Also known as: unun, unbalanced-to-unbalanced transformer
An unun (from unbalanced–unbalanced) is a transformer that changes impedance between two unbalanced lines, keeping both the input and output referenced to ground.1 Where a balun converts between a balanced and an unbalanced line, an unun stays unbalanced on both sides and does one job: match a source impedance (typically 50 Ω coax) to a very different load impedance, most famously the high feedpoint impedance of an end-fed wire antenna.
Overview
Like its balun cousin, an unun is usually built as a transmission-line transformer or an autotransformer wound on a ferrite toroid. The turns ratio sets the impedance ratio, which goes as the square of the turns: a 3:1 turns ratio gives a 9:1 impedance transformation. Because both ports share a common ground, an unun does not provide the common-mode suppression a current balun does — so an unun is often paired with a separate ferrite choke or a short counterpoise to keep RF off the coax shield. Key specs are the transformation ratio, the usable bandwidth (broadband transmission-line designs span many octaves), and the power rating set by the core.
Variants
- 9:1 unun — the classic match for random-wire and end-fed antennas whose feedpoint impedance hovers around a few hundred ohms; brings ~450 Ω toward 50 Ω so an antenna tuner can finish the job.
- 49:1 unun — used with resonant end-fed half-wave (EFHW) antennas, whose feedpoint impedance is very high (~2000–3000 Ω).
- 4:1 unun — a moderate step for antennas or networks near 200 Ω, and for interfacing some verticals and matching sections.
- 1:1 isolation unun — no impedance change, used mainly to place a defined reference between sections.
Relevance to SDR
Unnuns matter to SDR listeners who use long-wire or end-fed receive antennas, a popular choice for wideband HF and general coverage because a single wire covers enormous bandwidth. A 9:1 unun brings such a wire’s erratic, high impedance closer to the 50 Ω the receiver expects, raising the return loss and delivering more of the captured signal into the coax instead of reflecting it. The improvement is broadband, which suits the “listen everywhere” use case of an SDR better than a narrowband resonant match.
Because an end-fed wire relies on the feedline and ground as its counterpoise, an unun installation almost always benefits from a common-mode choke to stop the coax from becoming a noise-collecting radiator — the same noise-floor concern that drives balun and ferrite-choke use.
GopherTrunk is a receive-only software decoder with no analog hardware, so it never contains an unun; the device lives in the antenna system feeding the SDR. Its value to a GopherTrunk user is indirect but real: on the HF and low-VHF wire antennas sometimes used for POCSAG, utility, and other monitoring, a proper impedance match delivers a stronger, cleaner signal to the demodulator, improving the signal-to-noise ratio the decoder depends on.