Also known as: ground plane antenna, ground-plane vertical, GPA
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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.
Where to buy
For a fixed VHF/UHF scanning station, a commercial base vertical like the Diamond X50NA (around $85) is the easy upgrade over a stock whip: an elevated, self-contained gain vertical with its own decoupling section, so it mounts to a mast with no radial field to lay out and hears every trunking site at once. Its vertical polarization matches land-mobile traffic, and no antenna — this one included — can recover AES-encrypted audio; the antenna only decides how well the signal reaches the SDR.
For a wideband alternative, a higher-gain collinear, or the single outdoor pick, see the base scanner antenna page and the best scanner antenna guide.
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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. ↩
Frequently asked questions
Which ground-plane or base vertical antenna should I buy for scanning?
For a fixed VHF/UHF scanning station the Diamond X50NA (around $85) is a solid, self-contained base vertical: an elevated gain vertical with its own decoupling section, so it hears every trunking site at once without needing a rooftop full of radials. It is vertically polarized to match land-mobile P25, DMR, and NXDN traffic. If you want the widest possible frequency coverage instead of band-optimized gain, a discone or the outdoor pick on the base scanner antenna page is the alternative.
Ground plane versus discone for a scanner?
A ground-plane or gain vertical is optimized for one band (or a dual band) and gives more signal there; a discone trades gain for sheer bandwidth and covers a decade of frequency from one feedline. If you mostly watch one region’s VHF or UHF trunking, a base vertical hears better; if you sweep everything from air band to 800 MHz, a discone is more flexible.
Do I still need to lay out radials with a commercial base vertical?
No. A packaged base vertical like the X50NA supplies its own artificial ground — a decoupling sleeve or an integrated radial set at the base — so you just clamp it to a mast. A home-brew quarter-wave ground plane, by contrast, needs three or four radials about a quarter wavelength long to work against.
How high should I mount a ground-plane antenna?
As high and clear as practical. A ground-plane vertical is omnidirectional and radiates toward the horizon, so height and an unobstructed sky view matter more than aiming. Keep the feedline short and low-loss — a long thin coax run can throw away the gain you just paid for.