As part of another thread ( http://www.part15.us/node/2078), I posted "For example, suppose a current is induced in the wall and is flowing vertically. Would I not see this current if I stood on the other side of the wall? And, if so, wouldn't it produce a radiated field coming toward me? One interpretation can be that the wall thereby appears transparent to the incident wave and will not stop it." I was speculating that things which we "know" block or affect radio waves may not do so as we would think.
So, just for fun and not for real science, I tuned my trusty portable AM receiver to my part 15 AM station, cranked up the volume, and placed the receiver inside a metal ammo. canister and closed the hinged metal lid.
Now, everyone knows that metal blocks radio waves so what would everyone say if I asked everyone if I could hear the audio from my radio in the box?
The answer is yes, I did hear my audio loudly but muffled by the box.
Could it be that since the box is very very small compared to a wavelength that my "induced current" speculaton is true? Or could it be that since the lid on the box has a rubber moisture seal that the lid was not effectively electrically connected to the box despite the metal hinge and latch?
With my radio removed from the canister I then wrapped my radio tightly in aluminum foil and I could still hear my programming clearly though it was evident by the receiver noise in the audio that the RF was attenuated.
Conclusion: None, other than the observations.
Years ago I home built a 145 MHz repeater and needed to shield the receiver from the ambient field from the transmitter. I enclosed the receiver in a metal box with a lid and used feedthrough caps on all lines in and out of the box. With the antenna connector shorted the receiver would still hear the unwanted signal. Though the lid was secured by screws every inch around the perimeter the signal still leaked in. The solution was to make a copper mesh gasket to go between the lid and the box. There was no detectable leakage with this.
RF, like mice, can go places where we humans would predict it couldn't. This is why it is fun to experiment.
Neil
The RF penetration of metal is limited by the skin effect. Each skin depth attenuates the RF by 1 Neper, or 4.3 dB, which is 1/e, where "e" is the base of the natural logarithm. The ammo box has pretty heavy gauage steel sheet, and so the RF wouldn't penetrate the steel itself. (check your references to calculate the skin depth for steel.)
A welded watertight (a requirement to be RF-tight) steel box would very effectively shield its contents from RF. The RF gets into the box through the non-metallic cover seal, and through any other openings.
Ermi,
I agree. It would have been easy to jump to the conclusion that the RF was going through the metal to get to the radio, but as you point out the ammo. can has thick walls.
As related in my story about the repeater, RF shielding requires careful design and assembly. I was trying to convey two things. First, be careful interpreting observations and second, RF can get into places we normally wouldn't expect.
Neil
"A welded watertight (a requirement to be RF-tight) steel box would very effectively shield its contents from RF. The RF gets into the box through the non-metallic cover seal, and through any other openings."
Depends on wavelength.
John,
Yes it does depend on wavelength with smaller "mice" getting through "smaller" holes.
This whole idea of "transparent" metal arose from another discussion on this board where the effect of a fractional wavelength dimensioned metal obstacle on transmitting antenna performance was questioned. I suggested that the metal barrier could perhaps have no effect and therefore would be "transparent" though, literally, the RF passing through the metal was not the mechanism, yet the effect is the same.
I twisted this to a closed box and did an experiment and the two are definitely not the same situations and my comment about a transparent box was primarily for fun, yet my experience which I posted about shielding my repeater receiver was not. Adequate shielding can be obtained without welding but it is not trivial.
Neil