Time-domain electromagnetics and its applications

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Abstract
The purpose of this paper is to introduce the reader to the elments of time-domain electromagnetics, which includes baseband-pulse technology and target-signature analysis. Baseband pulses are video or carrierless pulses of very short duration, whose spectral content is concentrated primarily from zero frequency through the microwave region of the spectrum. Work in baseband technology began more than ten years ago at the Sperry Research Center with emphasis primarily on its use as an analytical tool, initially to explore the properties of microwave networks [1], and subsequently to determine the intrinsic properties of materials [2]. The experimental phases of these studies were aided by the pioneering development by the Hewlett-Packard Company of their sampling oscilloscope [3]. These techniques were further extended to experimentally analyze and synthesize antenna radiating and receiving elements [4], [5]. Success in the antenna area led to the development of an indoor ground-plane scattering range to measure the impulse properties of targets or obstacles [6]. This type of range obviated the need for an expensive anechoic chamber since time-gating techniques permitted unwanted reflections from the walls and ceilings to be easily eliminated. A sampling oscillosope and an instrumentation computer were used to process target-signature data as a function of illumination angle. At about the same time, analytical techniques were developed which permitted target-signature analysis to be carried out efficiently and accurately in the time domain [7]. Applications where threshold rather than target-signature data is sufficient were also investigated, and for these applications it became clear that because of cost, the sampling oscilloscope had to be replaced by a stable and fast-acting threshold device. To meet this requirement, two different types of tunnel-diode receivers were developed. The successful developmemt of these receivers, together with the design of inexpensive microwave delay-line ranging techniques, led to the evolution of BAseband Radar (BAR) or free-space time-domain reflectometry. BAR devices have been designed and recently demonstrated for various applications, including auto precollision sensing, spaceship docking, airport surface-traffic control, auto braking, tanker-ship docking, harborcollision avoidance, etc. These sensing applications cover ranges from 5 to 5000 ft [8]. In other applications, the development of high-speed subnanosecond logic has impacted the computer field, making higher speed computation possible. Further developments resulted in the construction of a subnanosecond, single coaxial cable scheme for multiplexing data between computer terminals [9]. More recently, baseband-pulse techniques have been applied to the problem of developing a short-range wireless communication link. Here, the low EM pollution and covertness of operation potentially provide the means for wireless transmission without licensing. We review the research areas described above in more detail and refer the reader to references and a comprehensive biblography where sources for detailed information can be found [10].

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