Multi-turn Magnetic Loop Antennas

There is suprisingly little desgin information on the multi-turn magnetic loop antenna. I did finally find some info that provides some insights into the performance trade-offs with this type of small loop antenna. One of the authors of an out of print book called “Magnetic Loop Antenna: Slightly Different Each Time” has a website with some key maths and an online calculator.  Here is a simple table comparing my realized 6.5 ft diameter loop for 40m vs the same conductor length in two turns with a diameter of 3.25 ft.

Turns 1 2
Frequency 7.03 7.03 Mhz
Conductor Diameter 7.9756 7.9756 mm
Loop Diameter 198.12 99.06 cm
Power 100 100 W
Spacing between turns n/a 25.4 mm
Wavelength 42.6m 42.6 m
Conductor length 6.2 6.2 m
Radiation resistance 0.0896 0.0224 Ohms
Loss Resistance 0.2 0.2 Ohms
Efficency 34.60% 11.1 %
Total Inductance 7 12.2 uH
Total Reactance 307.8 539.2 Ohms
Tuning Capacitor 73.6 42 pF
Q 1188 2681.7
Max Voltage 6000 12000 V
bandwidth 5.9 2.6 kHz

The effeciency drops by 67%. The capacitance required to tune drops by almost half but the volatge rating needs to nearly double. drops from 5.9kHz to 2.6 kHz. The performance over the large loop sis clearly lower but the rresult is better than a single loop of the same size.

What I need to find now is a model for the case where the turns are in parallel as a comparison.

8 responses to “Multi-turn Magnetic Loop Antennas

  1. John July 31, 2019 at 8:09 am

    I have a three turn magnetic loop, 2m in diameter for use on160m. It is constructed using LDF550 and PVC frame and a vacuum capacitor. The loops are spaced 200mm apart and the loop is fed with a coupling loop at the bottom ( The capacitor is also at the bottom of the loop ). I have operated on 160m for around 40 years and most success was using a 87ft top loaded vertical with a 25 ft capacity hat and 50 1/4 wave ground radials . RX was a beverage antenna. However moving to a small city block many types of antenna were tried but none came close to the three turn loop. As an experienced 160m op It’s performance given it’s size did give me a good workable antenna for both TX and RX.
    VK6IM John, 73

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  2. Bradley October 3, 2019 at 3:01 pm

    Is there any pros and cons to having more loops in the magnetic loop configuration .Just made one out of hardline 1/2 inch with 4 loops in a spiral if one looks at the antenna broadside.
    The inner loop diameter is 1,4 m.
    Was hoping it would do for 80-20 but 40-10 is fine .
    regards
    Bradley
    ZS5BG

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  3. Bradley October 8, 2019 at 2:45 pm

    Two questions: (1) which part of the L/C tuned magnetic antenna radiates ,L or C or the complex compilation of both .A fluorescent bulb will indicate the high voltage around the capacitor .
    Question (2) With a multi-turn loop using coax / hardline in my case , has anyone done extensive testing as to the effect of using the inner part of the coax as part of the antenna , adding it to the loopeither end , secondly leaving open cct , thirdly using this as an extension to multiply the number of turns by two .

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    • rjbono October 10, 2019 at 7:30 pm

      Question #1: antenna radiates from the loop as that is where the currents reside. This element has RLC components. Question #2: I have not seen much difference between connecting the inner coax conductor to the shield vs. leaving the shield unconnected. YMMV

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      • John October 11, 2019 at 8:16 am

        Regarding Question (2) I have a friend that has used it as a pad for extra capacitance required for operation on a lower band. He used this system with great success.

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  4. listener December 22, 2019 at 1:30 pm

    I left a reply, but it was removed. So one more and the last time:
    There is available old repaort titled: “The Impedance and Efficiency of Multiturn Loop Antennas” by T. L. Flaig, and the results presented there are not very optimistic 🙂

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  5. Pingback: MagLoop Antenna Construction Resources – KJ7DJR Amateur Radio

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