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Why does a chain link fence cause RF interference?

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tzoom84

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Question that popped in my head. Refer to a statement from Cisco regarding interference on 2.4 GHz signals from this page:
"Chain link fence, wire mesh with 1 - 1 1/2" spacing acts as a 1/2" wave that blocks a 2.4 GHz signal."
Click to expand...

I'm curious what physically about a metal fence causes interference when the spacing is on the order of a 1/2 or 1/4 wavelength? At first I was considering the concept of a Faraday cage where the cage itself effectively absorbs the electromagnetic wave (using the term "absorb" loosely). But doesn't the cage need to be enclosed to work? If that's the case, I'm confused how the fence (which may not be enclosed) causes interference. Or do the same "absorption"-like principles apply with the fence regardless? Would it behave differently if it was a solid thin wall of the same material?

Thanks again!
 

wavelength = speed of light/frequency or 300 X 10^6/2.4 X 10^9 = .1/8 m or 100/8 mm so according to my maths one wave length at 2.4 GHz is 125. mm or ~ 5 inches. So a quarter wave length would be 1 1/4 inches which is about right for a mesh. The problem that I see is that a chain link mesh fence looks like a series of diamond shaped apertures, its actually made of lengths of wire that are kinked and interleaved, so the mesh junctions are just some corroded wires that loop at these points. So are these actually quarter wave lengths lengths or kinked 10 feet lengths? The steel construction would however be very lossy so would dissipate some RF energy.
Frank
 

Technically, the fence does not "cause" interference, as in generating unwanted frequencies out of thin air. What it can do is reflect energy off of it. The phase angle and magnitude of that reflection can vary dramatically with a small frequency change. So if you are trying to send a widband signal with some sort of complex modulation format, as the signal takes up a wide bandwidth somewhere in that band the fence might add in a multipath drop out. that drop-out in phase or amplitude can cause bit errors, and if they get big enough, the network either goes down, or has to remodulate itself at a much lower data rate to live with the multipath interference.
 

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