Antenna radiation patterns of balanced electric dipoles and shielded magnetic loop antennas are obtained by measuring the relative wave amplitude with a small receiver antenna scanned around the exciter in a large uniform collisionless magnetized laboratory plasma in the whistler wave regime. The boundary effects are assumed to be negligible even for many farfield patterns. Characteristic differences are observed between electrically short and long antennas, the former exhibiting resonance cones and the latter showing dipole-like antenna patterns along the magnetic field. Resonance cones due to small electric dipoles and magnetic loops are observed in both the near zone and the far zone. A self-focusing process is revealed which produces a pencil-shaped field-aligned radiation pattern.

Antenna radiation patterns in the whistler wave regime measured in a large laboratory plasma

The last five years have witnessed remarkable progress in the theory and measurement of both the radiation and impedance properties of antennas in plasmas. Increased motivation for research in this area has been provided by the Space Shuttle program and by the prospect of nuclear fusion. The focus of attention has been on resonance cones in linear, anisotropic plasmas, including radiation patterns, wave interference, pulse propagation, reflections from boundaries and inhomogeneous media effects. Under nonlinear conditions, the focussed field of a resonance cone can significantly depress the plasma density. Under both linear and nonlinear conditions, the input impedance of dipole and loop antennas has been studied extensively, for both anisotropic and isotropic plasmas. A continuing challenge has been the as-yet-not-fully-explained experimental observation of linear, non-collisional enhanced resistivity of the sheath region around an antenna. Numerical impedance calculations employing simplified velocity distributions have shown particular promise. Ion and electron wave radiation patterns for various antenna shapes have been calculated and checked experimentally. The response of both single antennas and pairs of antennas to plasma fluctuations has been studied and found to have applications to plasma diagnostics.

The properties of antennas in plasmas

The interpretation of the experiment EIDI I, described in a companion paper, is undertaken. A relatively complete calculation of the impedance is done including the effect of the temperature of the plasma by using the Vlasov equation and hydrodynamic equations, the structure of the sheath using a simple model of it, the collision frequency computed for an equilibrium plasma, and an estimation of various other small effects (wake, ionospheric gradient, motion of ions, …). The result so obtained is in very good agreement with the experiment for the most part but a strong discrepancy appears in the resistance when X > 1. This indicates that the main features of the theory are correct except that a loss mechanism is missing; some hypothetical ones are discussed.

The impedance of a dipole antenna in the ionosphere: 2. Comparison with theory

In this paper, we proposed a semianalytical method for calculating the current distribution and input impedance of a very low frequency (VLF: 3-30 kHz) linear antenna of arbitrary orientation in a homogeneous anisotropic cold plasma. By considering the effect of the geomagnetic inclination angle, the kernel function, in this case, has a more complicated form and requires extra analytical techniques to deal with. The computations show that the amplitude coefficients for the ordinary wave are evidently greater than those for the extraordinary wave. We also found that the shape of the current distribution is not sensitive to the orientation of the antenna, but the total current moment on the antenna will be decreased when the inclination angle becomes larger. Moreover, due to the higher attenuation rates for both the ordinary and extraordinary waves at a propagation direction perpendicular to the magnetic field, the overall trend for the input impedance of the antenna is increasing with the geomagnetic inclination angle. It is then concluded that the optimal posture for a VLF space-borne linear antenna should be as parallel as possible to the direction of the geomagnetic field in order to achieve maximum antenna efficiency.

/pdf/vlf-current-distribution-and-input-impedance-of-an-1ns3nmr5it.pdf

VLF Current Distribution and Input Impedance of an Arbitrarily Oriented Linear Antenna in a Cold Plasma

A boundary-value problem for a dipole immersed in a warm isotropic plasma has been solved using the multiple water bag (MWB) and fluid models of the plasma. The MWB model contains information on the electron distribution function, and hence gives results in agreement with the kinetic theory of the warm plasma. Comparing results from the MWB model with those from the fluid model, we find that the latter are correct only when the antenna frequency ω is sufficiently above the plasma frequency ωp. The current distribution and the driving-point impedance are calculated for dipoles of several lengths. We find that the current distribution is triangular only when ω is well above ωp or when ω < ωl where ωl, is the frequency at which a resonance occurs in the driving point admittance; ωl is less than ωp and decreases with the increasing length of the dipole. The current distribution for frequencies close to ωp is oscillatory, with a complex wavenumber which is close to the wavenumber of the third-order Landau mode. Only for very short dipoles with lengths less than 10 Debye lengths or so can the current distribution be approximated by a triangular one for all frequencies. The calculated frequency variation of the dipole resistance with frequency based on the MWB model is similar to those measured experimentally; for frequencies just below ωp the theory in this paper predicts a much larger resistance for the dipole than previous theories.

A boundary‐value problem treatment of an electric dipole in a warm isotropic plasma using the multiple water bag model

The impedance of a short electric dipole antenna has been measured from rockets in the ionosphere. The EIDI I experiment was launched in conditions where the plasma was expected to be nearly isotropic (operating frequencies 4.48 and 6.09 MHz, f0F2 ⋍ 9 MHz, fH ⋍ 1 MHz). It was designed to detect and measure or to eliminate most of the unwanted effects which complicated the data reduction of previous experiments: outgassing, telemetry radiation, etc. The number of parameters measured simultaneously on board (impedances with variable bias and RF excitation level, resonance frequencies, Te, dc current drawn by the antennas, vehicle potential) and from the ground or an overhead satellite was large enough to provide numerous cross checks and to determine the sheath dimensions.

The impedance is very weakly spin-modulated over most of the flight; the collision frequency is very low (v/ω ⋍ 10−5) unlike some of the previous experiments where collision losses were dominant. The measured values of the impedance are given as a function of X=f2p/f2 where the other parameters are known.

N. Vernet

Papers

The impedance of a dipole antenna in the ionosphere: 1. Experimental study