What is a Dipole Antenna?
A dipole antenna is one of the simplest and most widely used antenna types. It consists of two conductive elements (usually wires or rods) arranged collinearly, with a feedpoint at the center. The half-wave dipole, whose total length is approximately half a wavelength, is the most common configuration. It radiates electromagnetic waves in a doughnut-shaped pattern perpendicular to the antenna axis.
The half-wave dipole is a fundamental reference antenna in radio engineering. Its gain of 2.15 dBi (relative to an isotropic radiator) serves as a baseline for measuring other antenna gains. The impedance at the feedpoint of a half-wave dipole in free space is approximately 73 ohms, making it reasonably well-matched to common 75-ohm transmission lines and close enough to 50 ohms for practical use.
Design Formula
The factor 0.95 (or 468 for feet/MHz) accounts for end effects and the velocity factor of the antenna conductor. Actual resonant length depends on wire diameter, height above ground, and nearby objects. Thicker wire results in wider bandwidth but slightly shorter resonant length.
Frequency and Length Reference
| Band | Frequency | Dipole Length |
|---|---|---|
| AM Broadcast | 1 MHz | 142.5 m |
| Shortwave | 14 MHz | 10.18 m |
| VHF (2m) | 145 MHz | 0.98 m |
| UHF (70cm) | 440 MHz | 0.32 m |
Dipole Types
- Half-wave: Most common, resonant, ~73 ohm impedance
- Folded dipole: ~300 ohm impedance, wider bandwidth
- Inverted V: Legs angled down, lower feedpoint, easier mounting
- Fan dipole: Multiple dipoles fed from same point for multiband operation
FAQ
Why is the actual length shorter than half a wavelength?
The physical length of a dipole is about 95% of the theoretical half-wavelength due to end effects. Current must go to zero at the wire ends, and the finite diameter of the conductor causes the effective electrical length to be slightly longer than the physical length. The correction factor varies from about 0.93 to 0.97 depending on the length-to-diameter ratio.
What feed impedance should I expect?
A half-wave dipole in free space has a feedpoint impedance of about 73 + j42.5 ohms at resonance. Near ground, this changes significantly. At half-wavelength height above ground, impedance can range from 50 to 90 ohms depending on ground conductivity. Trim the antenna for minimum SWR with your specific feedline.
How does height above ground affect performance?
Height dramatically affects both impedance and radiation pattern. At low heights (below quarter wavelength), radiation is primarily upward. At heights of half-wavelength to one wavelength, the pattern begins to develop lower-angle lobes useful for long-distance communication. Generally, higher is better for most applications.