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Mag Loop and Spiral Loop

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by VK3ZSC, Sep 17, 2021.

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  1. VK3ZSC

    VK3ZSC Premium Subscriber QRZ Page

  2. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Yes, there are significant differences.

    -The first antenna is an STL, Small Transmitting Loop, intended for...."Transmitting". Construction materials, methods and connections are critical due to the high voltages developed.

    -The second antennas is incorrectly called a "Spiral Loop". It is historically called a Frame Loop intended for receiving. Voltages are insignificant, and materials and construction are non-critical. This is relatively easy to build, and excellent for MW and Short Wave listening.
    PU2OZT and AK5B like this.
  3. VK3ZSC

    VK3ZSC Premium Subscriber QRZ Page

    I don't disagree with your comments but the reference article and several others address the high voltage issue and say it is suitable for transmitting. So with that in mind I re-ask my original question.
    "Is there any difference in characteristics between a magnetic loop antenna and a spiral loop antenna?"
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The voltage (and RF current) issues remain, although a spiral loop has more natural resistance, which has its own implications.

    I've never seen a spiral loop used for high transmitting power, but guess it's possible with a very substantial tuning capacitor (maybe a vacuum cap).
    K0UO likes this.
  5. VK3ZSC

    VK3ZSC Premium Subscriber QRZ Page

    Thanks for your reply. I'm mainly interested in characteristics like radiation pattern, impedance, gain, bandwidth, etc.
  6. K5ABB

    K5ABB Ham Member QRZ Page

    If you are serious about transmitting, the Small Transmitting Loop, commonly referred to as a Mag-Loop is where you need to go. Leigh Turner, VK5LT , an engineer and co-patenter of the modern STL has written the ultimate guide and explanation of this antenna for ham usage. It should answer all of your questions.

    Read here:
    PU2OZT, AK5B and WA4SIX like this.
  7. K4PP

    K4PP Ham Member QRZ Page

    The creator of the modern STL was Ted Hart, w5QJR. Mostly because he finally listed and defined all the equations used. He also did a bunch of testing to determine the distributed capacitance parameter and loop size vs. frequency limits of performing as an STL. QST June '86.

    Many hams thought the STL was junk until then. Some still do.

    You can also see original patent by Dunlavy

    Last edited: Sep 18, 2021
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  8. W9IQ

    W9IQ Ham Member QRZ Page

    I think it would be better to say he applied it to amateur radio use. Kraus had the antenna well defined in his 1950 book and I am sure there is earlier materials.

    What is often overlooked in amateur circles is the effect of more than one turn. It can significantly improve the efficiency. Kraus also documented the formulas for this effect.

    - Glenn W9IQ
    K4PP likes this.
  9. 5B8BA

    5B8BA Ham Member QRZ Page

    One notable difference is this comment from the link cited by Graham, VK3ZSC

    Important: ideally, like dipoles, this type of antenna should be installed at least 0.1 λ above the ground.

    Most STA are positioned significantly lower than the recommended height for the spiral (in one plane) antenna.

    See KK5JY's Small Transmitting Loop Project site at

    Paul 5b8ba
    Last edited: Sep 18, 2021
  10. EI7KS

    EI7KS Ham Member QRZ Page

    I intend to make a magloop by spiral winding 2 layers of thin 50 mm wide copper tape around a loop of black polyethylene 38 mm OD water pipe ( overall diameter 1.2 metres) The distance between the windings will be approx 10- 15 millimetres.
    The resultant will be a loop inductance larger than the one of a solid copper loop of the same diameter . I then can experiment with RF energy transfer via the usual (approx 1/5 dia) inner loop and a so-called Alex loop arrangement.
    The intended use is for low RF power.
    The rather fragile very thin copper tape will be sealed with varnish for outdoor applications.
    Using a transceiver with 2 antenna inputs (manually switchable ) I then can use the magloop for RX only if necessary to reduce local E-noise ,using the doublet for TX or the other way around or use the magloop for RX and TX.

    Frank , EI7KS

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