D. V. Giri

JOLT: a highly directive, very intensive, impulse-like radiator

Ultrawideband (UWB) systems that radiate very high-level transient waveforms and exhibit operating bandwidths of over two decades are now in demand for a number of applications. Such systems are known to radiate impulse-like waveforms with rise times around 100 ps and peak electric field values of tens of kilovolts per meter. Such waveforms, if properly radiated, will exhibit an operating spectrum of over two decades, making them ideal for applications such as concealed object detection, countermine, transient radar, and communications. In this paper, we describe a large, high-voltage transient system built at the Air Force Research Laboratory, Kirtland AFB, NM, from 1997 to 1999. The pulsed power system centers around a very compact resonant transformer capable of generating over 1 MV at a pulse-repetition frequency of /spl sim/ 600 Hz. This is switched, via an integrated transfer capacitor and an oil peaking switch onto an 85-/spl Omega/ half-impulse radiating antenna. This unique system will deliver a far radiated field with a full-width at half-maximum on the order of 100 ps, and a field-range product (rE/sub far/) of /spl sim/ 5.3 MV, exceeding all previously reported results by a factor of several.

Switched Oscillators and Their Integration Into Helical Antennas

In this paper, the problem of designing switched oscillators at four different frequencies (200, 300, 400, and 500 MHz) has been addressed. These oscillators are quarter-wavelength long coaxial transmission lines with a nitrogen spark gap switch at one end. Two of these switched oscillators at 200 and 500 MHz with a charge voltage of 30 kV have also been fabricated. These two oscillators are modeled using PSpice and their output into a 100 Ω load is estimated and tested by fabricating a 100 Ω transmission line. Use is made of a modified commercial helical antenna with a bandwidth of 400-600 MHz and a switched oscillator has been integrated into this helical antenna. Measurements have been made of the S11 , the voltage into the antenna, and also the transient fields at two distances. Indeed, starting from electrical power from a 12 V battery, electrical field strengths in excess of 10 kV/m with damped sinusoidal waveforms at 500 MHz (for example) have been demonstrated.

TEMPORAL AND SPECTRAL RADIATION ON BORESIGHT OF A REFLECTOR TYPE OF IMPULSE RADIATING ANTENNA (IRA)

The IRA considered here is a parboloidal reflector fed by conical TEM lines. Such an IRA has been analyzed in the past for its performance characteristics such as the prepulse or feed step in the impulse, assuming a step function excitation. In this paper, we extend the analysis to include the diffracted fields from the launcher plates and the circular rim of the reflector. The diffraction from the launcher plates can be viewed to consist of two parts. One has the plate edge diffraction followed by diffraction associated with the total currents on the plates. The leading terms in all of these diffracted signals arriving at an observer in the far field have been determined. In addition, the predominant portion of the radiated field consisting of the feed step and the impulse has been analytically Fourier transformed to get the boresight radiated spectrum. Experimental results from a prototype IRA system are also included.

Intermediate and far fields of a reflector antenna energized by a hydrogen spark-gap switched pulser

Previously, the design, fabrication and testing of a pulser with a parabolic reflector antenna, known as the prototype impulse-radiating antenna (IRA) has been presented. The paraboloidal reflector was fed by a TEM structure that in-turn was energized by a /spl plusmn/60 kV, /spl sim/100 atm. hydrogen switch operating in a burst mode at up to 200 Hz. The TEM structure also incorporated an electromagnetic lens to ensure a near-ideal spherical TEM wavelaunch. Some of the measured characteristics of this system were: (a) a peak electric field on boresight of 4.2 kV/m at a range r=305 m; (b) an uncorrected pulse rise-time (10-90%) of 99 ps; and (c) a boresight electric fields FWHM of 130 ps. The radiating system has now been more fully characterized with additional measurements and computations of near field, intermediate and far fields on the boresight. While the far fields from such a radiating system have been known for some time, the intermediate field analysis was only published recently. This method substitutes the radiated field from a paraboloidal reflector by the radiation field from the TEM structure reflected in the parabolic mirror. Although this work is limited to fields on the boresight at any distance from the antenna, the authors have been able to extend the analysis to the frequency domain. It has also been verified that the intermediate fields asymptotically tend to the far-field expressions, as the range r is increased. Good agreement between calculated and measured fields has been obtained for the prototype IRA in the near (r=5 m) and in the far field (r=305 m).

Evaluation of resistors for transient high-voltage applications

Applications for transient, high-voltage pulsed power technologies are on the increase. High-voltage resistors are an essential component of such systems, especially in the proof-of-concept and prototype testing. The authors have recently procured and tested certain resistor samples, supplied by Kanthal Globar and HVR Advanced Power Components. Results of a detailed evaluation of the HVR resistors are presented in this paper. Two types of HVR high-powered resistors have been tested to determine hold-off voltage, frequency variation and resistance to high voltage. The resistors were tested in a coaxial geometry driven by a two stage Marx generator. The voltage and current were measured by calibrated sensors. The high-voltage pulse resistance of each resistor is then determined on a pulse by pulse basis by dividing the maximum voltage by the maximum current in the time-domain. The two samples (HVR-10, HVR-12; washer type) were nominally 10 and 12 ohms with resistivity of 28 and 80 ohm-cm respectively. The variations in the low-voltage to pulsed high-voltage resistance were 9% for the HVR-10 and 18% for the HVR-12. With an average applied field of 65 kV/inch or 25.6 kV/cm, the resistors flashed in air, but not in pure SF/sub 6/ and N/sub 2//SF/sub 6/ mix. These resistors were found to be satisfactory for transient applications.

Early Developments in Sensors and Simulators at the Air Force Weapons Laboratory

Personnel at the Air Force Weapons Laboratory introduced many innovative concepts in electromagnetics (EM) and created new antenna and sensor designs that made possible much of the wideband electromagnetics technology we have today. Many practitioners in high power EM are familiar with wideband sensors and simulators, but may not be aware of their origin. The purpose of this paper is to describe some of the concepts from which many of the designs evolved and provide some interesting insight into the mind of Dr. Carl Baum, who created them.

High-Altitude Electromagnetic Pulse (HEMP) Risetime Evolution of Technology and Standards Exclusively for E1 Environment

There are many different definitions of the risetime of a transient waveform. In the context of high-altitude electromagnetic pulse (HEMP) standards, the 10-90% risetime of an idealized double exponential waveform has been defined and used for many decades. However, such a risetime definition is not strictly applicable to the transient voltage out of a pulse generator, since no practical switch can close in zero time. In this paper, we discuss various definitions and their applicability. More importantly, pulse power technology has evolved over five decades and the achievable risetimes have come down from 10s of nanoseconds to 10s of picoseconds. As a corollary, the highest achievable voltage gradient has been going upwards of 1015 V/s. In this paper, we review the definitions of risetime, and trace the evolution of technology and HEMP Standards, exclusively for the E1 environments.

An Overview of the Natural Frequencies of a Straight Wire by Various Methods

There are a number of different methods for computing the natural frequencies of a straight wire antenna or scatterer. The most accurate method is that derived from a numerical solution to the integral equation for the wire current. Due to the relative difficulty in obtaining this solution, however, several different approximations to natural frequencies have been developed and used in the past. Most recently, a paper by Meyers, has described a variational approach for computing these natural frequencies. This paper provides a brief description of each of these methods for determining the natural frequencies of a straight wire, and then presents a comparison of the result of each method for a wire of diameter to length ratio (d/L) ranging from 0 to 0.1.