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@mcdanlj@social.makerforums.info
2025-09-01 20:31:29

This is the third full build iteration of my miniature #QRP unun supporting both EFHW and "random wire" antennas. At this point, it is still fiddly to build, but I'd expect an experienced #HamRadio DIY / homebrew enthusiast to find the build not terribly difficult.
There's nothing…

Photograph of tiny QRP unun showing size of overall assembly including integrated coax. BNC connector for scale! The corners of the box have M3 countersink screws. There are three knobs on the side; a purple knob for the counterpoise connection, and red (high voltage!) knobs for "random wire" and end-fed half wave connections,
Inside view of unun autotransformer showing construction details, next to a ruler for scale, showing that the entire box is less that 3.5 cm across. There is a half-inch ferrite in the box, wrapped in kapton tape, with a 14-turn autotransformer wrapped around it. There are three M3x8 brass screws through the edges, connected to the transformer with ring terminals. The screws are held in place by captive nuts. An RG316 coax segment comes in through the side of the box to feed the autotransformer…
Side view of assembled unun, without a terminal knob, showing how thin it is. The letter "R" is visible on the side of the box, indicating that the adjacent terminal is intended for a "random wire" antenna.
A photo showing a comparison of this version of the unun design to the previous iteration, configured for deployment. The previous iteration was built on a SO-239 bulkhead connector, with a PL-259-to-BNC connector attached.