Half-Wave Dipole
Understanding the Half-Wave Dipole
The half-wave dipole is where antenna theory begins. Its behavior is fully described by Maxwell's equations, and it serves as the reference for gain measurements and the building block for Yagi, log-periodic, and many array antennas.
Half-Wave Dipole Properties
- Length: L = lambda/2 (adjusted slightly shorter for finite conductor diameter).
- Input impedance: 73 + j42.5 ohms at exact lambda/2. Trim slightly short for purely real impedance.
- Gain: 2.15 dBi (1.64 linear).
- Radiation pattern: Omnidirectional in the H-plane (donut-shaped). Null along the dipole axis.
- Bandwidth: 5-10% for VSWR < 2. Increases with thicker conductors.
Frequently Asked Questions
What is a half-wave dipole?
Two quarter-wave conductors fed at center. Impedance: 73 ohms. Gain: 2.15 dBi. Donut-shaped pattern. The fundamental resonant antenna and reference for all antenna measurements.
Why is the dipole impedance 73 ohms?
73 ohms comes from solving the integral of the far-field radiation pattern and equating radiated power = I^2 * R_rad. The 73 ohms is the radiation resistance, which is the ratio of radiated power to the square of the feed current.
How do you match a dipole to 50 ohms?
A 73-ohm dipole has VSWR = 1.46 in a 50-ohm system. This is acceptable for many applications. For better match: use a folded dipole (300 ohms, then a 4:1 balun to 75 ohms), a gamma match, or a matching network.