Also known as: bicone antenna, biconical dipole
A biconical antenna is a dipole whose two straight arms are replaced by two conductive cones meeting point-to-point at the feed.1 The flare is the whole trick: a thin-wire dipole is resonant and only works well near one frequency, but fattening the arms into cones smooths the antenna’s impedance so it stays reasonably matched over a very wide band — often several octaves. The pattern remains dipole-like — omnidirectional in the plane perpendicular to the axis — so the biconical gives broadband coverage in every horizontal direction at once, which is why it is a standard antenna for EMC test labs and wideband monitoring.
How it works
An ideal infinite biconical structure has a constant characteristic impedance set only by the cone angle — it behaves like a transmission line that happens to radiate, with no resonance to tie it to one frequency. Real cones are finite, so reflections from their ends introduce some frequency dependence, but the antenna still holds a usable match over a huge range compared with a thin dipole. Widening the cone angle lowers the impedance and further flattens it; the designer picks an angle that trades bandwidth against a convenient match to the feed line.
The radiation mechanism is otherwise that of a fat vertical dipole: currents flow along the cone surfaces, and the antenna radiates broadside with a doughnut pattern and nulls off the ends. Gain is modest — comparable to a dipole — because bandwidth, not gain, is the point. Cages of rods or spokes are usually substituted for solid metal cones to cut weight and wind load without changing the electrical behaviour, since the current mainly hugs the outer edge.
Variants
The most important relative is the discone antenna: take a biconical antenna and replace the upper cone with a flat disc, which acts as a ground plane. The result is an unbalanced, monopole-style radiator with the same multi-octave bandwidth and vertical omnidirectional pattern, but fed against a disc instead of a matching cone — mechanically simpler and the form most scanner listeners actually own. A related broadband form is the bowtie, a two-dimensional (flat) biconical often used for TV and UWB. Where more gain rather than pure bandwidth is wanted, the log-periodic is the directional broadband alternative.
Relevance to SDR
Because a wideband SDR can watch a huge slice of spectrum at once, an antenna that stays matched across that whole slice is genuinely useful, and the biconical family fills the role. In practice most SDR users deploy the discone version as a set-and-forget receive antenna covering roughly VHF through the low microwave range, so a single antenna serves airband, marine, land-mobile, and utility monitoring without swapping. Calibrated biconicals are also the reference radiators in EMC emissions testing, where flat broadband response matters more than gain.
For GopherTrunk specifically, a discone or biconical is a very practical antenna: it covers the VHF and UHF land-mobile bands where P25, DMR, NXDN, and TETRA trunked systems live, and its omnidirectional pattern suits scanning a multi-site system whose towers lie in different directions. The trade-off is the usual one — its broadband, low-gain nature means it hears everything but favours nothing, so a weak distant site may still call for a tuned or directional antenna.
Sources
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Biconical antenna — Wikipedia, for the flared-dipole construction, broadband impedance behaviour, and the discone relationship. ↩