Magnetic Loop Feed Question

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KK4NSF, Dec 18, 2015.

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  1. KK4NSF

    KK4NSF Ham Member QRZ Page

    OK... In a typical magnetic loop, the smaller feed loop is opposite from the variable capacitor. For instance, on my antenna the variable cap is at the top, and the feed loop is at the bottom. Is this 180 degree separation important, OR could the feed loop be positioned in the center of the main loop?

    Now.... we all know about Magnetic Loops and there are a lot of opinions about whether they are efficient or not. That is not what this discussion is intended to be. Nor is it a question about Gama matches, or transformer matches.... or any form of coupling other than using a smaller loop. Those issues have been discussed to death, and I've got mountains of information already. My question is simple: can the feed loop be centered in the main loop instead of at 180 degrees to the variable capacitor and the antenna still function properly?

  2. N3OX

    N3OX Ham Member QRZ Page

    You can put it in the center and get a match. It might have to be a different size.
  3. AK5B

    AK5B Ham Member QRZ Page

    Yes, but it might not be optimum. My 10-20M loop that I built this summer works best on 20m when the coupling loop is near the center---but on 17-10M it is best when the coupling loop is slid down the support and placed an inch or so above the loop bottom.

    No other change is necessary for me to achieve a nearly perfect SWR other than feed loop placement.

    73 and have fun with your STL; they can really amaze you when optimized by proper construction and placement.

  4. N3OX

    N3OX Ham Member QRZ Page

    Good point. I do recall that to get best SWR across the widest range on my loop I had to do the same. I even squashed it a bit to run along the bottom conductor:

    Mine did at least 40m up through juuuuust the top of 12m.
  5. KL7AJ

    KL7AJ Ham Member QRZ Page

    I think it's primarily for balance. You could have it on the SAME side, but that would make for a crowded situation, I believe.
  6. KK4NSF

    KK4NSF Ham Member QRZ Page

    Interesting. looking at the photo, I see what you mean. What that tells me is that your feed loop is a little small in relation to your main loop. What is the ratio of feed loop to main loop?

    When placed opposite the tuning cap, a ratio of 5:1 works well. If i move it to the center, I wonder if a larger feed loop (maybe 4:1) would be required?
  7. KL7AJ

    KL7AJ Ham Member QRZ Page

    That's a really interesting question. Intuition tells me the tightest coupling would be with the small loop dead center...But I'll have to build a prototype and see!
  8. KK4NSF

    KK4NSF Ham Member QRZ Page

    The question is indeed a good one, and since I couldn't work out a solution on paper..... I did the obvious thing: I conducted an experiment. Using my "indoor" loop, I moved the feed-loop to the center point and tested it. ........ drum roll, please........

    It doesn't work. While it still seems to resonate on recieve, it won't tune up to a workable SWR.

    Thinking that I might need a bigger feed loop I made one 25% bigger, and it still won't tune. SO.... my conclusion is this: Yes, the feed loop position does indeed make a difference. If you use the standard 5:1 ratio for main loop / feed loop, the feed loop needs to be near the main loop.... as is commonly used, and not in the center.

    So there you have it. A result. That's SCIENCE, man!
  9. AK5B

    AK5B Ham Member QRZ Page

    That may be true most of the time---but doesn't explain why my feed loop works best near the center on one band but not the four others---and it is 20% the size of the main loop.

    Like Abraham Lincoln, A9ABE (SK) once said, "You can achieve a flat SWR on some of the bands some of the time but not all of the bands all of the time... [assuming the feed is in a constant, fixed position]."
  10. KK4NSF

    KK4NSF Ham Member QRZ Page

    I'm not sure why that is. To figure it out, we'll have to make some adjustable size feed loops, and move 'em around a bit to see what's going on. I may work on that after the Holidays.

    I'm not sure I agree with old Abe. While it may be true for wires, it's not quite right concerning tunable loops. This latest loop of mine achieves a flat SWR from 6mhz up to 18mhz with no problem.

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