Magnetic Loop Aerial for 20m

Introduction

There are some excellent magnetic loop related sites on the web, many of which explain the theory behind loop construction better than I could hope to. Experimentation also plays a large role in the development of loop aerials. This page is therefore not intended to provide a "how to" guide - more a "here's what I tried" account.

Construction of this aerial in ongoing as of January 2009 - further developments and photography will be posted as construction continues.

Magnetic loop aerials have become popular in amateur radio for various reasons: Size - Magnetic loops tend to be smaller than other aerials constructed for the same wavelength. Narrow Bandwidth - the high Q of the magnetic loop can be helpful in rejecting interference from adjacent frequencies.

Materials

The loop conductor is fashioned from LDF 5-50 Heliax cable, using only the outer conductor. Support for the loop is provided by a cross structure, fashioned from plastic drainpipe. The loop is brought to resonance using an EEV vacuum variable capacitor, rated at 1000pF, 3KV, 40A - bought from the lovely chaps at Colormor Valves, Tubes, Components.

Measurements

λ - Wavelength, in metres.
f - Frequency, in Megahertz.
c - Speed of Light, in thousands of kilometres per second.
l - Loop circumference, in metres.
d - Loop diameter, in metres.

It possible to find various theories for calculation of magnetic loop length. Opinion seems to vary somewhere between one quarter and one eighth wavelength. I will be using the ¼λ design.

I decided to cut my loop for 14.250MHz - approximately central in the "phone" section of the 20m band.

So, the our loop circumference must be 5.26m, yielding a diameter of 1.67m.

Construction

Support Structure
For the vertical support, I bought a standard two-metre plastic downpipe from a local home/garden store. The horizontal arm is a two-metre stretch of 40mm plastic pipe. A 40mm hole was drilled through the vertical section, to allow the smaller-bore pipe to be inserted.
Affixing the Loop
A second hole (this time 28mm in diameter) was drilled in the vertical pipe, parallel to the horizontal support and a loop-radius below the crosspiece junction. The LDF 5-50 loop is thread through this hole to anchor the loop at its lowest point. With the loop now anchored at its lowest point, the two open ends were anchored at the top of the vertical pipe using plastic cable ties. (Any ideas for a better way of doing this?) Cable ties were also used to affix the two sides of the loop to the horizontal support.
Tuning Capacitor
Much consideration was given to fixing the tuning capacitor to the loop. Originally, six inches of the LDF 5-50 plastic outer was removed from each end of the loop and the bared copper shield was flattened and affixed to the capactor using jubilee clips. Although this clearly worked (we managed to contact W3A) it left sharp, jagged edges. Richard, G1SLE, came up with a better method. Using the silver-coated twin braid from some RG-214 left over from the Faraday loop, he "lashed" the bare ends of the LDF 5-50 and used the jubilee clips to affix the braid to the capacitor. This not only looked better but removed the sharp edges and lowered SWR.
Feeding the Loop
Some trawling through the multitude of magnetic loop-related sites on the web revealed two profilific methods of feeding the loop. Gamma Match and Faraday Loop. My aerial is fed using the Faraday Loop method as I've read several reports that it provides better protection from interference than the Gamma Match. The Faraday Loop is again constructed from coaxial cable. For this I used 50Ω UR-67 coax.
Tuning up
Due to the inherent narrow bandwidth of magnetic loop aerials, accurate tuning is essential. The transceiver is tuned to the desired frequency and the variable capacitor is adjusted slowly. As the loop approaches resonance, received audio levels increase dramatically. At this point, finer adjustment to the capacitor is performed to achieve minimum SWR. Acceptable levels of SWR are a subject of debate at my local radio club but I was able to reduce reflected power to a level undetectable by my meter. After some operation with the loop, I'm of the opinion that the variable capacitor has too large a range for use on 20m. Real care has to be taken during tuning as it's difficult to make fine enough adjustments. I hope that the addition of a stepper-motor control circuit will allow for finer control, although even a 0.9° step may be too much!

Log

Loop-Related Sites

W2BRI Magnetic Loops - The first loop site I came across. Brian has constructed a number of loop aerials for 20m, 40m & 80m and provides an interesting blend of theory and technique. His FAQ page is a great place to start if you're new to the subject.

PA3CNJ - An interesting site with excellent photography and information on constructing both Gamma Match and Faraday Loop feeds.

PAØRWE - Magnetic Loops with emphasis on the ABXKOPPEL feed system. I haven't tried this yet but it's on my list of things to do.

More to follow.