Automatic Magnetic Loop Tuner – SWR Bridge In A RF Shielded Box

Since we are dealing with RF we have to keep it away from all other components and equipment in our shack. The only thing you want to generate or pick-up RF with is your antenna. In our case a magnetic loop antenna.

The purpose of the SWR Bridge, or tandem match, is to measure the difference between the forwarded and reflected power. To do that, we have to place the SWR Bridge between our antenna and our transceiver. Ideally, this should be done as close as possible to the antenna. This, however, is not very practical, as you won’t be able to read the values on the screen.

That’s why its placed inside the automatic magnetic loop controller unit and also why we need to create a shield that will keep the RF inside the SWR Bridge box.

To be able to measure the differences between forward and reflected power, we need to connect the SWR Bridge to the Teensy, pad A10 and A11. If we would just drill two holes in the tinned box and wired the SWR Bridge directly to the Teensy, the RF could still escape and cause trouble in our tuner or even in the shack. By using feed through capacitors, we can connect the SWR Bridge to the Teensy and measure both forward and reflected power, while still keeping the RF in the shielded box.

Let’s zoom in on the electric diagrams, the source code and the PCB design to know how the SWR Bridge must be connected to the Automatic Magnetic Loop Controller PCB’s. This implies both Loftur as mine PCB designs.

Sontheimer Bridge. 200 W version.

From the ML.h file:
// AD inputs for Forward and Reflected Power (SWR measurement)
const int Pfwd = A10;
const int Pref = A11;

Looking at the SWR Bridge, or Sontheimer Bridge, it’s fairly easy to understand. Connect your transceiver to RF In, connect the antenna to RF Out. On the other side of the board, we have FWD (pin 1), GND (pin 2) and REF (pin 3). This is the part where we need to keep attention. On the PCB the pinout position is different for GND and REF.

Connect SWR Bridge pin 1 (FWD) with pin 1 on the PCB, SWR Bridge pin 2 (GND) with pin 3 on the PCB and SWR Bridge pin 3 (REF) with pin 2 on the PCB. After our signals went through some resistors and capacitors, they can be connected via another 3 pin header to the Teensy 3.2 pads A10 and A11. Pin 1 (FWD) is connected to Teensy pad A10, and pin 2 (REF) to Teensy pad A11.

SWRBridgeBox

Automatic Magnetic Loop Tuner – SWR Bridge

While reading the frequency from the radio, and having the “antenna characteristics” in the tuner’s memory, there is no real need for an SWR bridge to be used.
However, we can add a basic power/SWR-meter and auto-tune function if we do so. For this SWR bridge, Loftur proposed to use a Tandem Match over a Bruene Bridge.

He wrote the following in his Automatic Magnetic Loop Controller BOM and building instructions:

A Tandem Match coupler may be a bit more accurate than a Bruene Bridge, however it is more difficult to achieve very good
isolation between the Forward and Reverse outputs, as this is dependent on how similar the two transformers are. On the other
hand, this is not intended to be a super-precision instrument, the below circuit is certainly as good as the average HAM grade
Power/SWR meter.

The picture here below is a partially finished kit from KitsAndParts.com with the adaptation proposed by Loftur
Swr bridge

Modifications to the kit from Kits and Parts:
20 turns wound on the two ferrite cores. This will make the meter suitable for 100W (power dissipation in R1 and R2 is 0.25W max
at 100W), should be able to handle 200W SSB without upgrading the resistors to 1/2W type.
R3 and R4 are shorted, C1 and C2 are exchanged to 4.7nF capacitors (example: Digikey 445-4746-ND) to speed up the response of
the outputs. This is necessary for fast SWR tuning. C3 and C4 are omitted.
Note the different order of the pins at the J1 connec tor, when compared with the VSWR sense connector on the Controller PCB.

Resistor values R15, R16 and R17, R18 on Controller PCB:
For a useful range of 0.5 – 200W, R15 and R16 on the Controller PCB should be 18 kohm 1/4W (example: Digikey 18KQBK-ND) and
R17, R18 should be 22 kohm 1/4W (example: Digikey 22KQBK-ND).

Due to the number of components that needed to be replaced or tossed away, and since there was some interest from other OM’s of our club, I designed an SWR Bridge kit with the correct components and sufficient copper wire for the 20 turns per toroid. Since the calibration resistors are omitted from this design, it’s important to make the toroids as equal as possible. A small deviation is not an issue. The tuner will tune to the lowest SWR point. Even if the reading is a bit off, the antenna will still be tuned to the minimum SWR.

SWR Bridge Kit
Drop me an email at on5ia@uba.be if you are interested in this SWR kit specially made for the Automatic Magnetic Loop Tuner.

Automatic Magnetic Loop Tuner – Soldering Components

When all components were delivered, it was time to put them all together. The effort I spent writing custom references for all components, at the time of ordering, was well worth it. Now each bag of components like resistors, capacitors and trimpotmeters was labeled with the correct reference name for the project (R1, R2, C1, U1, etc..).

Automatic Magnetic Loop Tuner

My two friends were armed with a soldering iron, and I was providing them the ammunition in the form of electronic components.

