Also known as: balun, balanced-to-unbalanced transformer
A balun (from balanced–unbalanced) is a component that joins a balanced line or antenna — one where two conductors carry equal and opposite currents — to an unbalanced line such as coax, where signal rides on a center conductor against a grounded shield.1 Connecting the two directly lets current flow on the outside of the coax shield, unbalancing the antenna and turning the feedline into an unwanted radiator; a balun forces balance and can, with a turns ratio, also transform impedance.
Overview
Balance matters because a balanced antenna, like a center-fed dipole, assumes its two halves carry symmetric currents. Feed it with coax directly and the shield offers a third path: common-mode current flows on the outside of the braid, so the feedline both radiates (distorting the pattern) and receives (bringing household noise to the receiver). A current balun presents a high impedance to that common-mode current while passing the differential signal, restoring symmetry. A voltage balun instead forces equal and opposite voltages at its output; current baluns are generally preferred for pattern cleanliness.
Variants
- 1:1 current (choke) balun — many turns of coax on a ferrite core, or a coiled-coax “ugly balun.” No impedance change; its job is purely common-mode suppression at the feedpoint.
- 4:1 balun — transforms 200 Ω down to 50 Ω (or 300 Ω to 75 Ω), matching folded dipoles and off-center-fed antennas. Built as a transmission-line (Guanella/Ruthroff) transformer or a wound autotransformer.
- 9:1 and other ratios — used for high-impedance wire antennas such as random- wire and end-fed designs.
- Half-wave coax (bazooka) and sleeve baluns — narrowband baluns made from a measured length of line rather than a ferrite core.
A closely related device, the unun, transforms impedance between two unbalanced lines; a balun differs in that at least one side is balanced.
Relevance to SDR
Any listener feeding a balanced antenna — a dipole, folded dipole, loop, or T2FD — with coax should use a balun at the feedpoint. The receive-side payoff mirrors the transmit-side one: a current balun keeps common-mode noise off the coax shield, which for weak-signal SDR work often lowers the noise floor by several dB and cleans up the radiation pattern so nulls and gain behave as designed. Ratio baluns additionally correct the impedance mismatch that would otherwise raise SWR and cost signal.
GopherTrunk is software and includes no analog hardware, so a balun is entirely part of the antenna install ahead of the SDR. Its benefit to GopherTrunk is the same as any front-end improvement: a properly balanced, low-noise feed delivers a cleaner signal to the demodulator, improving the signal-to-noise ratio the trunking decoder relies on to hold a control-channel lock. Baluns are common as small in-line modules or as the potted transformer inside a receive antenna’s feedpoint box.