Investigation On Propagation Characteristics Of Ultra-Highpower And Ultra-Wideband Electromagnetic Pulses In Finite Space

With the miniaturization of electronic and electrical devices, they are more sensitive to and seriously interfered by electromagnetic pulse (EMP). Especially, after the emergence of some new-style weapons such as the high-power wideband EMP launcher and EM bomb, countries around the world initiate a new round competition in the research of electromagnetic interference (EMI) and protection techniques. It is evident that the investigation for EMP's characteristic is definitely important. In practical use, EMP power is greatly reduced by the impedance of the land or sea when propagating. As a result, characterize how the high-power wideband EMP propagates along impedance surface is of huge merit for both EMI and electromagnetic protection.Firstly, Chapter One starts from introducing various widely studied high-power wideband EMP waveforms nowadays. Then, EMP's different definitions and standards popular in academic and military realms are presented. As the conclusion, the thesis lists the mathematical expressions of some typical high-power wideband EMPs.Secondly, the basic model for the radiation of a dipole above impedance surface is developed in Chapter Two. As is well known, the impedance surface (such as ground, sea, forest, etc) usually has electric and magnetic parameters which are not only highly lossy but also dispersive, i.e. frequency varying. So the problems related to EMP propagating characterization in finite space become extremely complicated. As a solution, by studying the dipole's radiation model above impedance surface, my thesis extracts the surface's equivalent impedance that considers some critical information including the permittivity and permeability of the surface and the spectrum of the pulse. Therefore, the proposed model and method is more accurately and efficiently in simulating practical propagation problems.Next, the third chapter presents numerical examples for high-power wideband EMPs which propagate along the impedance surface. It should be noted that conventional dipole– impedance surface model is often designed to analyze all the attributes at single frequency. However, wideband EMPs need investigation of their propagation and attenuation in a relatively broad frequency band. Through frequency sweep and inverse fast Fourier transform (IFFT), an EMP's time domain characteristics can be obtained.In the last chapter, the proposed method is extended into the application of wire antennas'radiation analysis. Indeed, the radiation of an arbitrarily shaped wire antenna can be equaled to the summation of series of dipoles'radiation. Based on such an approximation, the dipole model interpreted in previous chapters can be employed here. Thus, if only the current distribution along a certain wire antenna is known, we replace the original radiation source with a set of dipoles, with different current values, at different positions. Furthermore, time-domain (TD) method of moments (MoM) and finite-difference time-domain (FDTD) are commonly used EM numerical methods to solve unknown currents on antennas of arbitrary geometries.