Demodulation of Amplitude Modulated RF Waves in a Plasma at Resonance

Abstract

A laboratory simulation experiment was performed to study the mechanism of the detection of audio waves from nearby radio broadcasting stations by spacecraft‐borne audio detectors in the ionosphere, where the carrier frequencies of the detected radio stations seem to depend on the altitude of space probes within the ionosphere. Two electrodes were immersed in a low density plasma, and an RF wave amplitude modulated by an audio wave was fed to one of them while the other electrode was used to detect the audio wave. The detection of the audio wave is a result of the rectification and integration by the sheath around the electrode (antenna), which has a nonlinear voltage‐current characteristic. The dependence of the carrier frequency on the altitude is explained as a result of series resonance of an inductive plasma with the sheath capacitance. This is just the same as the ordinary resonance probe and the resonance frequency is slightly smaller than the plasma frequency. For a higher modulation frequency, the audio output has a phase lag caused by the integration by a high plasma and sheath impedance. The audio output has a strong dependence on the potential of the receiving electrode (antenna) with respect to the plasma, and it reverses the polarity at the potential where the second derivative with respect to the voltage of the voltage‐current characteristic of the receiving electrode in plasma, is equal to zero. For higher potentials, the amplitude of the audio output shows a large broad maximum, and more than an order of magnitude larger output can be obtained than with the ordinary resonance probe.