{"id":230,"date":"2025-12-01T13:04:50","date_gmt":"2025-12-01T13:04:50","guid":{"rendered":"https:\/\/willwoodman.co.uk\/wordpress\/?p=230"},"modified":"2025-12-15T13:05:29","modified_gmt":"2025-12-15T13:05:29","slug":"really-weird-twisted-sisters","status":"publish","type":"post","link":"https:\/\/willwoodman.co.uk\/wordpress\/2025\/12\/01\/really-weird-twisted-sisters\/","title":{"rendered":"Really Weird Twisted Sisters!"},"content":{"rendered":"\n

I\u2019ve been building a QMX+ HF transceiver kit recently, and one feature really caught my attention: the \u201creally weird twisted sisters transformer\u201d (RWTST).
It\u2019s a great reminder of how, at RF frequencies, the smallest physical details can make or break circuit performance.

Earlier versions of the QMX+ used a conventional output transformer. But some builders noticed that performance dropped off at the higher end of the HF bands. After some clever troubleshooting – swapping transformers between \u201cgood\u201d and \u201cbad\u201d units, it became clear the issue lived in the transformer itself.

The culprit? Parasitic capacitance.

When RF-carrying wires are wound closely together, they naturally form small capacitors. In the original design, this unintended capacitance caused a parasitic resonance above ~28 MHz, limiting power output.

Ross (EX0AA) investigated and came up with the RWTST: a transformer where the primary and secondary wires are tightly twisted together before winding. Counter-intuitively, increasing the capacitance intentionally shifted the resonance to a more favorable frequency – improving performance in the problematic 10m band.
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Traditional Transformer<\/figcaption><\/figure>\n\n\n\n
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The RWTST<\/figcaption><\/figure>\n\n\n\n
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The Finished Article<\/figcaption><\/figure>\n<\/figure>\n\n\n\n


It\u2019s a brilliant example of the \u201cart meets physics\u201d nature of RF design. Sometimes the solution isn\u2019t a new component\u2026 it\u2019s simply twisting wires in a different way.

Amazing how, in the RF world, even the geometry of a winding can be the difference between success and failure.<\/p>\n","protected":false},"excerpt":{"rendered":"

I\u2019ve been building a QMX+ HF transceiver kit recently, and one feature really caught my attention: the \u201creally weird twisted sisters transformer\u201d (RWTST).It\u2019s a great reminder of how, at RF frequencies, the smallest physical details can make or break circuit performance. Earlier versions of the QMX+ used a conventional output transformer. But some builders noticed […]<\/p>\n","protected":false},"author":1,"featured_media":231,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[9,15,14],"tags":[],"_links":{"self":[{"href":"https:\/\/willwoodman.co.uk\/wordpress\/wp-json\/wp\/v2\/posts\/230"}],"collection":[{"href":"https:\/\/willwoodman.co.uk\/wordpress\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/willwoodman.co.uk\/wordpress\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/willwoodman.co.uk\/wordpress\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/willwoodman.co.uk\/wordpress\/wp-json\/wp\/v2\/comments?post=230"}],"version-history":[{"count":1,"href":"https:\/\/willwoodman.co.uk\/wordpress\/wp-json\/wp\/v2\/posts\/230\/revisions"}],"predecessor-version":[{"id":234,"href":"https:\/\/willwoodman.co.uk\/wordpress\/wp-json\/wp\/v2\/posts\/230\/revisions\/234"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/willwoodman.co.uk\/wordpress\/wp-json\/wp\/v2\/media\/231"}],"wp:attachment":[{"href":"https:\/\/willwoodman.co.uk\/wordpress\/wp-json\/wp\/v2\/media?parent=230"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/willwoodman.co.uk\/wordpress\/wp-json\/wp\/v2\/categories?post=230"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/willwoodman.co.uk\/wordpress\/wp-json\/wp\/v2\/tags?post=230"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}