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Antenna definition and one type

Short answer

An antenna converts guided electrical energy into radiated waves and vice versa. The half-wave dipole (length ≈ λ/2) has sinusoidal current distribution, omnidirectional doughnut pattern with 78° HPBW, directivity 1.64 (2.15 dBi), radiation resistance ~73 Ω, and linear polarization.

Give the definition of an antenna and explain the properties of any one type of antenna that you have studied during your electromagnetism course.

Definition

An antenna is a device that converts electrical energy from a transmission line into electromagnetic waves radiated into space (transmitting) and converts incoming electromagnetic waves into electrical signals (receiving). It is the transition between guided waves and free-space waves.

Half-wave dipole antenna: properties

A half-wave dipole consists of two straight conductors, each about λ/4 long (about λ/2 in total), fed at the centre.

 <------ λ/4 ------> <------ λ/4 ------>
 ===================o o===================
                    feed
                 (centre)
  • Current distribution: approximately sinusoidal, maximum at the centre and zero at the ends: I(z) = I₀ cos(kz).
  • Radiation pattern: E_θ ∝ cos(π/2 cosθ)/sinθ. It is doughnut-shaped: zero along the axis and maximum in the plane perpendicular to the wire (omnidirectional in azimuth).
  • Half-power beamwidth: 78° in the elevation plane.
  • Directivity: 1.64 (2.15 dBi); the gain is close to this because the efficiency is high.
  • Radiation resistance: about 73 Ω (input impedance 73 + j42.5 Ω; it becomes purely resistive when the length is trimmed to about 0.95 × λ/2).
  • Polarisation: linear, parallel to the dipole axis.
  • Bandwidth: fairly narrow (about 10 %); thicker conductors increase it.
  • Effective aperture: A_e = 0.13λ².
  • Widely used in TV and FM radio reception, and as a reference antenna for gain measurements.

✓ Answer checked by Claude Opus 5.5 (high effort)

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