steel, where skin
depth is much greater, and at lower frequencies, it's a big problem.
Jim W6RMK
----- Original Message -----
From: <K7LXC@aol.com>
To: <norsan@bright.net>; <TowerTalk@contesting.com>
Sent: Sunday, June 01, 2003 5:02 PM
Subject: Re: [TowerTalk] Re: Tower Faraday shield?
> In a message dated 6/1/03 12:15:02 PM Pacific Daylight Time,
> norsan@bright.net writes:
>
> > To be a Faraday shield the enclosure must have no holes larger than a
> > wave guide at cuttoff to the highest frequency you want protection
from.
> > By this definition a tower is not much of a Faraday shield.
>
> Then what's the hole size we're talking about at 14 mHz?
>
> Cheers,
> Steve K7LXC
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
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