Abstract: An amplitude modulated transmitter is disclosed for use in the frequency range generally below 50 mega-hertz. The transmitter is provided with a series diode modulator which is provided with a corrected modulating signal which biases the diode in proportion to the modulating signal. The modulating signal is derived from comparison in a difference amplifier between the desired audio input signal and a signal derived from the output of the output amplifier by means of a directional coupler. The directional coupler samples the magnitude and phase of the current and voltage at the output and feeds a signal proportional to the inciden as contrasted to any reflected component, of the output signal.