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.


3 thoughts to “Automatic Magnetic Loop Tuner – SWR Bridge In A RF Shielded Box”

  1. Are you using any type of capacitor between the FWD/REV on the SWR board and the main board? Looks like there is something there.

  2. Hello Paul, I’m using feedthrough capacitors of 1000 pF. This way we can keep the RF inside the box and still read the voltage differences between FWD and REV.

  3. Hello! I’m in the process of getting on of these together and have my mainboard design complete, but have a couple of questions:-

    1. Do you recall the dimensions of your SWR Bridge board that I could use as a starting point? I can’t find any reference anywhere either for your board or the one done by Kits and Parts.

    2. Instead of “overhead wires” from A10 and A11 of the Teensy to a header connector I have just incorporated tracks on the main PCB since looking at the Teensy 3.2 Development Board used there seem to be no reason why a set of header pins can’t be added alongside the other pins to plug in A10 and A11 at the same time. DO you foresee any problem with that?

    3. Paul Gaffney (above) made reference to what you said are “feedthrough capacitors”. I can’t seem to locate them anywhere. Could you explain where they are and what voltages the 1000 pF caps are?

    Thank you!

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