Directional Coupler Project

A while back I started to think about a way to monitor and display forward and reverse power, both for my remote station as well as my home station. I have meters at the home station, but this is about expanding the capabilities, beyond just showing power and SWR.
Beyond that, I also want to incorporate this into my dabbling into Node Red. I’m thinking that this could be used for realtime tuning for a loop antenna, should I ever build one.
So I buddied up to Google and started researching directional couple circuits and designs, and in particular, designs where I can read the forward and reverse power in real time, calculate SWR, have an intuitive and informative display, and even use the data to do other calculations and make logical decisions from those readings.

I was looking at the Telepost LP-100 for ideas and inspiration, as well as the Wavenode system. I came across K6JCA write-ups and some Loftur TF3LJ/VE2LJX articles showing his work with a tandem match coupler. Finally, WA2T has a website with an elegant design and set of instructions, along with board design files. These are located under RadioStuff on This looks simple and elegant, and easy to build. The RF is then measured with some AD8307 logarithmic detectors and the result is fed to a microcontroller as a voltage. These detectors allow for a wide dynamic range in terms of power measurement across a broad frequency range. Covering the entire amateur HF bands is well within the capability of this detector. I found a source (Amazon, Aliexpress) for small breakout boards for the AD8300-series chips, with an SMA connector and headers for power and output. These boards allow for a shielded RF connection between the coupler and the detector, which helps keep the microprocessor isolated.

Loftur’s display and function are good, but I didn’t need those all features, so I focussed on the coupler and detector elements only. I also came across some other articles showing a circuit board design for the coupler, so that board became the basis of my build project.

The bill of materials includes some ferrite toroids, and a number of surface mount components.
I sent off the gerber files to same folks that make our Morse Tutor kit circuit boards, JLCPCB. Several weeks later, my small order of 5 boards arrived. It’s annoying that shipping these to Canada cost more than the boards…
An order to Digikey completed the bill of materials, and I’m ready to tackle the build and practice my surface mount skills along the way. I made sure to order a few spare components. Just in case…

The board measures 55mm wide, so you can see the minuscule scale of the other parts.

Winding Toroids for the Coupler.

These are on T50-43 and T82-43 cores wound with 26 gauge magnet wire. There are 24 turns on each core.
It’s interesting to note the windings are in opposite directions on the two cores so we can accurately measure forward and reverse power.

Toroids ready

Soldering the parts

I used a hot air station for soldering the surface-mount parts.

There is a mixture of 2512, 1206, and awful small 0805 inductors. The challenge is to not have the hot air blow the parts off the board.

Soldering on a resistor
All the surface mounted parts installed

Next step, install the toroid-based transformers.

This is still the early beginnings of this project, so I’ll leave you hanging right here, right now. I’ll post updates as the build progresses…