Keith G. Balmain

The impedance of a short dipole antenna in a magnetoplasma

A formula for the impedance of a short cylindrical dipole in a magnetoplasma is derived using quasi-static electromagnetic theory. The formula is valid in a lossy plasma and for any dipole orientation with respect to the magnetic field. The dipole impedance is found to have a positive real part under lossless conditions when the quasi-static differential equation is hyperbolic; this indicates that the quasi-static theory predicts a form of radiation. It is shown that the quasi-static theory can be interpreted in terms of scaled coordinates and that a cylindrical dipole in a magnetoplasma has a free space equivalent with a distorted shape. A conductance correction term obtained from Langmuir probe theory is shown to be significant. Laboratory measurements of monopole impedance are compared with the theoretical calculations.

Surface Discharges on Teflon, Mylar and Kapton

FEP Teflon, Mylar and Kapton H are compared from the point of view of charged-area scaling of discharge current pulse properties. The properties measured are peak current, pulse duration, released charge and energy dissipated in a load resistor, all for 20 keV incident electrons at a current density of 80 nA/cm2. In general it is found that the three materials are more similar than dissimilar, and furthermore that this similarity extends to discharge damage as well, the damage being identified as a combination of subsurface filamentary tunnels and surface grooves. The experimental observations are used to motivate a theory of discharge arc propagation in which the propagation velocity is controlled by the rate of removal of excess charge via wave propagation along a filamentary tunnel filled with an overdense plasma.

Multipath performance of adaptive antennas with multiple interferers and correlated fadings

This paper presents an approximate analytical cumulative distribution function of the output signal-to-noise-plus-interference ratio (SINR) of an adaptive antenna operating in multipath environments with multiple interferers and correlated fadings. Previously, approximate analytical results were only available for the case of one interferer and independent fadings between antenna branches, whereas in other cases Monte Carlo simulations had to be used with many limitations including excessive computer time and inaccurate results for small probability levels. The distribution, expressed in terms of the mean eigenvalues of the system, is accurate in most cases investigated even though it is based on an approximation to the characteristic function of the output SINR. As a result, a closed-form expression of bit error rate (BER) for coherent phase-shift keying (PSK) has been derived based on this approximation.

Dipole admittance for magnetoplasma diagnostics

A method is presented for calculating the admittance of a short thin dipole antenna in a cold collisional magnetoplasma. The theory is quasi-static, and the results are slightly more accurate than in previous analyses. Numerical calculations of admittance as a function of frequency are presented for several different orientations of the dipole with respect to the magnetic field. For any orientation the calculations show that the plasma frequency and the upper hybrid frequency can be readily identified.

On the suppression of asymmetric artifacts arising in an implementation of the thin-wire method of moments

The original version of the thin-wire frequency-domain moment-method program developed by J.H. Richmond (1974) has been modified to suppress the computation of nonphysical asymmetric fields. Richmonds implementation uses piecewise sinusoidal expansion and testing functions, along with filamentary current approximations. The modified version, termed the bridge-current version, is described. The original program and the modified version are compared with each other and with simplified theory, where applicable, on the following symmetrical structures: a rectangular wire loop, a two-wire transmission line, and a log-periodic dipole antenna. The bridge-current version is shown to eliminate the computation of nonphysical asymmetric fields, to be essentially invariant with respect to variations in segmentation for the above-mentioned structures, and to produce results that compare well with simplified theories where applicable. It is noted that the bridge-current version is particularly advantageous for structures that include close-spaced parallel wires connected by short wire segments. >

The properties of antennas in plasmas

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.AnalyseLes cinq dernières années ont été le témoin d’un progrès remarquable dans la théorie et la mesure des propriétés de rayonnement et d’impédance des antennes dans les plasmas. Une motivation accrue pour les recherches dans ce domaine s’est manifestée à la suite de l’avènement du projet de navette spatiale et des projets de fusion nucléaire. L’attention s’est focalisée sur les cones de résonance dans les plasmas linéaire et anisotropique, incluant tous les problèmes de diagrammes de rayonnement, interférences d’ondes, propagation des impulsions, réflexions aux frontières et effets produits dans les milieux inhomogènes. Dans des conditions non linéaires, la focalisation du champ autour du cône de résonance peut conduire à une dépression importante de la densité de plasma. Dans les deux cas, linéaire et non linéaire, l’impédance d’entrée d’un doublet et d’un câble a été étudiée de façon extensive à la fois pour les plasmas isotropes et anisotropes. L’observation expérimentale, non encore pleinement expliquée, de l’accroissement de résistivité de la gaine d’une antenne (plasma linéaire non collisionnel), demeure un problème très stimulant. Des calculs numériques d’impédance, fondés sur l’emploi de fonctions de distribution de vitesses simplifiées, se sont montrés particulièrement prometteurs. Les diagrammes de rayonnement, dans les modes ionique et électronique, d’antennes de formes diverses ont été calculés et vérifiés expérimentalement. La réponse d’antennes uniques ou de paires d’antennes aux fluctuations de plasma a été étudiée et a été trouvée susceptible d’applications aux méthodes de diagnostic du plasma.

