Antennas › Monopole & Ground Plane

Monopole & Ground Plane Designer

Design a quarter-wave ground-plane antenna or a shortened whip with a loading coil. See how radial droop sets the feed impedance, how much power the ground system burns, and what inductance a short whip needs to resonate.

Inputs
Antenna
Frequency
End-effect factor K
×
Element diameter
mm
Radial system
Radial droop below horizontal
°
Number of radials
×
Ground system and feed
Ground loss resistance Rg
Ω
Feed line impedance
Ω
K shortens the element for its end effect and thickness: about 0.97 for thin wire, 0.95 for rod or tube, 0.93 for fat tubing. Rg is the extra series loss in the ground system; elevated resonant radials are typically a few ohms, a poor ground can be tens of ohms.
Results
Geometry and trends
Radiation pattern
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Dipole / Monopole Calculator → Antenna Unit Converter → 3D Pattern Visualizer → Antenna Theory →

How a monopole over a ground plane works

A quarter-wave monopole above a conducting ground plane radiates like half of a dipole: the ground plane supplies the mirror image. Over a perfect infinite ground it has a feed resistance of about 36.5 Ω (half of 73 Ω), a directivity of 5.15 dBi and a low take-off angle. Real antennas use a finite set of radials instead of a solid plane. Radials that run horizontally leave the feed impedance near 36 Ω; drooping them downward raises it, and a droop of about 45° brings it close to 50 Ω, so a classic ground-plane antenna can be fed directly with 50 Ω coax.

Element length and the end-effect factor

The physical length of a resonant element is shorter than a free-space quarter wavelength because of the end effect of the finite conductor diameter. A factor K of 0.95 is a good starting point for rod or tube; thicker elements need a smaller K. Always trim for the lowest SWR with an analyser, since nearby structures also shift the resonance.

Shortened whips and loading coils

A whip shorter than a quarter wave looks like a capacitor in series with a very small radiation resistance. A base-loading coil cancels that capacitive reactance, so the antenna resonates, but the radiation resistance falls roughly with the square of the height. The ground loss and the coil loss stay about the same, so efficiency drops quickly as the whip gets shorter. Use a high-Q coil, the best ground system you can build, and the tallest whip that is practical.