# Magnetic fields for communication or remote interaction with devices [closed]

My understanding is that the electric fields are primarily what is used in communication technology and what most antenna design based around. And I am aware of loop antennas that act as an induction coil for the magnetic field component of the electromagnetic wave. And that the and magnetic fields travel together orthogonal to each other.

I am curious about what other technology/systems there are that primarily relies on magnetic fields for communication and/or remotely interacting with devices? And what may be some of the limitations and advantages, in comparison to that of electric fields use for antennas?

## closed as off-topic by John Rennie, Kyle Kanos, ZeroTheHero, user191954, AccidentalFourierTransformSep 22 '18 at 21:26

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• That is a wrong understanding. One cannot radiate only electric or magnetic fields. In near field (non-radiative) communication one may approximately say that it is one or the other, capacitive or inductive but not when an antenna is involved. – hyportnex Aug 24 '18 at 14:49
• I don't think I said only one type of field is radiated, I did mention that I am aware they both travel together orthogonality. But my understanding for antennas is that the electric field is of primary importance, are you saying this wrong? – Dave Aug 24 '18 at 18:17
• You wrote "My understanding is that the electric fields are primarily what is used in communication technology and what most antenna design based around." An electric dipole responds directly to the electric but a magnetic dipole to the magnetic field, they are both used. It can be said that a microwave horn, especially the one with very high suppression of the "wrong" polarization, for example, is designed to both E and H fields. – hyportnex Aug 24 '18 at 18:41
• I see how that sentence can be misunderstood taken by itself, without the last sentence in the paragraph. I meant that when it comes to design of antennas, I thought that engineers primarily make use of the electric field component of the EM wave for design and calculations. Is that incorrect? And the example you give for the microwave horn, you say "It can be said... is designed to both E and H fields." You did not say that it is designed for both E and H fields. Are you saying that the E field is not the primary component of importance in antenna design? – Dave Aug 24 '18 at 21:41

We do have tools to measure the magnetic field so in theory we should be able to use it to transmit data. The reason it is not used over the typical method is that the magnetic field strength is very small compared to the electric field strength. $$B=\frac{E}{c}$$

See hypertonix below or here to see why my reasoning above was wrong.

So the electric field and magnetic field have the same strength. The reason we use the electric field is that it interacts strongly with charged mono-poles (electrons), and there are a bunch of them around. The magnetic field would interact strongly with magnetic charged mono-poles. Though it seems very unlikely that magnetic mono-poles exist and the smallest component is more likely a magnetic dipole which clearly does not interact as strongly with the H-field. So, it's the electric field that dominates the interaction of e-m waves with matter.

• thank you, where does that relationship come from? Is c in this case the speed of light? – Dave Aug 24 '18 at 18:21
• In vacuum $E=\sqrt{\mu_0/\epsilon_0}H$, or $E=cB$. It is not true that because $B=E/c$ the B field is small; actually, the magnetic energy in a propagating plane wave is exactly equal to that of its electric energy: $\frac{1}{2} ED = \frac{1}{2} HB$, equivalently in vacuum $\frac{1}{2}\epsilon_0 E^2 = \frac{1}{2} \mu_0 H^2$. The illusion that B is "small" is the result of the MKS system of units where a normal human size capacitor can have 1pF value. In MKS 1T is an enormous field, no human made radio wave has that kind of flux density but it is not needed; antenna engineers use H. – hyportnex Aug 25 '18 at 12:22
• Thank you for illustrating this point with the units @hyportnex. And for the added explanation for the interaction with regards to charged mono-pole (electrons) vs the magnetic dipoles (@ Finesagan). So, the strength of B field and E fields are equal. However if we want interactions primarily as a result of the magnetic field component, could it be said that more energy is required as it would be that the interaction is with a dipole, for comparing with electric field interactions with charged mono-poles? – Dave Aug 30 '18 at 11:04
• @Dave Remember: The energy carried by the plane wave is equally divided between magnetic and electric energies. It is up to us to design something that is more sensitive to one or the other. A linear wire parallel tot he electric field is more sensitive to the electric energy in the wave, a circular dipole whose plane is perpendicular to the electric field is more sensitive to the magnetic field. Your ear is sensitive to neither but your eyes are to both... It is possible and was done (Bell Labs) to design antennas sensitive to both electric and magnetic fields, so called "energy antenna". – hyportnex Aug 30 '18 at 13:24