Theory, design, and applications of the most fundamental antenna structure in wireless communications. From Hertz to modern WiFi.
Understand electromagnetic radiation mechanism and current distribution
Calculate radiation patterns, impedance, and directivity
Design dipole antennas for specific frequency bands
Implement practical dipole configurations and matching
The dipole antenna consists of two conductive elements fed at the center, forming the simplest and most widely used antenna structure.
A dipole antenna is a symmetrical antenna consisting of two straight conductive elements (arms) arranged collinearly with a small gap at the center where the transmission line is connected. The total length l determines its electrical characteristics.
For a thin dipole with sinusoidal current distribution, the current along the antenna is given by:
where k = 2π/λ is the wave number, Im is the maximum current, and z is the position along the antenna axis (-l/2 ≤ z ≤ l/2).
Important: The current must be zero at the ends (z = ±l/2) and maximum at the center for a half-wave dipole.
The dipole antenna was first demonstrated by Heinrich Hertz in 1886 during his experiments proving the existence of electromagnetic waves.
Total length l = λ/2. Most popular configuration with input impedance ≈ 73 Ω.
Half dipole over ground plane. Image theory applies. Impedance ≈ 36.5 Ω.
Two parallel dipoles connected at ends. Input impedance ≈ 300 Ω.
Infinitesimal dipole (Hertzian dipole). Used as theoretical reference.
Length = λ. High input impedance, difficult to match. Null at broadside.
Cylindrical sleeve acts as balun. Wide bandwidth, commonly used in VHF/UHF.
The radiation pattern of a dipole antenna depends on its electrical length. Use the interactive tool below to explore how the pattern changes with dipole length.
Figure-8 pattern in E-plane, omnidirectional in H-plane. Maximum radiation broadside to antenna.
Null appears at broadside. Four lobes with maximum radiation at ±45°.
Six lobes, more complex pattern. Multiple nulls in the radiation pattern.
Where Rr is the radiation resistance
At resonance, reactance XA = 0
For half-wave dipole
Half-power beamwidth
Half-wave dipole impedance (~73Ω) differs from standard 50Ω coaxial cable.
Balanced-to-unbalanced transformer required to connect coaxial cable to dipole.
Dipole bandwidth affected by wire diameter and length-to-diameter ratio.
Identify center frequency f₀ (e.g., 100 MHz for FM radio)
λ = c / f₀ = 300 / f(MHz) meters
l = 0.5 × λ × k
where k ≈ 0.95 (velocity factor for wire dipole)
Typical: λ/100 to λ/1000. Thicker wire = wider bandwidth
Implement balun and matching network for 50Ω or 75Ω feed line
Broadcast reception, "rabbit ears" TV antennas
2.4 GHz and 5 GHz wireless communications
Base station antennas, sector antennas
VHF communication, navigation systems
The dipole antenna remains the fundamental building block of antenna engineering. Understanding its radiation mechanism, impedance characteristics, and design principles provides the foundation for more complex antenna systems used in modern wireless communications.