Contemplating a big magnetic loop antenna.

  • Would a wide copper strip be better than a tube with the same conducting surface?
  • Where will all the current be due to the skin effect?
  • Would a flat ring be better or a wide ring, when looking at the broad side of antenna?

Ohmic loss is very important for this type of antenna. (Interested, because new design from DL5MCC http://www.dl5mcc.de/ does not need high voltage variable capacitor.)

By "big" i meant at least 2 meter diameter.

  • $\begingroup$ The PDF is in German, and the page you linked to is too large for translate.google.com. That doesn't affect our answering, but it would be interesting and helpful if we could read about this mag loop in English. $\endgroup$ Nov 9, 2020 at 0:34
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    $\begingroup$ @mikewaters dl5mcc.de/mla/downloads/… I think this is it $\endgroup$ Nov 9, 2020 at 3:05
  • $\begingroup$ ohmic losses are only really important if you are transmitting. are you? $\endgroup$ Nov 9, 2020 at 4:22
  • $\begingroup$ A magnetic loop is less than 1/10th wavelength in circumference: electrically "small". In what sense is the antenna you are contemplating "big"? $\endgroup$ Nov 9, 2020 at 4:24
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    $\begingroup$ Hi! Um, geometry of the conductor very much depends on the frequencies you will use this at. So, without you telling us which band this antenna is for, all we can do is throw a bunch of physics at you and tell you to "solve for your frequency". Don't think that would be great! So, please tell us the bands. $\endgroup$ Nov 10, 2020 at 9:33

2 Answers 2

  1. On a closed tube, the current only flows on the outside.

  2. On a strip, the current flows on both sides. It might not be equally distributed, but I don't believe that there is enough difference between the current density between the center and the edges to matter.

  • $\begingroup$ The third question, which I assume is basically a tube with a wide slit, I'll leave for others. $\endgroup$ Nov 9, 2020 at 1:35
  • $\begingroup$ I came to the original question after seeing youtube.com/watch?v=yYavPrYGibU It shows both variants. "Flat" inner element, "wide" outer loop, as seen in video. $\endgroup$
    – frkfan
    Nov 9, 2020 at 12:07

For a theoretical answer to the first two questions, the skin effect is caused by magnetic fields generated by the currents repelling each other, so the current flows as far away from current of the same direction as it can get. The overall result spreads the current out over the entire outer surface, with an exponential weakening of the current as you go deeper in, which is described as skin depth or penetration depth. All of this is frequency dependant and there is corresponding math to describe it. (Conductivity and magnitivity of the alloy also affect this.)

For the third answer, articles I've read from Cebik lead me to believe it generally doesn't matter what orientation flat strips are in as long as it isn't folded or layered or something like that.

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    $\begingroup$ So, for any (wide+thin) flat strip, does "as far away" mean only the very side edges carry most of the current? $\endgroup$
    – hotpaw2
    Nov 9, 2020 at 17:04
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    $\begingroup$ There's skin effect, indeed, but it's not caused by "charges repelling"; if that was the case, it wouldn't be happening here (as the charge is the same as on a conductor without current: at any time, you can pick any piece of conductor. The charge entering that piece of the conductor is the same as leaving it), and it wouldn't be frequency-dependent. $\endgroup$ Nov 10, 2020 at 9:31
  • $\begingroup$ You're right, it's not charge, it's magnetic field. I'll adjust the answer. $\endgroup$
    – user10489
    Nov 11, 2020 at 16:22
  • $\begingroup$ If there are sharp corners, higher voltages can form there. But the problem with the current all moving to the narrow edges is that you'd then have a current concentration which would move it closer not further. So yes, there will be some there, but more will be on the broad flat edges... think of it trying to distribute itself across the entire surface, possibly with more in some areas than others balancing concentration against maximizing distance from other areas with current. There will be some on the interior too -- this is called skin depth. $\endgroup$
    – user10489
    Nov 11, 2020 at 16:28

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