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I am reading a paper by Park and Mercier called Magnetic Human Body Communication (2015). They are looking into human body communication techniques and compare magnetic human body communication (mHBC) to capacitive coupling methods (eHBC.)

The way they apply their technique is different. In mHBC they place a coil around a part of the body as transmitter and the same is done for the receiver. A magnetic coupling takes place as the tranmitter changes electric signals through the transmitter coil.

In eHBC, they place a node of the transmitter on the body and the other node of the transmitter is a capacitive ground. The same is done for the receiver.

Now my main question is that, a changing magnetic field is always paired with an electric field, right? So in a sense, capacitive coupling is also an electric field (changing,) so how is that any different apart from the way they apply the devices. I would assume, in both cases, both a magnetic field and an electric field are generated since they come in pairs.

DOI: 10.1109/EMBC.2015.7318739

JRE
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Mart
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  • The way they apply the devices is answered in your own question; a coil for mag comms and plates for electric field comms. Other than that I'm unsure what your question is. It doesn't appear to have anything strictly related to the human body so maybe this is confusing you? – Andy aka Jan 02 '23 at 12:34
  • So the question is, apart from the method of applicating the system, are the methods of transferring data not exactly the same since magnetic fields are always paired with electric fields? I'm trying to formulate it correctly but having a hard time with that, sorry. – Mart Jan 02 '23 at 12:49
  • I don't understand the difference between *the method of applicating the system* **and** *the methods of transferring data* <-- I don't know what you are trying to say. – Andy aka Jan 02 '23 at 12:54
  • so, applying the system in eHBC is with an electrode, but with mHBC is with a coil. Fine. However, transferring in one is through capacity and the other through a changing magnetic field. How does a changing magnetic field behave different from a changing electric field in the body since both are paired? And if it is not different, How is eHBC different to mHBC apart from the coil and electrode. – Mart Jan 02 '23 at 13:10
  • The article (that you read) I'm sure will explain that magnetic field comms is much more resilient to human tissue attenuation than compared with electric fields. – Andy aka Jan 02 '23 at 13:16
  • But, if they are always paired, how can the one (magnetic field) not be attenuated that much by the body and the other (electric field) can be attenuated more? That would suggest that magnetic and electric field are independent? Is that true? – Mart Jan 02 '23 at 14:05
  • They are not an electromagnetic wave if that's what you mean. They are somewhat related but, their ratio can be massively adjusted by the "antenna". – Andy aka Jan 02 '23 at 15:29
  • ah, I think that is what i meant, but then they are not I assume? What are they then exactly? – Mart Jan 02 '23 at 15:34

1 Answers1

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Now my main question is that, a changing magnetic field is always paired with an electric field, right?

Yes, but what are their magnitudes?

In the far field from an antenna, they always have a particular ratio, the impedance of free space, which IIRC is about 377 ohms.

In the near field, which both eHBC and mHBC are, the ratio is heavily dependent on the type of antenna.

By choosing a loop / magnetic antenna, you get a strong magnetic field and negligible electric field, which will couple well to loops of conductors in the body.

By choosing a dipole antenna, you get a strong electric field and negligible magnetic, which will couple well to lines of conductor that intersect the skin at both ends.

Having chosen what structures in the body you want to couple into, the choice of antenna/applicator is made for you.

Neil_UK
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  • So, what I understand is that, although an changing electric field will always be paired with a changing magnetic field, they can vary in strength independent of the strength of the other? therefor, the electric field, for example can be attentuated more (or less) than the magnetic field depending on the medium? And in this case, the body is a better coupler for magnetic comm than for capacitive (electric field)? – Mart Jan 02 '23 at 14:13
  • Right, at low frequencies and short distances (really saying the same thing, related by speed of light), E and H fields are less tightly coupled and can take on ratios very different from Zo (377R) (or in general, whatever it is in the local medium, not just free space; it's very low in flesh). This will be the case for frequencies under, say, 100MHz, given typical human dimensions. Around that frequency or higher, wave effects will be harder to ignore / near-field will be impractically short range / wave effects can be harnessed instead of ignored. – Tim Williams Jan 02 '23 at 18:48