Magnetic Loop Antenna

For approximately two years I used a magnetic loop antenna with a diameter of 80 cm which works from 40 to 10 meters (7 to 30 MHz).

My magnetic loop antenna (ML-80)
This is my magnetic loop antenna ML-80 from Hilcock Projects lying on my balcony with open box with the motorised variable capacitor and w/o the motor control cable

Magnetic loop antennas radiate and receive the magnetic component of the electro-magnetic (radio) waves, and not the electrical component as other antennas. Usually they are much smaller than other antennas. That's why I choose this antenna type.

Other advantages of magnetic loop antennas:
- They are insensitive against electrical noise caused by other equipment (electrical motors, fluorescent lamps etc.) because this usually has only a low magnetic component.
- If they are equipped with a variable capacitor (as most of them are) they do not only work on one band or harmonic bands of the resonance. They also work on all bands between their minimum and maximum resonance frequency. That means they can also be used e.g. for frequencies outside the amateur bands, e.g. for broadcast listening.

I installed it as a stealth antenna among some trees in the garden. At the beginning I was a little sceptical about the performance. The efficency of magnetic loops decreases with the diameter / wavelength ratio. For 20 meters and above it does a good job but on 40 and 30 meters? With only 5 Watts output and a 25 m long cable between transceiver and antenna?
But I was pleasantly surprised! Okay it is really no DX antenna, but inside Europe it makes a great job. It is not always easy to get reply from CQ calling stations but all the more is feeling of success if they do!

This table gives an overview about the gain (or loss) on the different bands:

Band 40 m 30 m 20 m 17 m 15 m 12 m 10 m
Freq. 7 MHz 10 MHz 14 MHz 18 MHz 20 MHz 24 MHz 28 MHz
Gain [dBi] -4,9 -1,3 0,6 1,2 1,4 1,6 1,7
Gain [dBd] -2,7 0,9 2,8 3,4 3,6 3,8 3,9

(gain values taken from [external link] DM2BLE's homepage [in German language] for a comparable [external link] magnetic loop of 80 cm diameter [in German language] "0,8m AMA")

principle of magnetic loop antenna

Electrically a magnetic loop antenna is a parallel resonant circuit consisting of a capacitor (C) and an inductivity (L). In order to achieve resonance on different frequencies a variable capacitor is used. But how to change the capacity if the antenna (and the variable capacitor) is located on the roof? For this reason a lot of magnetic loop antennas (like mine) the capacitor is equipped with a motor which can be controlled remotely.

remote control of the magnetic loop antenna
Remote control of the magnetic loop antenna.
With the buttons "low" and "high" you can tune the resonance frequency. The tuning speed is variable so that you can easily switch between the different bands in "fast" position and fine tune in "slow" position.

In practical use you turn the capacitor and hear for maximal noise/signals on the choosen frequency. After you found the approximate resonance for fine tuning a CW signal is transmitted while monitoring the SWR (standing wave ratio) until it reaches the minimum. For that an SWR meter should always be looped in the antenna cable.

As you can imagine the band width of the resonant circuit of the magnetic loop antenna is not very wide. Thus sometimes a tuning is also necessary when the frequency is changed within one band.

Band 40 m 30 m 20 m 17 m 15 m 12 m 10 m
Freq. 7 MHz 10 MHz 14 MHz 18 MHz 20 MHz 24 MHz 28 MHz
Bandwidth [kHz]* 20 35 60 110 115 150 200

*) SWR ≤ 2:1

My magnetic loop is fed by a second smaller loop, the so called feeding loop. There are some other ways of how to connect a magnetic loop to the coaxial cable from the radio but this one is electrically and mechanically simple to build.
You can see both loops as the two coupled inductivities of a transformer. Some practical experience shows that the feeding loop shall have 1/5 diameter of the main loop.

feeding loop