How does a half-wave dipole antenna work?
A half-wave dipole is a center-fed conductor about λ/2 long. In free space its input impedance at resonance is about 73 + j42.5 Ω for an infinitely thin exact half wave; shortening it slightly (typically to about 0.47–0.48λ, depending on thickness) cancels the reactance. Its directivity is 1.64 (2.15 dBi) with a doughnut-shaped pattern having nulls along the wire axis and a 78° half-power beamwidth.[1]
How long should a dipole be?
A free-space half wavelength is λ/2 = 150/f(MHz) meters; practical resonant length is a few percent shorter because of end effects and conductor diameter. Amateur radio handbooks give approximately 143/f(MHz) m for wire dipoles; trim on a VNA for the installed environment.[2]
How is a dipole fed?
A dipole is balanced; feeding it from coax without a balun or choke lets current flow on the coax shield, skewing the pattern. Folded dipoles have about four times the impedance (~280–300 Ω) and pair with 4:1 baluns.[1]
What variants exist?
- Short dipole: Rrad = 20π²(ℓ/λ)² Ω, directivity 1.5.
- Sleeve dipole and coaxial dipole for vertical installations.
- Printed dipoles in arrays and Yagi–Uda directors/reflectors.
- Inverted-V and fan dipoles for multiband HF.
Frequently asked questions
What is the impedance of a half-wave dipole?
About 73 Ω resistive near resonance, plus a reactive part that depends on exact length.
Why is the dipole gain 2.15 dBi?
Its directivity is 1.64 relative to isotropic.
Does ground affect a dipole?
Yes; height above ground changes impedance and pattern.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.
- “The ARRL Handbook for Radio Communications”. ARRL. annual editions.