Also known as: discone, disc-cone antenna
A discone antenna is an ultra-wideband vertical antenna formed by a flat disc mounted just above the apex of a cone, with the feedline running to the disc and the cone.1 The shape is a distant relative of the biconical antenna — replacing one cone with a disc — and it delivers omnidirectional, vertically polarized coverage across a decade or more of frequency from a single feedpoint. That extreme bandwidth is why the discone is the archetypal scanner antenna.
How it works
The discone belongs to the family of frequency-independent antennas whose behaviour is set by angles and tapers rather than a single resonant length. A conventional monopole is resonant only near one frequency because its length is a fixed fraction of a wavelength. The discone instead presents a continuously changing radius from the feedpoint outward: at any frequency in band, some portion of the cone-and-disc structure is the right size to radiate. This keeps the feedpoint impedance roughly constant — near 50 Ω — across a wide range, so SWR stays low without retuning.
The cone’s slant height sets the low-frequency limit (roughly a quarter wavelength at the lowest usable frequency), while the disc diameter and the disc-to-cone spacing set the high-frequency behaviour, giving a usable range of about 10:1. The pattern is omnidirectional in azimuth and, like a vertical monopole, tilts toward the horizon with a null overhead; at the upper end of the band the main lobe rises somewhat and gain stays modest — a discone trades gain for sheer bandwidth, delivering roughly unity gain rather than the concentrated gain of a beam.
Practical discones use a skeleton of rods instead of solid metal for the disc and cone, which behaves almost identically while cutting weight and wind load.
Relevance to SDR
For general SDR scanning, a discone is often the single best all-round antenna: one feedline covers VHF, UHF, and beyond, so a listener can sweep aviation, marine, public-safety trunking, and business bands without swapping hardware. Its omnidirectionality means it hears every trunking site at once, and its vertical polarization matches land-mobile traffic.
GopherTrunk decodes whatever the front end delivers and imposes no antenna requirement, but a discone’s breadth suits GT’s multi-protocol, multi-band nature — you can leave it connected while monitoring different systems across the spectrum. The cost is modest gain: for a weak, distant single site, a directional Yagi or a band-optimized ground-plane will hear better, but for “one antenna, everything,” the discone is hard to beat.
Sources
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Discone antenna — Wikipedia, for the disc-over-cone geometry, ~10:1 bandwidth, near-constant impedance, and omnidirectional vertical pattern. ↩