Thesis Title: High-Frequency Technologies of Energy-Efficient, Wideband and Compact Antenna Systems for Portable Terminals and Advanced Wireless Communication NetworksThe explosive growth of wireless communications has significantly increased the need for compact antennas, which tend to behave as narrowband, inefficient and low-gain radiators. Reliable techniques for the design, fabrication and measurement of small antennas are highly desirable. The significance of antennas to the proper function of advanced mobile terminals cannot be overemphasized. Rapid advances in mobile and personal communication networks have caused antenna design targeting cellphones, portable computers, PDAs, smartphones and RFIDs to flourish. Contrary to the above, industrial and academic research has displayed little interest in the antenna system of a special category of portable terminals: nodes comprising Wireless Sensor Networks (WSNs). Wireless sensors are autonomous, tiny devices built around specifications that opt for the longest possible network lifetime. A vital prerequisite to the proper function of wireless microsensors is the availability of miniature, energy-efficient antenna systems covering the application band. The design, development, and characterization of such antennas are the major topics addressed in the Thesis. The antennas were built in printed circuit technology, since PCBs are a realistic and viable solution to the fabrication of wireless sensors.A study of Euclidean and Fractal printed monopoles during the initial stages of the Thesis showed that their achievable miniaturization limits are constrained by a shortcircuiting phenomenon. A new class of antennas that alleviate the problem are Analytical Geometry Antennas. The sinusoidal antenna was initially developed, which is inherently small, wideband and efficient. Particular attention was given to the effect of the ground plane (GNDP) which, contrary to what is generally understood, is an integral part of the overall radiator. A flexible figure-of-merit (FoM) was suggested, to merge the fundamental properties of small antennas into a single quantity. The genre of Analytical Antennas was augmented by the addition of Chebyshev antennas, which combine inductive and capacitive (top-hat) loading and demonstrate that the choice of analytical function can produce gains in antenna response. The study of Analytical Antennas was completed with the equally innovative, multi-band, generalized-sinusoidal Chirp antenna. The Chirp radiator introduces the concept of Analytical Self-Similarity. v 3 o soundtrack th ditri sunèjesn oi korufÐoi mousikoÐ dhmiourgoÐD kurÐw pì ton q¸ro tou prog rock/metalD pou prtÐjenti ed¸lfhtikX Anathema, Blind Guardian, Dire Straits, Dream Theater,