A theoretical analysis and experiments on one‐ and two‐element ring antennas in an isotropic compressible plasma are repeated. First, a two‐element ring antenna connected to the inner and outer conductors of a coaxial feeder is analyzed by decomposing the system into a zero phase type and a positive phase type. The analysis uses Fourier series expansion. Numerical results reveal that in both one‐ and two‐element systems the current is approximated by a triangular distribution, i.e., a charge with a uniform distribution, if the ring diameter is much smaller than the free space wavelength. In the case of two‐element systems, the charge distributions of elements #1 and #2 are out of phase. The input impedance of the two‐element ring is much smaller than that of the one‐element system. The measured and calculated radiation patterns of a one‐element ring for the electron plasma wave agree well except near θ = 90°. The side‐lobe level of the two‐element ring is much lower than that of the one‐element ring and the radiation characteristics are much improved. It is theoretically and experimentally confirmed that the radiation efficiency of the two‐element ring is much better than that of the one‐element ring for electron plasma waves.
A sleeve monopole antenna whose length is comparable to a plasma wavelength has been studied both theoretically and experimentally in the frequency region for which there is electron plasma wave propagation. In taking account of Landau damping, it is found that if the sleeve length is larger than 25 Debye lengths, for the monopole of length 4 cm with a radius of 1 mm, then the input impedance becomes insensitive to further increase in sleeve length. The theoretical input impedances are found to be in good quantitative agreement with the experimental results.
A short, coil-loaded two-element ring antenna including a transmission line, immersed in a warm, isotropic plasma, is investigated as a boundary value problem in the frequency range where electron plasma waves exist. The wave potential of electron plasma waves launched from a two-element ring antenna has been enhanced by a factor of 4-6 in comparison with the case of an unloaded one-element ring antenna, when a suitable inductive load is inserted at the feeding point of element 2, and the distance between element 1 and element 2 is chosen as an optimum element spacing. Theory is substantiated by the experiments. In addition, the unidirectional radiation patterns of the antenna for electron plasma waves are predicted by the present theory.
A sleeve monopole antenna in a warm plasma with length almost the plasma wavelength has been analyzed in the frequency region where electron plasma waves exist. The current distribution and the input impedance have been obtained. When Landau damping is not considered, the current distribution is similar to that for a dipole and oscillates violently near the plasma frequency. It approaches a triangular shape as the frequency is increased. The input impedance varies considerably near the plasma frequency depending on the sleeve length. On the other hand, if Landau damping is taken into account, the amplitude of the oscillatory component of the current distribution becomes extremely small. For the input impedance, the effect of the sleeve beyond 25 times the Debye length can be neglected. This result agreed well with the measured input impedance values. It is found that the sleeve as the external conductor of the coaxial feeder has the effect of reducing the input impedance.
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