The focus of this dissertation is on the development and circuit modeling of planar series-fed, linear- and circular-polarized microstrip aperture-coupled antennas and N-element arrays operating in C-band. These arrays were designed to be used as part of airborne or land-based frequency-hopped communication systems. One of the main objectives of this work was to maintain a constant beam angle over the frequency band of operation. In order to achieve constant beam pointing versus frequency, an anti-symmetric series-fed approach using lumped-element circuit models was developed. This series feed architecture also balances the power radiated by each element in the N-element arrays. The proposed series-fed approach was used in the development of four-element series-fed aperture-coupled arrays with 15% 10 dB impedance bandwidth centered at 5 GHz and a gain of 11.5 dB, to construct an omni-directional radiator. Omni-directional radiators with pattern frequency stability are desired in a multitude of applications; from defense in tactical communications, information gathering, and detection of signal of interest to being part of sensors in medical applications. A hexagonal assembly of six series-fed microstrip aperture-coupled four-element arrays was used to achieve the omni-directional radiation with 0.6 dB peak to peak difference across 360˚ broadside pattern and 0.6 dB gain variation at the specific azimuth angle across 15% impedance bandwidth. Given that each of these six arrays can be individually controlled, this configuration allows for individual pattern control and reconfiguration of the omni-directional pattern with increased gain at specific azimuth angles and the ability to form a directional pattern by employing a fewer number of arrays. Incorporating a beam-forming network or power distribution network is also possible. Wide 3 dB circular polarization (CP) bandwidth was achieved without external couplers and via only a single feed with a unique Z-slot aperture-coupled microstrip antenna. A single RHCP Z-slot aperture-coupled antenna has ∼10% CP and 10 dB impedance bandwidth. The series-fed network consisting of lumped elements, open-circuited stubs and transmission lines was subsequently developed to maintain more than 5% CP and 10% 10 dB impedance bandwidth in the series-fed four-element Z-slot aperture-coupled CP array that could also be used for-omni-directional radiation.

A novel approach to miniaturizing an 802.11b WLAN card using folded-flex ultra-thin substrates is presented. A 73 percent reduction in size was realized using hybrid circuits on FR4 and polyimide. There is even more potential for further reduction if more copper layers are used. The miniaturized 802.11b WLAN cards were used to design 802.11b wireless sensor nodes. A research test-bed was setup to study how 802.11b networked sensor nodes could operate in the field. There are many applications for such sensor networks like habitat monitoring, object tracking, seismic detection, military surveillance, or fire detection to name a few. This investigation focuses on the requirements, design, and performance of a miniaturized 802.11b wireless LAN sensor node that is reliable, can be deployed in large-scale, and has the endurance long-lived for surveillance applications. An aperture coupled microstrip antenna is investigated for 2.44 GHz wireless local area networks (WLAN) which has the advantages of being low-profile and compact. The most important parameters for antenna optimization have been determined through extensive simulation using Ansoft's HFSS and experimental verification. As a result, an omnidirectional antenna with a size of 36.2 mm x 32 mm x 4.75 mm has been realized using Rogers 5880 duroid (permittivity = 2.20 ; loss tangent = 0.0004) with 4.4 dBi of gain, > 80 MHz, and a return loss > -32 dB. These types of performance characteristics make the antenna highly desirable for both 802.11b and Bluetooth applications.

In this thesis, an analysis technique for the slot coupled patch antennas using MoM in conjunction with the closed form Green&rsquo / s functions is presented. Slot coupled patch antennas are fed by a microstrip open stub which is coupled to the patch through an electrically small slot. Current distributions over the microstrip line, slot line and the patch are represented by rooftop basis functions. First, a relatively simple structure, microstrip coupled slot line is investigated using the proposed technique. Then the method is extended to the slot coupled patch antenna geometry. By using the method, current distributions on the feedline and the patch are calculated for a generic slot coupled patch antenna. Then by using the distributions, return scattering parameters of the antenna is approximated with complex exponentials using Prony&rsquo / s method. A parametric study is carried out to observe the effect of each antenna component on the antenna performance. Current distributions and return loss calculations are repeated for modified antennas to observe and demonstrate the performance differences. All simulations are verified using HFSS&reg / software and the results available in the literature.

In this thesis, a wideband dual polarized microstrip antenna is designed, manufactured and measured. Slot coupled patch antenna structure is considered in order to achieve the wideband characteristic. Although rectangular shaped slot coupled patch antennas are widely used in most of the applications, their utilization in dual polarized antenna structures is not feasible due to space limitations regarding the positioning of two separate coupling slots for each polarization. For a rectangular slot, the parameter that affects the amount of coupling is the slot length. On the other hand when a H-shaped slot is considered, both the length of the center arm and the length of the side legs determine the coupling efficiency. This flexibility about the optimization parameters of the H-shaped slot makes it possible to position the two coupling slots within the boundaries of the patch antenna. Therefore, H-shaped slot coupled patch antennas are studied in this thesis. In order to investigate the effects of slot and antenna dimensions on the radiation characteristics of the antenna, a parametric study is performed by analyzing the antenna structure with a planar electromagnetic field simulation software (Ansoft Designer). By the help of the experience gained through this parametric study, a dual polarized patch antenna that can be used at the base station of a cellular system (DCS: 1710&ndash / 1880 MHz) is designed. Before manufacturing the antenna, dimensions of the antenna are re-tuned by considering a finite sized ground plane in the simulations. Finally, the antenna is manufactured and measured. An acceptable agreement is obtained between the measurement and the simulation results.