Faraday Cage Shielding by Frequency 4

[Scott543]

Can someone help me clear up a contradiction in my understanding of Faraday cage shielding?

1) Long waves penetrate better than short waves.
but...
2) Looking at graphs of Faraday cage shielding, the E field of long waves appears to be easily blocked by small thicknesses of metal.
(See below, from "Architectural Electromagnetic Shielding Handbook", by Leland H Hemming.

These two things seem contradictory, unless it is a function of the nature of Faraday cages. Can someone help me clear up the contradiction?

Thanks,

Scott

 

[Skogsgurra]

MMF (H-field, A/m) penetrates better at low frequencies. The reason is that the shielding effect is mostly (exclusively) due to eddy currents and eddy currents are not induced at lower frequencies.

EMF (E-field, V/m) penetrates better at higher frequencies. The reason is that the electric field is shorted by the metal. Since the shorting effect is less efficient at higher frequencies (skin effect and such things), those frequencies penetrate better at higher frequencies.

At a certain distance from the emitter antenna, the H field and the E field combine into a plane wave and then the shielding effectiveness is equal over a broad frequency range. Less efficient the thinner the shield, but also reaching higher in the frequency spectrum.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[Scott543]

Thanks, Gunnar. That's a very helpful explanation. Can you help me fully understand it by explaining an extreme example?

The military communicates with submerged submarines using the SLF bands that can penetrate into sea water. So, if the E field is blocked by the ocean, does the signal become only a magnetic field instead of electromagnetic? Is then the receiver only working off magnetics?

 

[Skogsgurra]

"Is then the receiver only working off magnetics?"

I am not so sure. I think that the electromagnetic wave needs both components to be able to propagate over long distance. It is only in the near field zone that H and E fields are "untangled". Even if water is not a very good insulator, I think it is good enough to permit SLF Waves to penetrate.

That said, it is very plausible that the submarines picked up only the H-field with their antennae, which had to be quite small in comparison to the 17,2 kHz (wavelength around 17 km) used by at least one of these stations. See

http://www.grimeton.info/

.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[VEBill]

Another useful extreme boundary condition is that visible light (also EM). If doesn't take much thin metal foil to block light perfectly.

(Once upon a time, somebody got confused by 'waveguide beyond cutoff'; he remembered it backwards. I held up a section of waveguide and peeked at him through it, smiling and blinking until he figured out that higher frequencies - light - were, in fact, passing straight through. Obviously not cutoff.)

 

[Higgler]

higher frequencies get into a box easier due to "waveguide cutoff phenomenon".
Box screws make it a High Pass Filter.
Always add absorber into a metal box that you don't want to seal from RF.
Salt water in a bag is the cheapest and best for a quick try just to see.

Good box designs have one lid, with gasket (EMI gasket) that keeps RF out, and absorber is almost always used inside to stop rf from coupling/bouncing around.

Example: two rf cables next to each other at the connectors, loads on connectors, -95 dB coupling between cables. Put exact same hardware inside a metal box 5"x3"x1", coupling now is -50 dB (exactly same positioning of rf cables as outside the box).
Hence an rf box will stop external crap from getting in, seal the stuff inside, but increase coupling from part to part inside. Hence all switches and amps have absorber on their covers and often times in other places.