A type of circuit operating at radio or microwave frequencies. At such frequencies, traditional discrete resistive and reactive components cannot be used because their dimensions are similar to the wavelength of the signal, resulting in a spatial variation of current and voltage in the component or circuit. Resistance and reactance are therefore realized using transmission-line elements, and the resistance and reactance are distributed in space along the circuit. The voltages and currents in the circuit are treated as travelling waves that flow along the transmission line. See transmission line; waveguide; power waves.