Noise Spectrum Received by an Antenna in a Plasma

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Abstract
A general expression for the noise spectrum received by an antenna in an isotropic plasma is derived by using a combination of plasma fluctuation theory and the reciprocity theorem. The special case of Maxwellian velocity distributions of the constituents is then considered. In the still more special case of thermal equilibrium Nyquist's theorem is shown to be satisfied. It is shown that a 'Maxwellian' energetic electron constituent carrying only a small fraction of the total thermal energy can enhance the spectral power density of antenna noise just above the plasma frequency. The maximum enhancement factor is equal to the square of the ratio of the 'thermal' velocities of the high‐ and low‐energy electrons. There is no such enhancement in the resistance of the antenna.