antenna input impedance

Antenna input impedance is what the transmitter 'feels' when it pushes power into the antenna's terminals — a combination of useful resistance (energy that flies away as radio waves) and reactance (energy that sloshes back and forth without leaving). It is the matchmaking number: if it doesn't match the feed line's impedance (usually 50 ohms), power bounces back toward the transmitter instead of escaping into the air, like shouting into a wall.

The resistive part splits into radiation resistance — the good kind, representing radiated power — and loss resistance from heat in the metal and ground. A perfectly tuned half-wave dipole shows about 73 ohms of nearly pure resistance, while a too-short antenna looks capacitive and a too-long one inductive. Designers add matching networks or trim the antenna length to drag the impedance onto the feed line's value, minimising reflected power and maximising what actually radiates.