Also known as: yagi, yagi-uda, beam antenna
A Yagi-Uda antenna (usually just “Yagi”) is a directional antenna made of one driven dipole plus several parasitic elements — a longer reflector behind it and one or more shorter directors in front — all mounted on a common boom.1 The parasitic elements are not fed; they re-radiate coupled energy with the right phase to reinforce the signal in one direction and cancel it in others, producing high gain and a strong front-to-back ratio. It is the classic “TV aerial” shape and the standard high-gain beam for point-to-point VHF/UHF work.
How it works
Only the driven element connects to the feedline; the rest are parasitic. When the driven dipole radiates, it induces currents in the nearby reflector and directors, and those elements re-radiate. Their length and spacing set the phase of the re-radiated field. A reflector is cut slightly longer than resonance so it looks inductive and its re-radiation adds in the forward direction while cancelling to the rear. Each director is cut slightly shorter, looking capacitive, and pulls the wavefront forward. Stack several directors and the reinforcement compounds, narrowing the beam.
The payoffs are directivity and rejection. Forward gain rises with boom length — roughly the number of elements — reaching perhaps 6 dBd for a three-element Yagi and well over 12 dBd for long designs. The radiation pattern becomes a narrow forward lobe with a suppressed rear, quantified by the front-to-back ratio, often 15–25 dB. That rejection is as valuable as the gain: it lets a receiver ignore interference and multipath arriving from other directions.
The trade-off is bandwidth and complexity. A high-gain Yagi is tuned for a narrow band; pushing it wideband costs gain. The driven element’s impedance is also pulled well below a dipole’s 73 Ω by the close parasitics, so most Yagis use a folded dipole or a matching device (gamma, hairpin) to reach 50 Ω.
Relevance to SDR
For an SDR trunking receiver a Yagi is the tool for weak or distant single sites. Pointed at one trunking site, it adds forward gain and, crucially, uses its front-to-back ratio and narrow beam to reject co-channel signals from other sites and reflections — cleaning up the constellation before the demodulator ever sees it.
The cost is that a Yagi hears well in only one direction, so it suits fixed monitoring of a known site, not scanning a whole region. GopherTrunk is a receive-only decoder and does no beam steering; the directivity is purely mechanical, set by where you aim the boom. Where omnidirectional coverage matters, a ground-plane or collinear vertical is the better match, and a log-periodic trades some Yagi gain for far wider bandwidth.
Sources
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Yagi–Uda antenna — Wikipedia, for the driven/reflector/director structure, parasitic phasing, and typical gain and front-to-back figures. ↩