Quarter-Wave Ground Plane Antenna Calculator
Enter your target frequency and build parameters to get practical starting dimensions for a vertical ground plane antenna.
What this ground plane antenna calculator does
This calculator gives you a practical starting point for a quarter-wave ground plane antenna. You enter frequency and a few design choices, and it returns suggested lengths for the vertical radiator and each radial, plus useful engineering estimates like wavelength, impedance trend, and mismatch SWR to 50 ohms.
It is meant for fast planning before cutting wire or tubing. Final trimming should always be done with measurement tools such as an antenna analyzer or SWR meter at the installed location.
Ground plane antenna basics
A classic ground plane antenna is a quarter-wave vertical radiator with several conductive radials at the base acting as an artificial ground. This design is simple, efficient, and widely used in VHF/UHF and HF installations.
Main parts
- Vertical element: The driven quarter-wave radiator.
- Radials: Usually 3 to 4 (or more), each near quarter-wave length.
- Feed point: Center conductor to vertical element, shield to radial hub.
- Mast/support: Non-conductive at the feed region is usually preferred for predictable tuning.
Why radial angle matters
With radials perfectly horizontal, feed impedance is often near 36 to 37 ohms. Drooping radials downward increases feed impedance. Around 45 degrees is a common target because it often moves impedance toward 50 ohms, which matches standard coax well.
Formulas used by the calculator
The calculator uses the speed-of-light wavelength relationship and then applies a user-defined velocity/end-effect factor for practical cut length:
- Wavelength: λ = c / f
- Electrical quarter-wave: λ/4
- Physical starting length: (λ/4) × velocity factor
It also provides a simple impedance estimate based on radial droop angle. That estimate is approximate and intended for planning, not final certification-grade design.
How to use the results in real builds
Step 1: Cut slightly long
Even with good formulas, nearby structures, feedline routing, and mounting details shift resonance. Cut each element a bit longer than calculated, then trim gradually.
Step 2: Install in final environment
Tune where the antenna will actually operate. A test on a workbench can differ significantly from rooftop or mast-mounted performance.
Step 3: Measure and trim
- Check resonance and SWR at target frequency.
- Trim the radiator in small increments.
- Adjust radial angle if feed impedance is not where you want it.
- Re-check after weatherproofing and final cable routing.
Typical practical guidance
- 2 meter band example: Around 146 MHz, quarter-wave dimensions are in the ~19 inch range before/after practical factor adjustment.
- UHF example: At 446 MHz, elements are much shorter, so construction tolerances become tighter.
- HF example: At lower frequencies, element lengths grow quickly and mechanical support becomes a major part of the design.
Common mistakes to avoid
- Ignoring nearby metal objects and mast coupling effects.
- Using poor feedpoint connections or inadequate strain relief.
- Skipping choke/balun considerations at the feedpoint when needed.
- Assuming calculator output is final instead of a starting point.
Final notes
This tool is intentionally practical: fast estimates, realistic defaults, and clear starting dimensions. For mission-critical, high-power, or multi-band systems, validate with simulation and professional measurement workflows. For hobby and field builds, these numbers will usually get you very close and save substantial trial-and-error time.