For those who haven’t seen it, in the movie “No Country For Old Men”, a character has stolen a briefcase full of money. Unbeknownst to him, the suitcase contains a transponder, and the villain is tracking him down. The suitcase gets stashed in a metal air conditioning duct, several feet from the opening, and the metal cover put back over the duct opening. The villain then happens to be driving past and the transceiver detects the suitcase. Mayhem ensues.
I had a problem with this. The ducting is quite clearly metal, and capped with a metal grate with overlapping slats typical of air conditioning vents. Isn’t this effectively a Faraday cage, and going to stop any radio transmission from reaching the outside world? There is no way that radio waves, traveling a straight line, can get to the outside world from the suitcase without passing through sheet metal.
Even if we allow the tiny chance they could get out through the slats, the area available is tiny and since the suitcase was about 5 feet back from the vent, the beam would be quite tight, not a 360o signal. Since the transceiver was operating every second, the odds of the villain driving past at 50km/hr being in front of the weakened beam at exactly the right moment seems improbable.
So is this a plot hole in the movie, or can radio waves from a 1980 model transistor transponder plausibly pas through sheet metal?
Farraday cages still have to be made out of a metal that conducts well. I can’t remember the scene that well, but ducts might be made of some form of steel that doesn’t conduct that well. Many alloys aren’t terrific conductors.
The reality is probably that nobody working on the movie had studied EM induction, though.
The holes in the Faraday cage have to be smaller than the wavelength of the EM radiation to be blocked.
I worked in a Faraday cage that was made of a double layer of copper window screening on a wooden frame. The cage blocked radio signals fine, but allowed light to pass without problem. We don’t know the wavelength the bug was using to communicate, or the dimensions of the holes in the grille. Or the power of the transmitter, come to think of it. They could have been compatible.
For typical radio wavelengths, the vent screen would act as a polarizer, not a complete blockage. And between diffraction and internal reflection, the radiation coming out of the vent would be pretty widely spread: Not a full 4pi steradians, of course, but pretty close to 2pi (a hemisphere).
I did this in the frequency control industry; the cage blocked incoming signals to prevent interference with precision frequency measurements. Would there be a significant difference in blocking outgoing versus incoming signals? I think it would be equally effective in both directions, but I’m not positive.
Correct, it works exactly the same in both directions. The way to think of it is that the mesh is literally opaque to any light with a wavelength bigger than the spacing of the mesh.
I repaired a Faraday cage a couple of years ago – I brought my cell phone and a portable radio in with me. If either one worked, I knew signal was getting in.
Despite the fact that the cage was double-walled and made of fine mesh, with a closing door fitted with phosphor-bronze “fingers” to make good contact, the cage still leaked – I could get a signal on the radio. It took a lot of work to finally track down an imperceptible tear only a few inches long in the screen. Once i “sewed” that back together, the leak was gone. The outer wall of the cage, I point out, wasn’t compromised.
So, yeah, a Faraday cage is harder to completely seal than you think. A haphazard one made by chance probably won’t stop a transponder signal. Unless, of course, you need it not to. It’s the Principle of Perversity.