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## [TowerTalk] Re: Tower Faraday shield?

 To: [TowerTalk] Re: Tower Faraday shield? jimlux@earthlink.net (Jim Lux) Mon Jun 2 00:35:06 2003
 ```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: To: ; 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. > > _______________________________________________ > TowerTalk mailing list > TowerTalk@contesting.com > http://lists.contesting.com/mailman/listinfo/towertalk ```
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