The Fundamental Radiator
The dipole antenna is where antenna engineering begins. Heinrich Hertz used a dipole to first demonstrate electromagnetic waves in 1887, confirming Maxwell's predictions. Today, the half-wave dipole remains the building block of antenna theory — its properties are analytically tractable, its construction is trivial, and its radiation pattern serves as the reference against which all other antennas are measured.
Radiation Pattern & Directivity
A half-wave dipole radiates most strongly perpendicular to its axis, creating a toroidal (donut-shaped) pattern. Along the wire axis, radiation drops to zero — these are the nulls. The directivity of 2.15 dBi means the peak radiation is 1.64 times stronger than an isotropic radiator with the same total power. This modest directivity is sufficient for many applications but motivates the development of arrays and reflectors for higher-gain systems.
Impedance & Matching
At exactly half-wave resonance, the dipole input impedance is purely resistive at approximately 73 + j42.5 Ω. In practice, the antenna is shortened slightly (to about 0.48λ) to eliminate the reactive component, yielding a purely real impedance near 70 Ω. This makes matching to 50 or 75 Ω transmission lines straightforward with a simple balun transformer, explaining the dipole's enduring popularity.
From Dipoles to Arrays
While a single dipole has limited directivity, arrays of dipoles can achieve remarkable beam control. The Yagi-Uda antenna uses parasitic dipole elements to create a directional beam. Collinear arrays stack dipoles vertically for omnidirectional horizontal gain. Log-periodic arrays arrange dipoles of varying length for broadband operation. Every advanced antenna design traces its lineage back to understanding the humble half-wave dipole.