PROPERTIES OF ELECTROMAGNETIC BEAMS GENERATED BY ULTRA-WIDE BANDWIDTH PULSE-DRIVEN ARRAYS

The characteristics of the beams generated by ultra-wide bandwidth electromagnetic systems are central to their practical applications. These characteristics include the rate of beam divergence, the beam intensity, and the energy efficiency. Analytical bounds on the characteristics of beams generated by an arbitrary pulse-driven array are derived and supported with numerical calculations. These bounds extend the meaning of near-field distances or diffraction lengths to the situation where the array driving functions can be broad-band-width signals. Particular attention is given to transmitting and receiving array systems which consist of elements that are not large in comparison to the shortest wavelength of significance contained in the signals driving them. The output signals of such systems are time derivatives of the input driving functions. They constitute higher-order beams whose coherence properties are degraded more slowly by diffraction than lower-order beams. The bounds define the extent of these enhancements. It is further shown that for certain measures of performance involving these beam characteristics, a localized wave pulse-driven array can outperform similar continuous-wave-driven arrays. A new type of array is required to realize these localized wave effects-one that has independently addressable elements. The enhanced localization effects are intimately coupled to the proper spatial distribution of broad-bandwidth signals driving the array; i.e., by controlling not only the amplitudes, but also the frequency spectra of the pulses driving the array.