Automatic Magnetic Loop Tuner

Here we are soldering the 14 pin female headers for the Teensy 3.2.

Automatic Magnetic Loop Tuner

Automatic Magnetic Loop Tuner

Check twice, solder once. Each component that was soldered to the PCB was checked. We had to be sure all values were correct and within the tolerance. It might seem like a bit of overkill, but faulty components do exist. One project almost failed because of a faulty resistor. If we didn’t check it upfront, we would have lost a lot of time debugging our project.

Automatic Magnetic Loop Tuner

The finished product of the day. We made 3 of these PCB’s in 4 hours time. That’s definitely not a speed record, but when you are having fun with friends, who cares about the time.

Automatic Magnetic Loop Tuner – Bill of materials (BOM)

The first thing we did on our first project meeting was having a look at the BOM. What components do we need to build this automatic magnetic loop tuner, and are they easy to find.

It seemed Loftur did a great job listing all the parts and most of them could be ordered at DigiKey, Mouser or eBay. It would be only later in this project I found out huge savings could be done if some parts were ordered at AliExpress.

I was soon appointed as logistics manager and started sourcing and ordering all the needed parts.

Following parts are worth to spend a few words on:

LCD screen
Display
If there is only one item of this project you should buy on AliExpress, it’s this 20x4A LCD display. Using the search function you can easily find this screen for less than $3. For Europeans, it can be even more advantageous when the exchange rate of the dollar is low. You can pick the one without the IIC/I2C Serial Interface Adapter Module as for this project you won’t need it. Link to item on AliExpress.

Stepper motor
Stepper motor
My advice is to buy the motor only when you know what type of capacitor you’ll use for your magnetic loop antenna. For an air variable capacitor, you might need extra resolution, for a vacuum variable capacitor you might need extra torque. Both can be achieved by using a stepper motor with a reduction gear. However, be careful with to much torque, you might end up damaging your vacuum capacitor if your end stop settings are not set correctly. Link to item on AliExpress.

Stepper motor driver chip/module
Here you have several options. Option one is Allegro since it is this IC that Loftur’s used in his electrical diagram and PCB design. However, there are two other options to make the stepper motor do its magical micro-stepping. The A4988 and the DRV8825. Loftur wrote the code in such a way it can be easily modified to take into account the type of ic or stepper module you are using. You only need to comment and un-comment some lines of code and recompile.

Screen bezel
If you want your automatic magnetic loop tuner to be a nice looking piece of equipment, this screen bezel is a must. Strangely enough, there seems to be only one manufacturer in the world who can provide these little simple, plastic and Lexan, screen bezels. At 3,5 dollar each, they are not expensive, but the shipping costs will make you think twice. On top of that, it’s coming from the USA, which is a huge red flag for the Belgian Customs, who are very keen on making sure you pay your import duties. In the end, more than 50% of the total cost is the tax. Not only because of the 21% VAT but especially due to the flat rate of 12 euro in “handling cost”.

PBC by Loftur
Automatic magnetic loop tuner PCB.
As none of the three of us wanted to etch the schematics on a copper prototype board, we just ordered the 5 first controller PCB via Loftur.

Later I made my own version of the PCB.

SWR bridge
SWR Bridge Kit
In the beginning, you want to be sure, that what you are making will work. So to play on the safe side, you copy what others have been done before. To incorporate the SWR auto-tune function into the automatic magnetic loop controller, Loftur proposed to use an SWR Bridge kit from KitsAndParts.com and make some adjustments to it. So that’s what we did for the first automatic magnetic loop controllers. Later I made my own PCB’s based on the Sontheimer Bridge design. You can read that story here.

Rotary encoder

We all know rotary encoders and we love them. It’s the big round knob on our radio we like to spin around to scroll over the band to spot that new DX or SOTA. For the magnetic loop controller, you can’t just use any encoder. It has to be an encoder that has sufficient resolution to be usable for this project. In some configurations, each encoder pulse is equal to a 1/8 step, so if your capacitor needs to be adjusted a few steps, you’ll have to turn the knob several times around if it has not enough pulses per rotation. We are using an encoder with 64ppr, Loftur is using a 128 encoder in his automatic magnetic loop controller. Cheaper 400 ppr alternatives can be found on AliExpress, but you have to take into account the much bigger size. More detailed info here: Rotary Encoder.

Teensy 3.2
Teensy 3.2
This little Arduino compatible device is the hart of the automatic magnetic loop controller. It is where the processing power and memory storage is coming from. They cost about the same as the rotary encoders. If you are lucky you can find cheaper compatible devices on AliExpress, but then you are not sure if they will work. As a matter of experiment, I bought two and they seem to be doing the job fine. You can save a few euro if you buy the Teensy version without the pins already attached. Soldering 28 pins can be done in two minutes, and since we are soldering anyway, why wouldn’t we do this ourselves?

Previous Chapter: Automatic Magnetic Loop Tuner – The Beginning
Next Chapter: Automatic Magnetic Loop Tuner – Soldering Components

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 electrical 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.