All Portable Discussion Zone build #2
For our second build-along project, we decided to go for a Stockton bridge SWR meter, again a G-QRP project from the 2023 buildathon.
The SWR bridge is pretty simple: RF goes in and out through a couple of connectors on a nice bit of wire that passes through a binocular core. This core has a couple of 12t windings in each nostril, and it transfers the voltage over to the circuit, whilst sniffing both forward and backward voltage. A 50r resistor acts as reference, and a shottky diode rectifies the AC. Or this is my perception of how it works, in a pistachio shell.
I built mine in an afternoon, but had no life from it. Puzzled, I tinkered for a couple of weeks and made some posts on Discord.
AA7U kindly mailed me a working one that he built.
Having this on my bench, seeing how incredibly simple it is (for some reason looking at a version someone else made feels simpler) made me re-evaluate how I was testing things.
Drumroll for my idiocy: though I made some changes which improve it, the real issue turned out to be that
A: the signal generator’s 0.5v signal was insufficient to drive the circuit.
B: the FT-817 does not emit RF in CW mode pressing the PTT – since I was worried about SWR, I had the meter on the radio showing SWR and not power. When I realised this, I switched it over to FM and I get the kind of behaviour I would expect. I ended up switching the pots out to 100k to have better control over the scale, and by running the rig into each input (moving the dummy load), I was able to set them the same, so that the 5w sits at the 3 on the scale. Plugging a 100r dummy load in shows about half scale reflected power, full scale forward power.
Filed under: DIY - @ May 4, 2026 12:15 am
Tags: all portable discussion zone, diy, HAM radio, red summit rf