Automatic Magnetic Loop Tuner – The Beginning

Somewhere in March 2016, I was in a QSO over our local Brandmeister DMR repeater with other club members. Every Wednesday evening we have a net where we discuss the things we did the past week regarding our hobby and talk about projects we would like to start.

At that time I was rather new to the club, as I only got my licence in February 2016. Two OM’s were talking about Magnetic Loop Antennas. One OM’s liked them allot because of them being less prone to QRM and wanted to build one for the 160m band, the other because of the limited space you need to set one up. They both had one issue though: it was not easy to keep them in resonance with the frequency of the radio’s VFO. Especially not when they were scrolling over the bands.

“How hard could it be to keep them tuned?” was the first thing that came into my mind, and I immediately started googling. The first result of my search query was the site of Loftur E. Jónasson – TF3LJ / VE2LJX: https://sites.google.com/site/lofturj/to-automatically-tune-a-magnetic-loop-antenna.

When it was my turn to push the PTT button, I informed my fellow OM’s. They were not very enthusiastic about my find. Soon I understood why. Although they were both specialists in radio’s and old school electronics they didn’t know how to program something in Arduino. They thought this project was off limits for them.

I’ve done quite a bit of programming in the past, but other than them, I was not very acquainted with electronics or electric diagrams. We soon realized we could make this project work if we combined our complementary knowledge and experience.

We agreed on a date, and one week later, we were sitting in the shack discussing how we would tackle this innovative project.

Sontheimer Bridge – SWR Bridge

For the Automatic Magnetic Loop Tuner project I needed an SWR Bridge (Standing Wave Ratio) which could handle 200 W. The existing kit of http://www.kitsandparts.com/ could be used if I made some adjustments. Two capacitors and two resistors needed to be thrown away, two capacitors needed to be replaced, and more copper wire was needed then what was provided in the kit. With a price tag of $12 and shipping cost of $15, that’s a waste of money and resources.

Since the design isn’t that complicated, and I’m always interested in learning something new, I decided to make the SWR bridge myself.

The SWR bridge is a tandem match, also known as a Sontheimer bridge. It’s named after it’s inventor, Carl G. Sontheimer.

I started to draw the electrical diagram in Eagle. Eagle, a part of Autodesk, from the creators of AutoCAD amongst other things, can be downloaded here for free. With the free full working version of Eagle you can design PCB’s (Printed Circuit Boards) with a maximum size of 80mm x 100mm and max 2 layers. More than enough for this little project.

When you are finished with the electrical diagram, you can start designing the PCB. It can be a real hassle if you’ve never done it. Fortunately there are a lot of people making tutorials on YouTube, describing how things should be done. I followed the three-part YouTube Eagle tutorial of Jeremy Blum.

Part 1: Schematic Design https://youtu.be/1AXwjZoyNno
Part 2: Printend Circuit Board Layout https://youtu.be/CCTs0mNXY24
Part 3: CAM output and DFM https://youtu.be/oId-h6AeXXE

You’ll need just a little over 90 minutes to watch the three of them. Jeremy explains how you can make your own PCB very well and guided me through the entire process.

Based on the Gerber files that are finally generated by Eagle during the CAM stage, you can ask your PCB manufacturer to produce your PCB. I must say I’m pretty happy with the result.

Sontheimer Bridge. 200 W version.

Now that I have the PCB, I only need to order the other components that need to be placed on it. I can’t wait to test this SWR Bridge and see how it performs.