In rapid wireless communication technology development environment, antennas, the interface among many wireless communications, are an indispensable component for wireless systems. Miniaturization and functionality stability (high tolerance to environmental variations) of the antenna are fast becoming the design trends in research and development of wireless communication systems. They are also the main objectives of this thesis. In the first part of this thesis, we designed two highly stable antennas, which can be used in notebook computers or tablet PCs. The antenna has self-balanced characteristics, where the environmental interference is minimized, in its performance/functionality and patterns. The first antenna design, which can be easily integrated into an RF front-end board, employed capacitive coupling, differential feed printed loop configurations. Comparing to the existing differentially fed antenna design, our designs are much more miniaturized: the antenna size was 13 mm ¡Ñ 27 mm, the ground size was 4.5 mm ¡Ñ 4.5 mm. Implemented on a low-cost FR4 board, the antenna reduced the leakage current formed on coaxial transmission line, due to the advantage of being differentially fed. The second antenna design, fed by coaxial cable (single-ended fed), and without a ground plane, excited only self-balanced modes. The radiation patterns of higher modes in this antenna design are complete and without side lobes. This antenna design also has wide bandwidth characteristics: at 2.4 GHz it had 380 MHz, and at 5.2 GHz it had 1270 MHz bandwidths of high tolerance (stability). The actual measurement validated our simulation results. In the second part, MIMO antennas were designed for 802.11n wireless standards with maximum transfer rates of up to 300 Mbps. First, we designed two small single antennas, which were applied later in MIMO antenna designs. The size of our MIMO antenna designs was only 19 mm ¡Ñ 30.3 mm. In MIMO antenna designs, we employed two methods to increase the isolation between the two MIMO antennas: one manipulated the ground plane size, in which the isolation reached 18.9 dB; the other utilized a decoupling metal, where the overall isolation reached 24.6 dB in all of the operating frequencies, with the best isolation being 31.4 dB. The frequency of the coupling/decoupling for the decoupling metal can be adjusted independently; thus not affecting the original resonant frequency and the return loss of the two MIMO antennas. Actual measurements conducted in the microwave chamber (Reverberation Chamber) have verified the channel capacity were effectively increased, the total radiation efficiencies were about 60%, and the effective diversity gain was about 7dB. The MIMO antenna designs can practically and easily applied in the USB dongles.

Phased array weather radar antennas with beam steering capabilities are suitable alternatives to weather radars with mechanically scanning reflector antennas. Dual-polarized phased-array weather radar antennas, however, demand careful assessment of the x-polar characteristics. The low x-pol radiation of polarimetric weather radar antennas is of significant importance for the proper classification and qualitative estimation of hydrometeors in illuminate volumes. Unfortunately, array antennas display changing x-pol contributions during the electronical beam steering process. Typically, the x-pol radiation will be substantially increased in the co-polar main beam direction but also in other angular directions. Consequently, it is a vital challenge to design arrays with low x-pol contribution during beam steering. In this dissertation a new phased-array weather radar concept is developed. The phased array system configuration can be used to substitute state-of-the-art weather radars with reflector antennas. Furthermore, a dense network of these phased-array radars can be used to substitute a network of high power weather radars, which are used nowadays. The research focus of this work is the development of a dual-polarized microstrip patch antenna with phased-array capability and very high polarization purity. In this regard, new graphical techniques are developed to investigate the causes and the reduction of the x-pol radiation of isolated (stand-alone) microstrip patch antennas. To further reduce the x-pol contribution of antennas, optimization methods have been investigated, evaluated and developed. For the first time in literature, differential-feed antenna arrays are compared to excitation optimized single-feed antenna arrays in their x-pol contribution in the boresight direction and during beam steering. In particular, two dual-polarized 4x8 antenna arrays have been developed and simulated by CST MWS, produced as multilayer PCB and verified at the compact antenna test range at RWTH Aachen. The results show that the x-pol contributions of arrays are significantly reduced for differentially-feed antenna arrays, even when beam steering is performed. During the azimuth scan of 120_ a record setting x-pol suppression of -45 dB and -36 dB could be measured for the horizontal and vertical polarization channels, respectively. / Wetterradarsysteme mit phasengesteuerten Antennen stellen eine echte Alternative zu Wetterradarsystemen mit mechanisch drehenden Reflektorantennen dar. Dual-polarisierte phasengesteuerte Antennen müssen jedoch sehr genau in ihrem Kreuzpolarisationsverhalten verifiziert werden, um für den Wetterradarbereich von Nutzen zu sein. Die Unterdrückung der kreuzpolaren Anteile von Radarantennen ist von fundamentaler Bedeutung, um Hydrometeore mit Hilfe von polarimetrischen Wetterradarsystemen klassifizieren und qualitativ bestimmen zu können. Die hohe Anforderung an Polarisationsreinheit ist mit aktuell erhältlichen Arraydesigns nur schwierig zu realisieren, da sich die Kreuzpolarisationsunterdrückung während des elektronischen Schwenks der Hauptkeule signifikant verschlechtert. Diese Dissertation stellt ein Wetterradar Systemkonzept mit phasengesteuerter Gruppenantenne vor, welches die aktuell genutzten Wetterradare mit Reflektorantennen ablösen könnte. Der Fokus der Arbeit wurde auf die Entwicklung einer Dual-polarimetrischen, polarisationsreinen und phasengesteuerten Mikrostreifenleiterantennen gelegt. Hierbei wurden neue grafische Verfahren entwickelt, die es ermöglichen, die Generierung der kreuzpolaren Anteile von isolierten Patchantennen (Einzelpatche) zu erklären und zu minimieren. Um die kreuzpolaren Anteile weiter herabzusetzen wurden Optimierungsverfahren für Arrayantennen erforscht, bewertet und neu entwickelt. Zum ersten Mal wurden differentiell gespeiste mit einzeln gespeisten Antennenarrays in ihrem Kreuzpolarisationsverhalten während des elektronischen Schwenks der Hauptkeule verglichen. Zwei Dual- polarimetrische 4x8 Antennenarrays (differentiell gespeist und mit optimierter Phasenansteuerung) wurden zu diesem Zweck mittels CST MWS entworfen, simuliert, als Multilagenplatine gefertigt und an der Antennentestanlage der RWTH Aachen vermessen. Die Resultate zeigen, dass die Kreuzpolarisationsanteile bei differentiell gespeisten Mikrostreifenleiterantennen in Gruppenkonfiguration, selbst beim elektronischen Schwenk der Hauptkeule, signifikant minimiert werden konnten. Für einen azimutalen Scanbereich von 120_ konnte eine exzellente Kreuzpolarisationsunterdrückung zwischen -45 dB und -36 dB messtechnisch für den horizontalen und vertikalen Polarisationskanal nachgewiesen werden.