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 Post subject: RF Attenuation imperfect faraday shield
PostPosted: Jan Wed 17, 2018 8:15 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Pasadena CA USA
I can not recall the formulas and can lot find my old books, but assume an almost perfect faraday box ( except for a hole as described below) with an rf source ( transmitter) inside. The box has a half wavelength hole in it. How much will this faraday shield with the half wavelength round hole attenuate the rf field? Thanks, jim


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 Post subject: Re: RF Attenuation imperfect faraday shield
PostPosted: Jan Wed 17, 2018 8:47 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: S. Dartmouth MA 02748-1225 USA
IMHO there will no attenuation. The wave will propagate out of the outside of the hole, spread equally around in 1/2 wave loops. Looking at cavity resonators used in microwaves to get some visual concept. I have used similar concepts in creating 50 mhz bandpass filters where the metal container is the inductor.

YMMV

Chas


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 Post subject: Re: RF Attenuation imperfect faraday shield
PostPosted: Jan Thu 18, 2018 5:09 am 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 9381
Location: Santa Rosa, CA
Quote:
Dangerous radiation from looking into your microwave oven?
Even though you can see into the microwave oven when your food is cooking, the microwaves are effectively blocked from getting out into the room because the holes in the metal screen on the microwave oven door are about 1 mm in diameter compared to a 120 mm wavelength for the microwaves. The wavelength of the microwaves is about 120 times the size of the holes, and can't "see" the holes to get out. This is an application of the fact that you cannot image anything that is smaller than the wavelength of the radiation you use to image it. You can see through the holes because at 500 nm wavelength, the visible light wavelengths are about 2000 times smaller than the holes.


Seems like the same deal with the "box." If the RF wavelength is a lot longer than the hole size, there won't be much leakage?

Rich
http://hyperphysics.phy-astr.gsu.edu/hb ... woven.html


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 Post subject: Re: RF Attenuation imperfect faraday shield
PostPosted: Jan Thu 18, 2018 8:52 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 671
Location: Pasadena CA USA
As I recall there is a formula to approximate the attenuation of the signal vs. hole size, but I can not recall it or find it. Anyone recall this?


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 Post subject: Re: RF Attenuation imperfect faraday shield
PostPosted: Jan Thu 18, 2018 12:03 pm 
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Joined: Jul Mon 26, 2010 8:30 pm
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Location: Annapolis, MD
try looking up "waveguide beyond cutoff" to see if that gives any clues......

_________________
-Mark http://pixellany.com

"It's always something". --Gilda Radner (1946 - 1989)


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 Post subject: Re: RF Attenuation imperfect faraday shield
PostPosted: Jan Thu 18, 2018 7:38 pm 
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Joined: Mar Wed 16, 2011 10:44 pm
Posts: 754
Location: Peekskill, NY
Also look up "slot antenna" as this is the other issue.

The long dimension of the 'hole' controls the cutoff frequency, NOT the short dimension.

A slot 0.001 inch wide and three inches long, is basically the same thing as a three inch
diameter hole - oh there's some issue with the polarization of the radiation inside the
box, but it's within a factor of two.


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