Edward F. Vance

Shielding Effectiveness of Braided-Wire Shields

An analysis of the shielding effectiveness of braidedwire shields is made using the parameters of the woven wire and the theory of electromagnetic coupling through small irises. The coupling through the rhombic-shaped holes in the braid is approximated by using the electric and magnetic polarizabilities of elliptical holes of the same width and length as the rhombus. The analysis develops the transfer impedance and the mutual capacitance of the shield. The transfer impedance is calculated and plotted for shields of different optical coverages. The variation of mutual capacitance and mutual inductance with the weave angle of the braid are examined.

One‐Sided Multipactor Discharge Modes

Modes of multipactor discharge occurring in a biased gap are described in which secondary emission from only one electrode is required to sustain the discharge. An analysis of these modes and an experimental corroboration of the theory are presented.

EMP Coupling Through Cable Shields

Recent research in electromagnetic coupling between the interior and exterior of coaxial cables is described. Both tubular shields and shields with apertures are discussed with reference to the physical coupling mechanisms and their mathematical representations in terms of transmission-line models.

EMP Coupling to Power Lines

A transmission line model of coupling to power lines has been developed that shows the effect of ground conductivity, angle of arrival, polarization, and power line geometry on the open-circuit voltage at the end of the line. With this information, and the characteristic impedance of the line, the current and voltage to any termination may be calculated. This theory has been checked with experimental measurements and shows quite good agreement with measured values of current. The measurements were performed on scale models of power lines. The soil conductivity was scaled appropriately. The results show that, although the transmission-line model is not strictly applicable at the highest frequencies of interest, the error introduced by this approximation is less than the experimental error (¿ ± 10 percent). Measurements have also been made of the propagation of pulses down lines showing effects of attenuation, dispersion, junctions, bends, and spurs. Where available, theoretical calculations have been compared to measured results. From these measurements it is concluded that transmission-line models can adequately be used to predict propagation effects.

ELECTROMAGNETIC INTERACTION WITH LARGE SYSTEMS

ABSTRACT The use of shield topology Co partition large systems into tractable pieces is discussed. The interactions of externally generated transients with large systems are discussed from the aspect of the numbers of operations that must be considered. The segments of interaction associated with multiple shield imperfections, multiple levels of shields, system states, and abnormal modes lead to enormous numbers of analytical operations that must be performed to evaluate a family of responses. Practical reduction in this number, by invoking the principle of effectively impervious shields, is described.

Internal Voltages and Currents in Solid-Shielded Cables

Analytical techniques for determining the current induced in long conductors near the surface of the ground by surface fields have been developed by Sunde and Wait. These techniques have been verified for insulated and bare conductors in experiments conducted with a CW transmitter, and computer programs have been developed to calculate the currents using a version of Sundes theory. Until recently, however, cable-current theory was limited to determining the total current induced in a cable. For many applications where shielded or multiply shielded cables are used, however, it is important to know what voltages and currents are induced in the signal-carrying core of the cable. The purpose of this note is to describe one technique that has been developed to compute these internal currents and voltages. The technique described in this paper combines the shielding theory of Kaden and the transmission-line theory of Sunde. Classical shielding theory is used to determine the axial electric-field strength at the inside surface of the shield from the current flowing in the shield. This axial electric field is then used in the equations for a transmission line with a distributed driving source to compute the current in the core and the core-to-shield voltage. (In the case of multiple shields, multiple applications of these two steps are required to progress from the outer shield through the inner shields to the core.

Current Induced in Buried Conductors

ABSTRACT Laboratory measurements of the current induced in buried conductors were made and are compared to predictions based on the transmission-line approximation. The measurements were made in a test region of salted soil that was illuminated with a repetitive transient wave from a Large Linear antenna. The small wires buried in the salted soil are electromagnetically similar to larger conductors in typical natural soils. The actual variation of induced current with angle of incidence, depth of burial, and wire diameter are compared with the variations predicted from the transmission-Line approximation.

Correlation Between Clear Air Turbulence and Aircraft Electrical Activity

Abstract : The results of a cooperative effort by Stanford Research Institute, United Air Lines, and Air Force Cambridge Research Laboratories to determine the correlation between regions of clear-air turbulence (CAT) and aircraft electrical activity are described. Corona discharges from precipitation static dischargers on DC-8 aircraft were monitored and correlated with CAT encounters. A significant correlation was found to exist between CAT encounters and periods of electrical discharge. It is suggested that these electrical discharges may be caused by electric fields in the region of clear-air turbulence, particulate matter in the region that charges the aircraft, or a combination of both. A meteorological analysis was made of the regions where turbulence associated with electrical activity was observed. This analysis indicated that the CAT incidents were associated with jet streams. (Author)