Magnetic Loop Question

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W8NSI, Nov 22, 2016.

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  1. W8NSI

    W8NSI Ham Member QRZ Page

    I have looked at other MagLoop questions and dont find an answer so I thought I would toss it out to the magloop gurus.

    What would be the effects to feed the mag loop at or next to the tuning capacitor instead at the opposite side?
    Normally the side where the feed loop is located will be a low impedance low voltage point. The other side where the capacitor is located is a HIGH voltage point. If it is fed at the capacitor tuning point then that becomes the low voltage point and a simple link coupling should be possible to be used. A more closely spaced tuning cap should also be able to be used. Have I reasoned this out properly?
    Point A is where the capacitor usually goes. Instead I place it at point B along with a small number of turns of #12 wire. A coupling loop attached to the coax is inserted into the opther loop. The capacitor should be able to adjust for lowest swr.


    73 de w8nsi jim
  2. N5CEY

    N5CEY Ham Member QRZ Page

    That point is the highest voltage point of the loop. Even at qrp levels the voltages can exceed 1kv
    It's also the highest impedance point versus the lowest at the opposite side. Would be very hard to match.
  3. W8NSI

    W8NSI Ham Member QRZ Page

    What point do you mean. On my picture I have labeled two points A and B.
  4. W8NSI

    W8NSI Ham Member QRZ Page

    OK lets get to some fundamentals of the loop as it is normally pictured. A very small loop, smaller than even a fraction of a wavelength is bent into a loop with an open section. A large variable capacitor is placed across the open ends. A coupling loop is placed at the opposite side at the physical middle of the loop. A delta match or a gamma type feed can also be used. The shield side of the coax can even be directly attached to the middle of the loop. The capacitor is usually adjusted for max noise on the receiver to get into the neighborhood of a sharp resonant peak. The open ends with the capacitor are a very high voltage point on the normal configuration loop.

    Now we shift the open ends with the capacitor around to the link coupled feed point, as in my drawing. The High voltage point should still be on the opposite side of the loop (at point A on my drawing). I guess I will have to build this to see what happens.
    Last edited: Nov 22, 2016
  5. WA4SIX

    WA4SIX Ham Member QRZ Page

    1. Adding a coil at the feedpoint increases ohmic losses.
    2. I do remember someone in my extensive studies on STLs that tried a feed near the capacitor that never got it to work well. He tried ferrite, wrap, gama.
    3. The only way I have seen to have the capacitor & feedpoint on the same spot, is to have a double/spiral loop.
    4. The reason for having the feed opposite of the capacitor is to have it at a current node, instead of a voltage node.
    Is there a specific reason for trying this? If so, let us know & see what can be worked out.

    AK5B likes this.
  6. VK3YE

    VK3YE Ham Member QRZ Page

  7. W8NSI

    W8NSI Ham Member QRZ Page

  8. W8NSI

    W8NSI Ham Member QRZ Page

    Here is some info I found after having the initial page from VK3YE.

    It looks like it will take a 1.5 to 1 turn ratio to match 50 ohms when the coli is placed next to the capacitor. Of course the loop in this article is a quarter wave loop, not a very small mag loop as was initially discussed. in this loop the feed point impedance according to the author is in the neighborhood of 22 ohms.
  9. AK5B

    AK5B Ham Member QRZ Page

    Just the first reason that Ed gave a few days ago would be enough for me to completely avoid the feeding/coupling method you propose.

    Ohmic losses are the bane of STLs; the idea when constructing one is to use the best construction practices possible so that these performance damaging losses are minimized. The only exception would be if you were building one as a SWL or only to use for listening (then it is not nearly so critical to performance).

    Having said that, one of the great things about STLs is that they lend themselves well to all sorts of variations and experimentation. Perhaps if you are prepared to make changes as you proceed you will eventually build something that totally strikes your fancy and achieves your goals.

    I'm also curious as why you wish to feed your loop as you propose.

    73, Jeff
  10. W8NSI

    W8NSI Ham Member QRZ Page

    Can you or Ed compute the ohmic loss of 2 turns of #10 wire around a toroid? One side is silver soldered to the loop and the other to the clamp on the capacitor. The coupling loop hooked to the coax is also wrapped around the toroid but has no bearing on the copper losses you are worried about. I do not see ohmic losses of a 4 or 5 inches maximum amount of wire as an insurmountable problem.

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