I've been playing around with some contactless bank cards and an RFID reader app on my phone. As expected, if I wrap the card in foil, the reader no longer detects it. But I was surprised to find that if I place a layer of foil on a flat surface, put the card on top of it and the phone on top of that, it also fails to detect the card. Why does placing a barrier behind the card prevent communication?
[Physics] Shielding RFID with aluminium foil
electromagnetismradio frequency
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It really depends on what you mean by "magnetic," because there are different kinds of magnetic properties.
Materials like iron are ferromagnetic, which means that once you align the individual magnetic dipoles in the material, they will tend to stay aligned even without an external magnetic field. Ferromagnetic materials are the ones that permanent magnets are made out of, and they are probably what most people think of when they imagine a magnetic material. There are only three elements (as far as I know) that are ferromagnetic: iron, cobalt, and nickel, although other elements can be combined to make ferromagnetic polyatomic crystals.
Other materials that aren't ferromagnetic can (and typically do) have interesting magnetic properties, though - in other words, just because a material isn't a ferromagnet doesn't mean it doesn't interact magnetically at all. Paramagnetism is one such interaction. When you put a paramagnetic material in a magnetic field, its individual dipoles tend to align with the magnetic field, and thus with each other, thereby making the material magnetic. When this happens, the paramagnetic material is attracted to the magnetic field. The difference is that when you take the external magnetic field away, the individual dipoles in a paramagnetic material don't retain their orientation. Instead, thermal motion takes over and reorients them randomly. So a paramagnetic material only has a net magnetic moment while it is in an external magnetic field.
If Magneto is able to control magnetic fields, then that would potentially allow him to control all sorts of magnetic materials - not just ferromagnets (iron etc.) but also all paramagnetic and perhaps diamagnetic materials, since he can create the external field necessary to magnetize those materials. In fact, all materials, even non-metals, are diamagnetic to some (small) extent. However, paramagnetism and especially diamagnetism are generally much weaker effects than ferromagnetism, so it stands to reason that Magneto would have a harder time controlling non-ferromagnetic materials.
The closest thing to a scientific explanation for Magneto's abilities that I can come up with is that he's able to generate magnetic fields that are strong enough to have a significant effect on ferromagnetic and some of the more paramagnetic materials, but with diamagnetic materials, the magnetic fields he can produce are not strong enough to override other natural forces acting on those materials. Of course, I'm sure that wouldn't really hold up if you really look at the comics or the movie closely... but with comic books you probably don't want to ask too many questions ;-)
Generally those sort of passive rectifying tags need antenna which are long in one dimension. They have to a be a reasonable fraction of a wavelength - and you need relatively long wavelengths for long range at reasonable power.
For example the Recco tags for ski clothing would work and are about 2" long. One issue may be the angle of them relative to your search - you need the antennae to be perpendicular to your detector for a strong signal.
I don't know their exact range but they do helicopter based searches for skiers buried under snow so a few 100m in air should be possible
Best Answer
The metal is detuning both the tag's antenna and depending on how close the phone is, the phone's RFID antenna too. When a piece of metal is placed in the near field area of an antenna it becomes coupled to the antenna and it's resonance frequency drops, the impedance decreases (causing a large signal loss) and the bandwidth widens (Q decreases). In an RFID tag, these little radios probably have very little transmitter power and a small reference ground plane. Both of these things make detuning the system very hard to have enough signal to communicate.
This is a very common problem in antenna design as many people around the world witnessed with iPhone4 as demonstrated in this video. Anything conductive can do this. So your hand with water in it, detunes your cell phone. Designers have to be aware of these cases and make sure the design is robust enough to overcome some of these conditions.