What is a ground-plane antenna?
A ground-plane antenna is a quarter-wave vertical monopole whose ground is formed by several (often three or four) radial wires about λ/4 long, rather than a solid plane. With horizontal radials the feed impedance is well below 50 Ω; drooping the radials downward toward about 30–45° below horizontal raises it to near 50 Ω.[1][2]
How many radials are needed?
For an elevated ground plane, a few resonant radials (three or four) form an adequate counterpoise because the system behaves much like an asymmetric dipole. For a vertical at ground level, the radials couple to lossy earth and many (tens of radials) are used to reduce ground loss, as in MF broadcast practice of 120 buried radials.[1]
Why droop the radials?
Sloping the radials downward makes the structure more like a vertical dipole: the impedance rises toward that of a dipole and the pattern becomes more symmetric. A sleeve or skirt instead of radials gives the coaxial (sleeve) dipole.[2]
What about feed-line radiation?
With few radials, current can flow on the outside of the coax shield. A choke balun at the feed point or a set of λ/4 decoupling radials reduces this; see common-mode currents.[1]
Frequently asked questions
Can radials be shorter than λ/4?
Yes, if loaded, but bandwidth and efficiency usually suffer.
Are buried radials resonant?
No; ground contact detunes them, so length is less critical and more radials help.
Is a ground-plane antenna omnidirectional?
Yes in azimuth, with low-angle radiation useful for VHF/UHF ground coverage.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “The ARRL Antenna Book for Radio Communications”. ARRL. multiple editions.
- “Antennas for All Applications, 3rd ed.”. J. D. Kraus and R. J. Marhefka, McGraw-Hill. 2002.