Quarter-Wave Antenna Length

Formula

Description

A quarter-wave antenna (also called a quarter-wave monopole or ground plane antenna) is one of the most common antenna types in amateur and commercial radio. Its electrical length is one quarter of the wavelength at the operating frequency, adjusted by the velocity factor which accounts for the fact that electromagnetic waves travel slower in conductors than in free space. The velocity factor for bare wire is approximately 0.95, while coaxial elements may have VF of 0.66-0.82 depending on the dielectric material.

Variables

• l — Physical antenna length (m)
• f — Operating frequency (Hz)
• VF — Velocity factor (0-1, typically 0.95 for wire)

Practical Notes

A quarter-wave monopole over a ground plane has approximately 36Ω impedance and requires a ground plane or counterpoise for proper operation. For the 2-meter amateur band (146 MHz), the quarter-wave length is about 0.49 m. For HF bands, antennas become much longer: 40 meters band (7 MHz) needs about 10.2 m. Quarter-wave stubs are also used as notch filters and impedance transformers in feed systems.