chapter  2
3 Impedance and radiation resistance
Pages 1

Because voltage and current appear in an antenna

(a minute voltage and current in the case of a

receiving antenna and a much larger voltage and

current in the case of a transmitting antenna) an

aerial is said to have impedance. Here it’s worth

remembering that impedance is a mixture of

resistance, R, and reactance, X, both measured in

ohms (Ω). Of these two quantities, X varies with

frequency whilst R remains constant. This is an

important concept because it explains why

antennas are often designed for operation over a

restricted range of frequencies.