Also known as: collinear array, colinear antenna, stacked dipole
A collinear antenna is an array of several radiating elements arranged end-to-end along a single vertical line and driven in phase, so their fields add toward the horizon.1 Unlike a Yagi, it stays omnidirectional in azimuth; the gain comes purely from compressing the vertical radiation pattern into a flatter, disc-like lobe. It is the standard high-gain base-station vertical for VHF/UHF.
How it works
A single dipole or monopole radiates in a fat doughnut — much of its energy goes at steep angles into the sky and the ground, wasted for terrestrial work. A collinear stacks two or more half-wave elements vertically and feeds them so their currents are in phase. In the horizontal plane the fields from all sections add directly; at angles above and below the horizon they arrive with path differences that make them partly cancel. The net effect is array gain: the same total power is squeezed into a thinner, lower-angle lobe, raising signal strength toward distant stations near the horizon.
The engineering problem is keeping the elements in phase. Simply butting half-wave rods together would put adjacent sections out of phase, because current reverses every half wavelength. Collinears solve this with phasing sections between the radiating elements: quarter-wave stubs, folded loops, or coaxial phase-reversal sections (as in the popular “coco” coaxial-collinear) that flip the phase back so every radiating segment adds in step. The more sections stacked, the higher the gain — typically 3 dBd for a two-element collinear up toward 6–9 dBd for tall commercial verticals — but the vertical beamwidth narrows accordingly.
That narrow vertical lobe is the main caveat. A high-gain collinear aimed flat at the horizon can underperform for very local, high-angle signals — a nearby site up a hill may sit in a pattern null. Height and a clear horizon are what let the low-angle gain pay off.
Relevance to SDR
For a fixed SDR scanning station a collinear is the go-to when you want more range without giving up omnidirectional coverage. Because it hears all directions at once, it suits scanning a region full of trunking sites, and its extra low-angle gain pulls in weak distant signals that a plain quarter-wave ground-plane would miss. Its vertical polarization matches land-mobile P25, DMR, and NXDN traffic.
GopherTrunk is a receive-only decoder and benefits from the improved signal-to-noise a collinear provides, with no configuration needed. The trade-offs versus other antennas are the usual ones: a discone covers far more bandwidth at lower gain, and a Yagi gives more gain but only in one direction. A collinear occupies the middle ground — band-limited, omnidirectional, and higher-gain than a single vertical.
Sources
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Collinear antenna array — Wikipedia, for the in-phase stacked-element principle, phasing sections, and horizon-directed gain. ↩