Magnetic Loop Construction – Part 1
So I got a little time over the weekend to make some progress on my magnetic loop antenna. I constructed this antenna from 1″ copper pipe in a square shape with 90 degree elbow. The loop is 30″ square.
At this time it will be rockbound on 20 meters, centered around 14.060 – so the bandwidth will be very limited. I want to do this so that I can do some testing before I go all out and convert it into a multi-band loop, which will hopefully work on 20-10 meters (with maybe 30 if I am lucky).
Making the Coax Stub Capacitor
So what I did was take a piece for RG-213 coax and cut it about 30″ long. Then on one end I removed about 2 1/2″ of the shield and pulled the braid away from the core. I taped it all up really well with multiple wraps of electrical tape, leaving about 1/2″ of braid and 1/2″ of conductor (sticking out of core).
After sanding the copper to a nice shine, I took some stainless steel hose clamps and clamped the braid to one side of the loop and the center conductor to the other side.
This piece of coax becomes the capacitor which will be used to tune the loop to the desired frequency. Right now it is longer than needed so it will resonate well below the 20 meter band.
The Feed Loop
I then took some RG-8 and made the feed loop. For my loop size the feed loop is about 1/5th the circumference of the main loop – so about 24″. I added a bit for the coax connector. I decided on a shielded Faraday loop after reading that they are quieter on the mag loop email list.
To construct this loop I formed the circle and soldered the center conductor to the braid at the bottom of the circle. Then at the top of the circle I removed about a 1/2″ of the braid only. I taped up all connections and exposed braid, etc.
I quickly taped the Faraday loop to the main loop with some painters tape because I wanted to see how it looked just sitting on the bench. Before I do the final tuning I will hang the loop on the wall of my garage – its final home. I was just excited to see where it was resonant!
So I hooked up the antenna analyzer and started a slow sweep from 20 meters down looking for the SWR dip. THERE IT WAS SWR 1.2 @ 11.131 MHZ – clear as day!
The next thing I am going to do is mount the loop to a piece of 1x wood and then mount that on the wall at its final location. Loops are sensitive to their surroundings, so you always want to tune them at the location they will be when completed.
Then with the analyzer connected I will start cutting 1″ chunks off the coax until I get close to 14 MHZ – the final tuning will be done by sliding the braid up and down the core to get the frequency exactly where I want it.
Once it is operational I will spend some time testing with the Reverse Beacon Network and making contacts. If it works well, ultimately I will construct a butterfly capacitor for tuning and build an Arduino stepper motor controller so that it can be remotely tuned from the shack.
I can’t wait to see how it works and if the noise floor is substantially lower than using my Portable QRP Vertical.
If you would like to construct something similar (although he shows a Gamma Match for the feeding) you should check out W2BRI‘s pages – great instructions with pictures to make everything easy.