Everything you need to build the automatic magnetic loop tuner in just one picture. On the Flickr page you can see the annotations. To reach the Flickr page, you need to click on the picture.
After some soldering, drilling, cutting, filing, screwing, programming and painting, … it will look like this:
There are about 10 working days between finishing the PCB design in Eagle and the reception of the finished product at my doorstep. I have always wondered how the Sontheimer bridge PCB and the A4988 PCB were manufactured.
Scotty Allen from Strange Parts went on a factory tour and made a pretty extensive video about the production process of a PCB. If you have 26 minutes of time, check out his youtube video below. (If you have less time you can fast forward some parts 😉 )
I took some time to solder another PCB for an Automatic Magnetic Loop Tuner. I made some pictures of the final result which I would like to share with you.
This is a view from the top of a populated PCB. The Teensy 3.2 and the A4988 stepper module are not yet installed. This loop controller won’t need the end stop feature. Therefor D2, D3, R25, R26, C19, C20, C21, C22, T3 and the 3 pin header for the end stop connector, are not installed.
C1 is located underneath the Teensy 3.2. This capacitor can be soldered upright. There is plenty of space, so no need to lay it flat on the board.
Here we have both the Teensy 3.2 and the A4988 stepper module installed. Note that a jumper is placed over JP1, but that JP2 is left open. JP1 connects “Sleep” and “Reset”. JP2 is for future use. When using a DRV8825 stepper module it gives us the possibility to connect the unused Teensy 3.2 pin 19 with pin MS3 of the DRV8825. With some additional programming to the software, this could enable the 1/32 step stepper resolution. This however has not yet been programmer, nor tested.
If there is one thing I would change to this board it’s the re-positioning of the LM7805 voltage regulator. The ground plane in facing inward which makes it difficult to attach a heat-sink. This can be solved easily by bending the legs of the LM7805 so the ground will face upwards, or by connecting it from the underside of the PCB and connecting the ground to the metal enclosure the whole PCB will be put in.
On this last picture, you can see capacitor C1 has plenty of space and is enjoying the company and security of the Teensy 3.2 😉
Using the original schematic as a starting point, I reworked the electric diagram and incorporated a A4988/DRV8825 module.
You can download a pdf file of the reworked schematic here: MagLoopTuner.pdf
The two jumpers in the schematic deserve a little explanation:
JP1 connects the sleep and reset pins of the A4988/DRV8825 module board. I’ve seen schematics where these pins are soldered together, but I preferred to use a jumper. This way you have the possibility to add a reset switch, or if you think it’s not needed, just bridge the two contacts.
JP2 was incorporated for future use. It connects the unused pin 19 of the Teensy 3.2 with pin MS3 of the A4988/DRV8825 module. Without this connection, the maximum resolution of the magnetic loop controller is 1/8th step. With some extra coding, we could drive the stepper motor as precise as 1/16th step (with the A4988) or even 1/32th step (with the DRV8825). This coding is not yet done, and I’m also not sure if this fine stepping is useful for this project, but I thought it was a nice idea to experiment with it. So for now, you can leave JP2 open.
If you are not using end stop switches,then there is no need for D2, D3, R25, R26, C19, C20, C21, C22 and T3.
Depending on the type or brand of rotary encoder, the A or B phase can be reversed. However, the VCC and G must always be respected. You can check the correct wiring of A and B when you scroll through the menu. Turning the rotary encoder clockwise must increase the menu option, the steps and the frequency. When this is correctly done, you can start checking the wiring of the stepper motor. In order for the backlash and auto-tune function to work correctly, the capacity must go down, when you turn the rotary encoder clockwise.
Don’t need the SWR / power meter function? Then you don’t need R15, R16, R17, R18, C25, C26 and R20, R21 and R22 (and their corresponding switches) either.
Please check Loftur’s project page for a more detailed explanation, the BOM and building instructions: https://sites.google.com/site/lofturj/to-automatically-tune-a-magnetic-loop-antenna
For any inquiries or questions about the PCB, please send an email to on5ia@uba.be or post a comment on this blog.
The design Loftur published on the project site was built around two A4975SBT ic’s. Although they are available at digikey.com they are rather expensive if you only need 2. Mouser.com even doesn’t have them in stock. When you need more of them to build multiple automatic magnetic loop tuners, you could start looking for them on AliExpress. A starting seller (who needs good reviews for credibility) will offer you 10 pieces for less than 12 euro. They have a 1,5 A continuous output current and have a step sequencing from 1 full step until an eighth of a step. When you are using a multi turn variable vacuum capacitor or any capacitor connected to a geared motor, this will do just fine.
Allegro A4975SBT.
If the steering cable is too long, and it’s resistance is becoming too high, you need to provide more than 1,5 A so your motor would still receive enough current to generate sufficient torque for turning the capacitor. Depending on the type of capacitor, you might need more than 1/8 of a microstep. In both cases you should look out for something more powerful or precise than the A4975STB’s. Two alternative stepper driver modules are the A4988 or the DRV8825. On top of being more precise and / or more powerful, they are more easier to find and a lot cheaper than two A4975STB’s. Just like with the NEMA 17 stepper motors, these modules are widely used in all kinds of 3D printers and DIY CNC machines. There are plenty of suppliers on AliExpress, offering them for prices close to one euro.
The A4988 is capable to deliver 1A per phase without a heat sink or forced airflow and is rated for 2A per coil with sufficient additional cooling. On top of all kinds of over- and under-protection intelligence (voltage, temperature, current, crossover-current, short-to-ground and shorted-load) it features five different step resolutions: full-step, half-step, quarter-step, eighth-step, and sixteenth-step.
A4988 stepper module.
The DRV8825 has a pinout and interface that are nearly identical to those of the A4988 stepper driver module, so it can be used as a higher-performance drop-in replacement for those modules in our automatic magnetic loop tuner. It can deliver up to approximately 1.5 A per phase without a heat sink or forced air flow and is rated for up to 2.2 A per coil with sufficient additional cooling.
DRV8825 stepper module.
It will not sound as a surprise to you when I tell you these two modules don’t fit in the original PCB layout Loftur designed. Joerg, a user in the loop controller yahoo group found a solution for that and made an adapter board to use a A4988 or DRV8825 module with Loftur’s print.
Joerg made me realise it wouldn’t be to difficult to adapt the original design and make a new PCB especially for the A4988 or DRV8825 stepper driver modules. After some hours redrawing the schematics and moving around the components I came up with following PCB design.
It’s 67mm x 100mm and features besides the space for a A4988 or DRV8825 module, 3 (T1, T2 and T3) 2.2 A common mode chokes (CMS1-8-R). Loftur proposed these chokes as an upgrade for the 1 A common mode chokes of Wurth Electronics (744227S) originally used in his first design.
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. Idealy 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 Teense 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.
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.
While reading the frequency from the radio, and having the “antenna characteristics” in the tuner’s memory, there is no real need for a 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
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 a 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.
Drop me an email at on5ia@uba.be if you are interested in this SWR kit especially made for the Automatic Magnetic Loop Tuner.
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.
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.
Golb.be 🇧🇪
@golbbe