An antenna is a transducer that converts between guided electromagnetic waves (in a transmission line) and free-space radiation. Every antenna has a reciprocal receive and transmit behaviour described by the same parameters.

Key Parameters

• Gain (G) — ratio of radiation intensity in the peak direction to that of an isotropic radiator, measured in dBi.
• Directivity (D) — same as gain but ignoring losses. G = η·D where η is radiation efficiency.
• Beamwidth (HPBW) — angular width of the main beam between half-power (−3 dB) points.
• Bandwidth — frequency range over which VSWR < 2 (or another specification).
• Polarisation — orientation of the E-field: linear, circular, or elliptical.
• Radiation resistance — equivalent resistance that would dissipate the same power as is radiated.

Half-Wave Dipole

The reference antenna. Total length ≈ λ/2 (slightly less due to end effects). Gain = 2.15 dBi. Radiation resistance ≈ 73 Ω. Omnidirectional in the plane perpendicular to its axis.

Patch (Microstrip) Antenna

A rectangular conductor on a PCB substrate, resonant when the patch length L ≈ λ/(2√εreff). Gain ≈ 5–8 dBi, bandwidth 2–5% (can be increased with thicker substrates). Widely used in GPS, WiFi, and cellular handsets due to its flat, PCB-compatible form factor.

Array Factor

When N identical elements are arranged in a line with spacing d and progressive phase shift δ, the combined pattern is the element pattern multiplied by the array factor AF(θ). The main beam can be steered electronically by changing δ without physically moving the array — the principle of phased array radar and 5G beamforming.