Magnetic loops

I am not going to explain the theory of this antenna since it is very well described in literature and on many web sites (see my links page) but, more practically, I would like to give here an overview of the successful designs I have tested and some useful tricks.

However, just as a short introduction, I would like to remind the key characteristics about these antennas since they are quite different to other more common antennas such as dipoles, verticals, etc and because knowing these differences can help in using them better:

Based on these characteristics, one needs to understand that, under specific circumstances, they can achieve a better efficiency as compared to a classic dipole or a vertical antenna, for example when it is not possible to implement a good grounding system and/or when the space to put a full size antenna is not available. An interesting comparison for example can be: how a magnetic loop can compare with a mobile shortened antenna on a balcony ? To be honest, the answer is not simple, we need to think about bandwidth (frequency can be changed in case of a remote loop), efficiency, available ground system in case of the vertical wipe, etc…and you can reach different conclusions based on different operator wishes, bands and locations.

There are various situations in which these antennas can be tried and used so I think helpful to indicate first that my interest in these antennas have mainly been to achieve a multiband system that can be used in an apartment either indoor or on the window or balcony, not a fully remote system since it keeps being accessible by hand from time to time, for example to change from one band to another: this can explain the sizes and technical choices I have made.

Primary loop

Nothing special to say about the loop itself except that its diameter determines the usable frequency range and the diameter of the tube section used to build the loop h as major impact on its radiating efficiency. Note also that the loop can be made with a copper or aluminium ribbon instead of a tube (in that case, you can replace the perimeter by two times the width of the ribbon in the formulas); in fact, the shape of the conductor does not matter much, what is critical is to provide the biggest apparent diameter or exposed surface for this conductor. Whatever you can use, try to go to the biggest diameter (or width in case of a ribbon) you can achieve knowing that there are obvious limitations in term of weight, mechanical resistance and cost !

Feeding the main loop

There are many ways to feed a magnetic loop (gamma match, tuning variable capacitor, secondary loop and possibly others).

Some loop examples

80 cm diameter loop for 20 to 10 meter bands

This loop is an octogone in copper tubing of 22 mm diameter equipped with a split stator variable capacitor.

loop 80 cm

loop 80 cm CV

160 cm diameter loop for 40 to 20 meter bands

This loop is made from RG-8 coaxial and is tuned with one CV for band switching and a second CV for band tuning. I deliberately took advantage of the softness of the coaxial in the lower part to twist the loop to move it out of the window into its operating position, something that would not have been possible with a rigid loop. Furthermore, the two arms in V position can be folded on the boom and the loop can be folded for storage. Not very nice but very handy !

loop 160 cm

loop 160 cm CV

Both loops are remotely tuned with DC motors and appropriate reduction gear.

The two loops have mostly been used either in indoor or attached in a vertical position just as on the pictures on an apartment window, hanging outside, for the curriosity of my neighborgs !

Various tricks

servo CV

Some calculations

Here is below a link to an Excel spreadsheet that I have developed based on classic formulas published in litterature. I designed this very simple spreadsheet to generate performance parameters of a given design on multiple bands at one time, which helps to accelerate quite a bit the various modelling and testing. The only information you need to enter are the diameter of the main loop and the diameter of the tube you plan to use for the main loop. The spreadsheet calculates the main performance parameters for the most common Ham bands but you can change the frequencies as needed. It also gives you the best size of the secondary loop and the upper usable frequency (based on the inductance and capacity of the main loop).


F5RDS magnetic loop spreadsheet

Using this spreadsheet, I have computed and summarized in the table below some design efficiencies just to give an idea of comparative performance between them.

main loop diam. (cm) conductor diam. (mm) material Radiating power efficiency (%) / Loss relative to a loss-less antenna (dB)
10 m 15 m 20 m 30 m 40 m
80 10 RG-8 85% / -0.7 dB 67% / -1.8 dB 33% / -4.8 dB 13% / -8.8 dB 4.1% / -14 dB
20 copper tube 20 mm 92% / -0.36 dB 80% / -0.96 dB 49% / -3.0 dB 23% / -6.4 dB 8 % / -11 dB
30 copper tube 30 mm 90% / -0.44 dB 86% / -0.66 dB 59% / -2.3 dB 31% / -5.1 dB 11% / -9.4 dB
100 10 RG-8 92% / -0.37 dB 80% / -0.98 dB 49% / -3.1 dB 23% / -6.4 dB 7.7% / -11 dB
20 copper tube 20 mm 96% / -0.19 dB 89% / -0.52 dB 66% / -1.8 dB 37% / -4.3 dB 14% / -8.4 dB
30 copper tube 30 mm 95% / -0.13 dB 92% / -0.35 dB 74% / -1.3 dB 47% / -3.3 dB 20% / -7.0 dB
120 10 RG-8 can't work - max 26 MHz 87% / -0.59 dB 62% / -2.1 dB 34% / -4.7 dB 13% / -9.0 dB
20 copper tube 20 mm can't work - max 28 MHz 93% / -0.31 dB 77% / -1.2 dB 51% / -3.0 dB 23% / 6-.5 dB
30 copper tube 30 mm LIMIT 29.7 MHz 95% / -0.21 dB 83% / -0.8 dB 61% / -2.2 dB 30% / -5.2 dB
160 10 RG-8 can't work - max 19 MHz can't work - max 19 MHz 80% / -0.99 dB 55% / -2.6 dB 26% / -5.9 dB
20 copper tube 20 mm can't work - max 20.3 MHz can't work - max 20.3 MHz 89% / -0.52 dB 71% / -1.5 dB 41% / -3.9 dB
30 copper tube 30 mm can't work - max 21.4 MHz can't work - max 21.4 MHz 92% / -0.38 dB 78% / -1.1 dB 51% / -2.9 dB


As a conclusion, I would like to say that these loop antennas have allowed me to be active on some bands in locations where the other types of antenna I tried failed. As always in real life, things are never completely black or white, but with a wide variety of gray nuances, so these magnetic loops are not the only possible answer to all situations and issues, otherwise everyone would be using them today ! If you are enough lucky to live in an area where you can put large, full size antennas, you don’t really need them; why would you bother with such narrow bandwidth for example ? But if you are facing antenna restrictions or lack of space on your roof, if you want to put your antenna on your balcony, or if you plan some portable operations without deployed long wires and carrying long masts, or even for the fun of experimenting new things, I encourage you to give it a try, you will be surprised to see that, yes, they work !!!

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