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At the bottom of webpages in the advertisement swamps I keep seeing this image of an antenna, accompanied by an offer that I won't be clicking on.

But the antenna itself looks quite intriguing. It's a circular, horizontal loop of copper with the ground braid of a coaxial cable connected at one point. The center conductor connects to what looks like tap that runs a short distance around the loop, connecting to it just shy of 90 degrees away from where the braid connects to it.

Assuming that this is a legitimate antenna design (and it might not be):

  1. What kind of loop antenna is it? What is it called?
  2. What is the relationship between the diameter of the loop and the wavelength at which it is used?

enter image description here

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    $\begingroup$ I'd originally asked this in electronics and posted in the Ham Shack, but it was suggested that this might be a better fit here. $\endgroup$ – uhoh Nov 12 '19 at 0:30
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It is a 2-meter halo antenna as built by Mike Fedler N6TWW: the image appears to have been lifted from his page describing the build. It's 40" in circumference, and the boom length (slightly more than the diameter) is 14.5" long. It's tuned for 144.25 MHz. The name "halo" comes from a combination of the antenna's appearance and an abbreviation for half wave loop.

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    $\begingroup$ Excellent. One end of it is split, though you cannot see that in the photos there (only in the text). There is a 1.5" gap opposite the ground point. $\endgroup$ – Mike Waters Nov 12 '19 at 20:00
  • $\begingroup$ Wow, thank you for tracking down the origin of the photo and adding the specifics! $\endgroup$ – uhoh Nov 12 '19 at 21:37
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That is a horizontally polarized omnidirectional halo antenna.

The relationship between the diameter and length (λ/2) is simply diameter÷π.

Some halos have a series capacitor to the right (where the picture is cut off). Others are open. Pictures from Google search.

They were in common use on automobiles back when 6 and 2 meter AM was popular in the 50s and 60s. However, they are not that common now, since FM is now vertically polarized.

Having said that, they would be an excellent choice in areas where VHF SSB is used, since that is mostly horizontal polarization.

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    $\begingroup$ Thanks for the speedy answer! Any thoughts on the second part; "the relationship between the diameter of the loop and the wavelength at which it is used?" I've looked at en.wikipedia.org/wiki/Halo_antenna#Size_is_relative but that passage isn't very clear to me. $\endgroup$ – uhoh Nov 12 '19 at 1:11
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    $\begingroup$ That might not be a halo. Halo has a break opposite the feedpoint. This is certainly a loop of some sort and the extra cooper piece in the middle is just a gamma match. $\endgroup$ – Chris K8NVH Nov 12 '19 at 12:07
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    $\begingroup$ @uhoh a halo has a circumference of half a wavelength, so a diameter of L/2pi (half wave loop) -- if this is a halo $\endgroup$ – hobbs - KC2G Nov 12 '19 at 19:06
  • $\begingroup$ @hobbs-KC2G Thanks, but your comment might reach more people if it was posted as an answer. Comments cannot be found in a search. $\endgroup$ – Mike Waters Nov 12 '19 at 19:40
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    $\begingroup$ @ChrisK8NVH It certainly looks that way! However, the link in this answer indicates that there is a break opposite the feedpoint. $\endgroup$ – Mike Waters Nov 12 '19 at 20:11

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