Also known as: monopole, quarter-wave monopole, marconi antenna
A monopole antenna is a single straight conductor, classically a quarter wavelength long, driven at its base against a conducting ground plane.1 The ground plane reflects the element to form the electrical equivalent of a half-wave dipole, so a physically short rod behaves like a much larger antenna. Because it is vertical and omnidirectional in azimuth, the monopole is the workhorse form for mobile whips, base verticals, and countless scanner antennas.
How it works
A quarter-wave rod on its own is not resonant, but placed over a large conducting sheet it is completed by its electrical image. Image theory says that a vertical current above a perfect conductor is indistinguishable, in the region above the plane, from that current plus a mirror-image current below it. The rod and its image together look like a centre-fed half-wave dipole, so the monopole resonates at a quarter wavelength — half the physical size of a dipole for the same band.
Two consequences follow from radiating over only the upper half-space. First, all the transmitted or received power is concentrated above the plane, so a monopole has about 3 dB more gain than the equivalent dipole (5.15 dBi versus 2.15 dBi for the ideal case). Second, the feedpoint impedance is halved: an ideal quarter-wave monopole presents roughly 37 Ω at resonance, close enough to 50 Ω coax to give a usable match without a transformer. The radiation pattern is omnidirectional in the horizontal plane and vertically polarized, with a null straight up and peak response toward the horizon — ideal for hearing distant land-mobile sites.
The catch is that the “ground plane” must actually conduct. A real ground plane is never perfect, so its size, conductivity, and the return path it offers all shift the resonance, the pattern, and the take-off angle. On a vehicle the metal roof serves as the plane; on a base vertical a set of radials does the same job electrically.
Relevance to SDR
The monopole and its variants dominate practical SDR scanning. A quarter-wave whip cut for the target band — a length in centimetres of roughly 7125 divided by the frequency in MHz — is the cheapest antenna that actually works for VHF/UHF trunking. A ground-plane antenna is simply a monopole given its own artificial ground, and a mobile whip is a monopole using the vehicle body as the plane.
GopherTrunk is a receive-only decoder and imposes no antenna requirement, but because land-mobile P25, DMR, and NXDN signals are vertically polarized, a vertical monopole matches their polarization and rejects horizontally polarized interference. Getting the element high and clear of obstructions, and giving it an adequate ground plane, improves signal-to-noise more than anything downstream in the radio.
In practice
- Size the ground plane. A plane at least a quarter wavelength in radius approximates the ideal; smaller planes raise the take-off angle and shift resonance upward.
- Elevated feed changes impedance. Sloping the radials of a ground-plane antenna downward raises the ~37 Ω feedpoint toward 50 Ω for a better SWR match.
- Watch common-mode current. Without a good ground return, RF flows on the outside of the coax and distorts the pattern; a choke or proper radial system prevents it.
Sources
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Monopole antenna — Wikipedia, for the quarter-wave monopole, image theory, and its ~37 Ω feedpoint and 3 dB gain over a dipole. ↩