How antennas radiate

An antenna is a transducer: it converts a guided electrical signal on a wire or cable into electromagnetic waves travelling through free space, and vice versa.

From current to waves

When an alternating current flows in a conductor, it sets up changing electric and magnetic fields around it. At low frequencies those fields stay bound to the wire. But when the conductor is a meaningful fraction of the signal’s wavelength, the changing fields detach and propagate outward as a self-sustaining wave that travels at the speed of light.

Reciprocity

The same structure works in both directions. A wire that efficiently launches a wave will, when a passing wave washes over it, efficiently induce a matching current for a receiver to detect. This principle, called reciprocity, means an antenna’s transmit and receive behaviour (its pattern, gain, and impedance) are identical.

Why size tracks wavelength

Because radiation only becomes efficient when the conductor is comparable to the wavelength, antenna size is tied to frequency. A half-wave dipole for the AM band is hundreds of metres long; the same design for Wi-Fi is a few centimetres. The half-wave dipole is the canonical example we use to anchor every other idea in this course.