Magnetic Loop AntennasBy Jose B Rivera - N2LRB
April 23, 2017 - It’s been a year now since I started learning about and then begin to build my magnetic loop antennas. Magnetic Loop Antennas are excellent radiators given their size. They are made with the following elements. Instead of a vertical or horizontal radiator, you have a large loop which connects to a variable capacitor. The loop does not provide the energy for the antenna. A small loop 20% of the length of the large loop is what powers the antenna.
In most incarnations of the magnetic loop, the small loop physically touches the large loop. The small loop can be either at the top of the large loop opposite the capacitor or at the bottom of the large loop also opposite the capacitor. Where ever you put the small loop, the capacitor needs to be opposite of it.
There is one strange aspect of the small inner loop. That being that its center conductor must be attached to its outer shield. It took me a while to accept what I was seeing on various loop antenna design web sites. I thought, surely, this will short out the connection and possibly kill the radio. But for some reason, it does not do so.
Some of my first loops were the MFJ QRP Loop Tuner, model MFJ-9232. It worked well both in my apartment and out of doors. The MFJ-9232 comes with its own outer loop (no inner loop is needed). Though you can attach any other diameter size wire or pipe if you like. It is QRP though, nothing over 20 watts I believe.
In making my own loop I had to purchase a variable capacitor online. I got one from RF-Parts. It is a 14-70PF Variable Capacitor, part number 73-112-15. It is just a few inches long but has wife spacing between its blades. I was able to obtain a wooden box for it. Home Depot did not have a plastic electrical box it’s size. So I went to a hobby shop and got the wooden box.
I then attached two S0-239s, one on each side of the box. The S0-239s is where the two ends of the large outer loop attach. One could instead opt to have exposed cable ends without any type of connector. You just have to make a way to mechanically keep it attached to the capacitor enclosure.
I also purchase a 100:1 Planetary Reduction Drive. Most people are happy with a 5:1 or 6:1 reduction drive, but since these antennas can have a very short tuning point, I figured why not have smaller increments to work with have? I obtained the drive from Oren Elliot and the 25:1 and 100:1 drives both go for $100. This week I ordered a 6:1 drive just to see how well they work. I find the 100:1 hard to turn and it is supposed to be that way. But a MagLoop I purchased from W2LI Tri County Radio Assoc. in New Jersey had a 6:1 reduction drive that turns much easier than my 100:1. In the future I can see buying a 25:1 but for the most part the 6:1 are 1/6th the price of the 100:1 and probably what I will purchase from now on.
In March 2016 I started putting together some of my first magnetic loop antennas. They worked, from inside my bedroom, but were not structurally sound. I had to hold them up straight using one hand and then tune with the other. It was hard to do. But I did find out that the antenna worked! I made contacts with it from inside my bedroom and one or two from the roof of my apartment building.
In November of 2016 I purchased the wooden box, made holes for the SO-239s, inserted the variable capacitor, soldered the necessary connections and had a good enclosure for the capacitor. I also order various lengths and thicknesses of cables for the outer loop. I started out with RG-8, the thick one which Radio Shack used to sell. Moved on to RG-213 which is thicker, then to LMR-400 and finally to LMR-600. The length of these cables varied from 10 to 16 feet. The 16 footers are the most efficient for the 20 meters band, where I operate. It is hard to manage structurally. The 10 foot lengths of LMR 600 stay rigid and need no horizontal support to keep their round shape.
I am very happy with my latest homemade magnetic loop antennas. My 10 footers (in length) are easy to handle, deal with. I have also re-tried the very first variable capacitor I ever purchased. Its 12 inches long and works on 40 meters.
I am waiting for warmer weather and better band conditions to test my loops out of doors, up on the roof of my apartment building and in Central Park.