What is a half-wave dipole antenna?
A half-wave dipole is a straight conductor about half a wavelength long, fed at the center. With a sinusoidal current distribution it has a directivity of 1.64 (2.15 dBi), an E-plane half-power beamwidth of about 78°, and an input impedance of about 73 + j42.5 Ω at exactly λ/2; trimming it slightly shorter (about 0.47–0.48λ, depending on thickness) makes it resonant near 70 Ω.[1][2]
How long should a half-wave dipole be?
The free-space half wavelength is 149.9/f(MHz) meters. Because of end effects and finite conductor diameter, practical wire dipoles are cut a few percent shorter; the amateur-radio rule of thumb is L(m) ≈ 142.6/f(MHz), equivalent to 468/f(MHz) feet, then trimmed while measuring.[3] Thicker elements need more shortening and gain bandwidth.
What is its radiation pattern?
The normalized far field is Eθ ∝ cos((π/2)cosθ)/sinθ, a doughnut with nulls along the wire axis and maximum broadside.[1] Mounted vertically it is omnidirectional in azimuth; horizontally over ground its elevation pattern depends strongly on height because of the ground reflection.[3]
How is a dipole fed?
A dipole is a balanced antenna. Connecting coaxial cable directly allows current on the outside of the shield, which distorts the pattern and makes the cable radiate. A balun (e.g., a 1:1 current balun or sleeve balun) or choke suppresses these common-mode currents.[1]
What are common dipole variants?
- Folded dipole: about 4× the impedance, wider bandwidth.
- Inverted-V: center raised, ends lowered; lower impedance, more omnidirectional.
- Sleeve and coaxial dipoles: coaxially fed vertical dipoles.
- Bow-tie and biconical: wideband fat dipoles.
- Printed dipoles on PCBs, often with integrated baluns.
Frequently asked questions
Is a half-wave dipole 50 Ω?
It is close to 70 Ω in free space; 50 Ω coax gives a VSWR of about 1.4:1, usually acceptable.
What is the gain of a dipole in dBd?
0 dBd by definition, 2.15 dBi.
Can a dipole work on harmonics?
Yes on odd harmonics with low impedance; at even harmonics the center impedance is very high.
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.
- “Antenna Theory and Design, 3rd ed.”. W. L. Stutzman and G. A. Thiele, Wiley. 2012.
- “The ARRL Antenna Book for Radio Communications”. ARRL. multiple editions.