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We are building a 4 ft diameter mag loop using 1" copper tube. The coupling loop will be at the top of the loop (we are using a large vacuum cap in a weatherproof box and it is too heavy to be permanently at the top of the loop in the high winds in Colorado).

Can we run the feedline within the copper tube from the top of the loop, exiting near the bottom of the loop tube? Is this a better solution than simply having the feedline drop across the nearfield?

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    $\begingroup$ I'm confused about why the coupling loop has to be at the top. Do you have a "top" switched with "bottom" somewhere in the explanation? Maybe a picture would help? $\endgroup$ – Phil Frost - W8II Sep 24 '18 at 14:59
  • $\begingroup$ Echoing Phil, a diagram would help. If I understand correctly, you're planning to put the capacitor at the bottom? This doesn't sound like a good idea - the high voltages there, and hence electric fields, will be close to the ground. You want the gap as high up as possible. Also, you definitely can't run the coax through the tube and out at the capacitor gap. Finally, the capacitor motor wires will be a challenge, if not isolated from both terminals of the cap. $\endgroup$ – tomnexus Sep 25 '18 at 4:50
  • $\begingroup$ I have no pictures of the build since it is still in the planning stage. However the concept is simple, place the cap and it's weatherproof box at the bottom to limit wind damage (we have 100mph winds in Colorado). However, I am now considering strengthening the support and placing the cap at the top of the antenna due to the divergent opinions of running the coax through the loop tube. $\endgroup$ – Rons Sep 26 '18 at 13:51
  • $\begingroup$ The motor wires do indeed need to be physically away from the cap. If the cap is moved to the top of the antenna it seems the stepper should be above the cap (outside of the loop aperture). This requires the motor leads to pass down in proxmity to the cap terminals. Any suggestions are welcome. $\endgroup$ – Rons Sep 26 '18 at 13:54
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It is a tough call.

Clearly the inside of the tube carries no RF current. This would suggest that routing the coax in the tube would be a safe approach.

But my concern is that where the coax exits the loop, there will be a strong tendancy for the uniform, high current on the outside of the loop to couple to the exterior of the coaxial shield giving rise to common mode currents on the coax. A secondary issue is due to the reduction of the loop surface area where the coax exits. This will raise the RF impedance in this area causing some reduction in efficiency and possibly localized heating.

The alternative route of letting it drop near the loop also provides the opportunity for coupling of common mode currents but at least there is a minor separation possible in this case.

In either case, a good common mode choke on the coax is warranted. You may wish to experiment with the two techniques. An RF clamp-on ammeter would help to remove ambiguity/subjectivity from the experiment..

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  • $\begingroup$ I thought that I read about folks running coax inside the loop. The article may be on a link on w0btu.com/magnetic_loops.html. The 404 bulldogtrust link has been changed, and bulldogtrust.com is the correct one. In any case, looking at the various STLs there may give the OP some ideas. Note the photo there with the tuning motor at the bottom. $\endgroup$ – Mike Waters♦ Sep 23 '18 at 22:41
  • $\begingroup$ I may be thinking about the motor leads that were running inside it, and not the coax. That has been done, and you can even run motor and limit switch leads inside it. See the article by Bill Jones KD7S is in the November 1994 QST (partially reproduced in my ARRL Antenna Handbook, 19th edition, Section 5). $\endgroup$ – Mike Waters♦ Sep 23 '18 at 23:08
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My apologies for my tardiness in updating the bulldogtrust site.

I am too skeptical to try coax inside one side of the loop tubing, due to high voltage and asymmetrical capacitance it would introduce at the exit point.

My simple minded solution, discovered first on a Russian website, uses capacitive feed at the loop's tuning capacitor. Each side of the tuning capacitor is coupled to each side of the coax through 10-50 pF of capacitance (depending on band and loop size) per side to match 50 ohm coax. With a kilowatt there is 6 or 8 kV difference between loop and coax there, so high voltage high Q capacitors are necessary. A practical solution is a series string of 3 or 4 small inexpensive USSR surplus doorknobs on each side of the tuning capacitor. A a series string of 3-5 39, 47 or 100pF at 3 kV from Ebay will provide trial and error tapping choices for getting a match. (I use a 15 kV vacuum tuning capacitor to handle my kW amp)

Hung from a tree branch, the capacitor at the bottom of the loop should have sufficient distance to ground to be both safe and efficient. (1 diameter is usually sufficient)

Incidentally, a reasonably efficient loop will be too sharp in tuning to use a DC motor. Stepper motors (NEMA 17 is sufficient) and controllers (T-500) from pololu.com with a generic quadrature rotary encoder is one way to go.

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