Study of compressed log-periodic dipole antennas

The behavior of log-periodic dipole (LPD) antennas, which are compressed along the transmission-line axis, is studied both experimentally and theoretically. Compressed LPD antennas are found to be efficient, of low gain, and frequency independent. With compression, the radiation pattern approaches dipole-like behavior and the bandwidth increases slightly. It is also found that LPD antennas exhibit anomalous frequency-dependent behavior in narrow bands of frequencies ( \approx 1/8 of a log period in width) when a reactive termination is used and when the scaling factor \tau is below about 0.92-0.93.

Application of Ring-Opened Poly(ferrocene)s as Protective Charge Dissipation Coatings for Dielectrics

Numerous satellites providing communication, defense, and meteorological services are currently in the Earths orbit, typically at altitudes of three to six earth radii. These come under the influence of charged particles (electrons, protons, and helium nuclei) originating from the surface of the sun. Satellites immersed in this ambient plasma attain electrical equilibrium via the formation of surface charges. The formation of these charges is to a large extent dependent on the development of photoelectrons originating from the surface of the spacecrafts dielectrics. However, in regions of low sunlight intensity or in periods of intense solar activities, negative charge accumulation can occur on the surface (from low energy electrons) and within the interior (from high-energy electrons) of the dielectrics comprising the satellites thermal blankets, cable coatings, and microelectronic devices. When the potential difference between the satellites dielectrics and the surrounding environment is such that the dielectric strength of the material is compromised, a high-energy arc discharge occurs. These arc discharges are of sufficient energy to generate magnetic interference, material breakdown, and device failure. In fact it is believed that the January 1994 and March 1996 failures on the synchronous-orbit ANIK satellites were due to such high-energy arc discharges. The financial implications of such electronic breakdowns are severe. For this reason a number of studies aimed at understanding the interactions of dielectrics with both high and low energy electrons have been undertaken. Also, it would be very desirable to develop a robust polymeric charge dissipating coating which, when applied to the surface of dielectrics, would remove or prevent the formation of any accumulated charge. Importantly, the coating should not possess sufficient electrical conductivity to adversely influence any nearby radio frequency devices. It should also be noted that charge dissipation materials have a wide array of other potential uses including coatings for circuit boards. Soluble, high molecular weight poly(ferrocene)s, 2, (E = Si, Ge, Sn, P, S, etc.) are now readily available via thermal, transition metal catalyzed or anionic ring-opening polymerization (ROP) of strained ferrocenophanes (1). Subsequent studies have shown that such materials possess a range of interesting properties, including high thermal stability and good processability. As a result of the interacting metal centers present in the polymer backbone, materials such as 2 (E = Si) display electrical conductivities of ca. 10 S cm when doped with I2. [8] Additionally, by varying the nature of the bridging moiety, one can tune the metal±metal interactions, and thus the electronic properties of the resulting material. As a result of these properties, pristine samples of poly(ferrocenylsilane)s, 3 and 4, were investigated as charge dissipation coatings. In this communication, we describe our initial, very promising, results in this area.

Compact traveling-wave physical simulator for human ESD

Existing travelling-wave electrostatic discharge (ESD) simulators satisfactorily reproduce the characteristics of a human discharge but have dimensions comparable to a human subject, so they are inconveniently large for practical ESD susceptibility testing of electronic equipment. This paper describes the design of a compact travelling-wave ESD simulator with benchtop-scale dimensions. The design process utilized frequency-domain measurements and computer simulations of wave attenuation on the part of the simulator representing the human arm. From these results, it was deduced that the ESD simulator arm could be shortened from its original human-scale length of 60 cm to a more compact 30 cm without significantly affecting the currents flowing on it. The part of the simulator representing the human body was designed on the basis of capacitance. The resulting travelling-wave simulator (arm and body) is approximately half the size of the original simulator. Measurements are presented comparing the compact simulator with a previously-designed full-size simulator and with a human test subject, with regard to arm currents, swept-frequency input impedance, and capacitance.

EMI-induced failures in crystal oscillators

An electromagnetic interference (EMI) induced failure mode pertaining to crystal-based voltage-controlled oscillators (VCO) has been studied. The failure consists of a transition to a frequency of oscillation that differs from the crystals fundamental resonant frequency, when the circuit is temporarily exposed to continuous or pulsed radio-frequency electromagnetic fields. The new state persists even after the EMI source is removed and leads to hang-up in digital systems. This mode transition has been observed experimentally. Its essential properties have been predicted theoretically and simulated numerically, using simplified oscillator models. The likelihood of observing such a failure in a noisy electromagnetic environment is assessed with respect to the radiated susceptibility levels given in MIL-STD-461B. >