Monday, February 17, 2014

Simple Magnectic Loop Antenna

A while back I mentioned my interest in mag loops and proposed at some stage to build an experimental antenna. 
After much searching I finally found and purchased a variable capacitor from my local radio club, I’m not sure of the exact tuning range but I figure it’s got to be approximately 20 -115pf, this should cover me for QRP work, but it obviously limits my power output. 

So now having the copper piping and the variable capacitor I’ve been able to begin building the antenna. As this is purely an experimental project I wasn’t going to make it as neat and tidy as some of the examples I’ve seen on the web. The idea was to build a simple loop connect it all up and see what happens. 

The first issue I had to work out was how to build it, as I had no experience with mag loops what so ever! But after hunting around on the internet I found plenty of examples and a handy calculator tool  to work out dimensions etc.

Getting round bending 9ft x3/4” of copper tubing was a bit of a problem, I had no plumbing tools so I had to have a think as to what I could substitute and eventually used my circular garden water butt which at roughly 3ft in diameter and set against a wall allowed me to squeeze the copper tubing in between the butt and the wall and gently pull the tubing in to a rough circle. It’s by no means perfect but as long as it circular I wasn’t too bothered. 

 It's not exactly round but it will do the job for this experiment!

For the smaller loop (Primary coupling), I used some thinner copper tubing measured length at 1/5th of the size of the larger loop and ¼” width, I could have used the same width tubing but as I had some of the ¼” handy and it was far easier to bend, I didn’t think that for this particular project it would cause any problems. 

The smaller loop threaded through the PVC tubing and ready to be connected to the coax.

Next I mounted both loops on an old piece of 7ft x 1 1/2” plastic tube.  Both copper loops were fed through holes I drilled midway along the length of the plastic piping so that they were held in place. The top ends of the larger loop were then attached with two pieces of thick copper wire and then soldered to the two connections of the variable capacitor. 

 The Variable capacitor connected up to each end of the main tubing

The smaller loop with both loose ends hanging down were also connected by thick copper wire and then via a choc block connected up to the coax cable which in turn is connected to the receiver or transmitter. 

Basically the whole set up is as simple as possible and took all of one day to complete. 

So, results – well I have to say I haven’t transmitted yet but I connected the mag loop up to my OHR100A and had the antenna sitting next to me as I tuned around 20 meters. No question about it as I tuned and peaked the variable capacitor the signals started coming in, it was working! 

Obviously I've got to do some more testing especially having a go at transmitting but so far considering I've never built a mag loop before I'm very pleased.