Magnetic antenna query

Started by daedalus, November 06, 2019, 11:38:52 pm

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daedalus

How does a magnetic loop "know" it is to pick up magnetic waves and not electric , probably something very simple and if so apologise for asking !

gil

I wish I knew...

The fact is that a radio wave is made of both an electric and a magnetic field at 90deg to each-other, since those are generated by the tank circuit, electric field by the capacitor and magnetic field by the inductor. A magnetic loop is said to "favor" the magnetic field... Whatever than means... I do not know how that happens, but it is an interesting question to investigate...

Gil.

daedalus

I posted the same query on an antenna forum and received this answer, now I just have to research to understand it ! It has also thrown a spanner in the works on my homegrown antenna theory (different thread):

"Far from sources, magnetic and electric fields are almost interchangeable: EM waves are made up of E- and H-, but if you know E you also know H. This situation does not apply near EM sources (such as antennas that radiate) or reflectors (such as conductors).

A small loop (by small we always mean the diameter of the loop relative to the wavelength of interest, so a 1 meter loop is very small at 10 kHz but very large for 10GHz) produces only magnetic fields. A small loop is basically an inductor; it has low impedance so charges cannot be separated. Magnetic fields flow around current; a loop is current and hence is circulated by magnetic field. The loop does not allow for charge separation (because it a short circuit); so very little electric fields are produced.

Contrast that with a short dipole antenna. Very little current flows, so very little magnetic fields are produced. The voltage across the dipole though can be large, since it is not shorted out. This means there is charge separation - which is what produces electric fields.

So there aren't magnetic and electric waves, but fields, that certain structures are much more sensitive too."

gil

Not very clear, but it does explain it somewhat. Very little electric field, but no doubt not zero, or there would be no RF produced, as far as I understand it. Interestingly mag loops can have a few thousand volts at the capacitor... That somewhat contradicts the statement.

Gil.

daedalus

I'm finding that as I dig into the fundamental antenna theory that it is very subjective, it seems that most of our modern antenna knowledge is empirical driven, the physics is not consistent.

This just spurs me on to dig deeper and hopefully at some point I will emerge with a clearer understanding.

gil

It is a vast subject... Let us know if you find anything ;-)
Probably a lot of antennas have been discovered by trial and error. The science is there to verify findings, but I don't think it can offer solutions. I believe a lot of informed intuition goes into designing an antenna...
Gil.