The design and analysis of miniaturized portable antennas for ultra-wideband (UWB) communications in the 3.1--10.6 GHz bandwidth are pursued using electromagnetic simulations, measurements and theoretical analysis.; Initially, four small, low-profile, printed microstrip ultra-wideband antenna designs namely: the elliptical-patch, half-elliptical-patch, crescent-patch, and half-crescent-patch antennas are investigated. Simulations and measurements show that in addition to return loss, each of the antennas has radiation characteristics which are suitable for the intended UWB communications including: a fairly consistent radiation pattern; a substantial gain (>2 dB) that does not vary drastically with frequency; and high radiation efficiencies (>85%). The crescent patch and half-crescent patch antennas are entirely novel and each offers an additional degree of freedom for design and economizes space on a printed circuit board.; Frequency domain spherical wave expansion (SWE) analysis reveal that at low in-band frequencies each of the antennas radiates essentially as an electric dipole extending along the axis of the microstrip feed, but at higher in-band frequencies, higher order multipoles emerge in the radiation.; The similarities observed in the radiation properties of the four antennas in spite of the differences in their patch structures; and mindful of their interrelationship, together with the results of SWE analysis and some known findings regarding wide angle conical antennas are used to postulate a breakthrough qualitative theory for the functionality of UWB microstrip antennas in general. A theory is also developed for vertical planar monopole antennas---the predecessor. In either case, the theory is used to explain a great deal of previous empirical observations about the class of antennas, and is justified by more experimental evidence, as well. Then it is used to derive optimization techniques for the relevant class of antennas. And for UWB microstrip antennas, it is used to evaluate their performance with respect to theoretical limitations on antennas, leading to new strategies for approaching these limits.
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