Quarter Wavelength Antenna Calculator
Calculate the starting radiator and radial lengths for a quarter-wave vertical or ground-plane antenna.
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Inputs
Live
Math
3
Related
Enter parameters and click Calculate to view results
Formula & Theory
Free-Space Quarter-Wave Length (m) = 75 / f(MHz); Practical Cut Length = (75 / f) × Shortening FactorThis formula is used to calculate antenna parameters for quarter wavelength antenna calculator.
Overview
Estimate the starting length of a quarter-wave vertical radiator and matching ground-plane radials. The calculator uses the ideal free-space quarter-wave formula and applies a shortening factor to account for practical conductor effects. Final tuning should always be performed after installation.
Input Guide
Enter Operating Frequency, Element Shortening Factor, Number of Radials exactly in the units shown by this quarter wavelength antenna. Check the operating band, unit prefix, and decimal position before calculating; these are the inputs used by the formula.
- Operating Frequency — use MHz.
- Element Shortening Factor.
- Number of Radials.
Output Guide
The results describe the calculated quarter wavelength antenna values for the inputs you entered. Check each value against the available space, selected components, feed system, and operating conditions before making a final design decision.
How This Calculator Works
The Quarter Wavelength Antenna uses Free-Space Quarter-Wave Length (m) = 75 / f(MHz); Practical Cut Length = (75 / f) × Shortening Factor. Supply Operating Frequency (MHz), Element Shortening Factor, Number of Radials in the displayed units, then use the calculated values as the first engineering target for this wire antennas design or analysis.
Design Notes
A quarter-wave vertical depends on its ground plane, radial configuration, mounting height, nearby conductors, and feed line routing. Radial angle primarily affects feed-point impedance, while conductor diameter and surroundings slightly shift resonance.
Build and Tuning Notes
Cut the radiator and radials slightly longer than calculated, assemble the antenna completely, then measure resonance with an antenna analyzer or SWR meter. Trim the radiator gradually until the desired resonant frequency is reached.
Frequently Asked Questions
How long is a quarter-wave antenna?
The ideal free-space quarter-wave length is 75 divided by the operating frequency in MHz, expressed in metres. Practical antennas are usually shortened slightly using a correction factor.
Should the radials be the same length as the radiator?
Using the same starting length for both radiator and radials is common practice. Final dimensions may differ slightly after tuning depending on the antenna design.
Why does radial angle affect impedance?
Horizontal radials typically produce a feed-point impedance near 36 Ω. Angling the radials downward raises the impedance toward approximately 50 Ω for easier coax matching.
Alex Warren
B.Sc. in Electrical & Electronic Engineering (EEE)
Alex specialises in antenna design and wave propagation. His expertise helps ensure these calculators present practical RF concepts, useful design estimates, and clear engineering guidance for students, HAM operators, and wireless professionals.