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.

w4lix

For Gil. I watched your video on operation of your mag loop with the toroid feed configuration. i noticed the problem you had with matching.

i wanted to pass to you, that i use a coupling coil one fifth the diameter of the large coil
and have not had a matching problem on my operating freqs.

my present loop has a 5ft diameter made from 19 ft 3/8 od, flexable copper tubing.
so using pi, we figure
19x3.14=59.69 inches(diameter)

59.69 x .2 (1/5th) = 11.93 inch diameter (coupling loop diameter)

11.63 x 3.14 = 37.50 (length of tubing, coax needed for coupling loop)

in a nut shell, you can figure the diameter of loop to use in space you have, then you can divide your diameter by 3.14 and get length of material needed to complete loop.

my config: using two 3.5 KV air variables. both in the 40-400pfds. i have them in parallel
which allows me to use 75m to 40m bands. i run 100watts to the loop. but the efficiency is 2%
on 75-80m band, about 25% on 40m. as freqs increase, efficiency can reach 90% on 20 meters.

this info from loop design software.
i am still evaulating the loop to verticals and dipoles. off the top, i am leaning toward those antennas, then the propagation favors me, and i am suprised at my sig reports.
on a 3ft loop on 40 meters, got a report of 20db over 9! wow.

software i am using is
small magnetic loop antenna calculator ver 1.22a
by steve yates  AA5TB
aa5tb@yahoo.com



gil

Hello. I have been told the same before. I might just make a tapped toroid feed... Not simpler by any means, but since I already started that way...

Gil.