Also known as: ground plane antenna, ground-plane vertical, GPA
A ground-plane antenna is a monopole — a quarter-wavelength vertical element — mounted over an artificial ground made of a few horizontal or sloping radials instead of a solid metal sheet.1 The radials supply the electrical image the monopole needs, so the whole assembly is a self-contained, omnidirectional vertical that does not depend on being near real earth or a vehicle body. It is one of the most common fixed-station antennas for scanning and amateur VHF/UHF.
How it works
The driven element is an ordinary quarter-wave monopole: it needs a conducting surface beneath it to mirror the element into an effective half-wave dipole. A solid sheet works, but so does a “skeleton” of a few radial wires, each about a quarter wavelength long. Three or four radials are enough to approximate the current distribution of a continuous plane, keeping the antenna light and wind-transparent while still supplying the return path.
Radial geometry sets the feedpoint impedance. With the radials horizontal, an ideal ground-plane presents roughly 37 Ω — the monopole value — a poor match to 50 Ω coax. Sloping the radials downward at about 45° raises the feedpoint impedance to near 50 Ω, giving a low SWR directly on standard coax without a matching network. Drooping the radials also lifts the main lobe slightly and lowers the take-off angle, favouring distant signals near the horizon.
The result is an omnidirectional azimuth pattern, a null overhead, and vertical polarization — matched to the vertical land-mobile signals a scanner listens to. Modest gain over a dipole comes from concentrating radiation into the upper half-space.
Relevance to SDR
For fixed SDR scanning of VHF/UHF trunked systems, a ground-plane antenna is often the best value: it is broadband enough to cover a whole band segment, omnidirectional so it hears all sites at once, and vertically polarized to match the target signals. Many commercial “discone-lite” and dedicated scanner verticals are ground-plane designs.
GopherTrunk decodes whatever the front end delivers and cares only about signal-to-noise. Because a ground-plane antenna can be built for pennies from wire and a coax connector, and mounted high with a clear horizon, it is a common first upgrade over the stock telescopic whip that ships with an RTL-SDR dongle.
In practice
- Cut for the band centre. Element ≈ 7125 / f(MHz) in cm; radials a touch longer than the element.
- Slope radials for match. Horizontal ≈ 37 Ω; ~45° droop ≈ 50 Ω for a clean match.
- More radials, diminishing returns. Four beats three; beyond about four the gain is marginal for an elevated ground-plane.
Sources
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Ground plane — Wikipedia, for the radial ground-plane antenna, its ~37 Ω horizontal feedpoint, and the 45° droop that raises it toward 50 Ω. ↩