SWR Bridge

12 Aug

This term really bugs me, as none of common designs are really measuring SWR (“Standing Waves”), they’re measuring the error ratio of an actual impedance compared to a reference. If they were really measuring standing waves, which they can’t be doing  because they measure even if connected directly to a load with no transmission line, the SWR on the transmission line would be assumed to be the ratio of the known impedance to the reference impedance. For example, in a 50 ohm system, a 2:1 impedance mismatch (25 ohms or 100 ohms) would correspond to a 2:1 SWR. This is another soapbox matter for another day.

Anyway, I designed this little board to go on the SO-239 connector of my uBitX transceiver and drive an A-d converter input. If there’s interest, I’ll publish the DipTrace CAD files and Gerbers and/or make it available directly from the board fab house. Seems to work, with over 20dB of directivity without any adjustments. This was with 100mW from my HP8640 signal generator. Calibration with power needs to be developed. Insertion loss is less than 0.2dB and input match is better that 20 dB & seems pretty flat over the HF range, but I don’t have any detailed numeric or other real-world data at other power levels to publish at this point. I’ll publish more as I characterize it, but got to get back on the Raduino Pill first.

SWR Bridge Schematic (pdf)

The schematic is from LTSpice, so you can run the simulation if you like.

Note that the ratio of R1 to R3 & R4 sets the full-scale DC output voltage. I have the values set to get about 3.3V @ 10 watts, which is right for the A-D converter on the Blue Pill board.

This is a sweep of the reverse port made with my Tektronix 495P analyzer w/tracking generator. Note that the sweep is referenced to the second graticule line from the top. 

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