Space antennas for whistler reception

Abstract

The performance of two satellite-borne antennas, an electric dipole and a loop antenna in detecting Whistlers and VLF emissions above the F 2 peak level of the ionosphere in the frequency range from 100 cps to 100 kc is evaluated. Both types of antennas are inefficient because of their necessarily small sizes. This study establishes a method of selection based upon signal-to-noise ratio. Theoretical relationships are determined, and the expected signal-to-noise ratio of each antenna is compared. The antennas are assumed to be immersed in the ionosphere at typical satellite altitudes. Because the frequency range of interest is wide, matching networks are impractical, and the antennas are assumed to be connected directly to the front end of the receiver. The capacitance of an electric dipole and the radiation resistances of both antennas are modified by the presence of a magneto-ionic medium, where the refractive index is given by the Appleton-Hartree relation. Loop inductance and antenna ohmic losses are not affected by the medium; they are determined solely by geometry. The signal and noise voltages at the input stages are calculated for both antennas; a comparison factor, defined as the electric-dipole S/N ratio divided by the loop-antenna S/N ratio, is determined. Generally, the various resistances of the input circuit are not at the same temperature, and the noise voltage over the bandwidth is derived from Williams' noise equation. The overall comparison factor is then found by including the receiver noise factor as affected by the antenna impedances. A sample calculation is performed by assuming typical antenna dimensions and satellite altitude. It is generally found that the electric dipole performs better than the loop antenna over the frequency range of interest.