The transmitting and the receiving patterns of a dipole antenna for an electron plasma wave have been measured by using the interference method The results agree fairly well with the theoretical patterns that were predicted from the fluid model analysis and also experimentally prove the reciprocity relationship between the transmitting and the receiving patterns, which is theoretically found to be valid in the plasma wave approximation The characteristics of the transmitting and the receiving efficiencies versus the dc bias voltage applied to the dipole antenna have been also experimentally investigated

Measurements of the transmitting and the receiving patterns of a dipole antenna for an electron plasma wave

Absolute electric field measurements of a 2 GHz electromagnetic wave in a nonuniform plasma are described. A remote measurement technique with minimum perturbations is used in which the information is obtained at the launching antenna from the backscattered signal of a small movable dipole in the radiation field. The weak scattering signal of the test dipole is discriminated against reflections from metallic walls and the plasma cutoff layer by introducing a small frequency shift (Δω/ω≃10−3) at the dipole. The dipole contains a passive nonlinear reactive element (varactor diode) which generates multiple sidebands when irradiated with two frequencies. The nonlinear behavior of the sidebands is used to obtain an absolute calibration for the electric field strength at the dipole. Mismatch effects due to the density dependent dipole radiation resistance are minimized by terminating the dipole with a high impedance.

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Nonlinear scattering dipole for electromagnetic field measurements in plasmas

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.

The sleeve monopole antenna in a warm plasma

The current distribution of a center-fed dipole cylindrical antenna of arbitrary dimension immersed in unbounded lossless warm plasma is considered, taking into account the commonly neglected end-cap effect. The approximated solution of the integral equation for the current distribution is obtained with the aid of Fourier analysis, and the expression is derived for the current distribution I(z) as the function of position z along the cylindrical surface of the antenna. Sample calculations are made for a half-wavelength dipole antenna with different values of thickness parameter ?=(a/l)<0.25 and of the normalized plasma density parameter po=(?p/?)2 <1. Here a and l denote the radius and the half-length of the antenna, respectively. ?p and ? denote the plasma angular frequency and the radiation angular frequency. The variation of the amplitude and phase of the current with the position, and the variation of radiation resistance with po and ? are examined and discussed in detail.

Current Distribution of a Cylindrical Antenna in a Warm Plasma

Experimental results on the input impedances and the current distributions of cylindrical antennas of various lengths in a large volume of hot lossy plasma are presented. A relatively simple theory based on an integral equation and a grid antenna model is developed. The theoretical results are in good qualitative agreement with the experiments. The theory is also compared to theories of some other researchers. Because of its simplicity, the present theory may prove useful in practical applications.

K. Chen

Papers

An experimental and theoretical study on the circuit properties of a cylindrical antenna in a hot lossy plasma