الفهرس 
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Abstract
ABSTRACT
Since the Federal Communications Commission (FCC) in USA released the unlicensed commercial use of a bandwidth of 7.5 GHz (from 3.1 to 10.6 GHz) for ultra–wideband (UWB) wireless communications, UWB is rapidly developed as a high data rate wireless communication technology. Consequently, UWB antennas have drawn more and more attention from both academia and industries worldwide. However, unlike traditional narrow band antennas, design and analysis of UWB antennas are facing more challenges and difficulties. A suitable UWB antenna should be capable of operating over an ultra–wide bandwidth as allocated by the FCC, with Equivalent Isotropically Radiated Power (EIRP) less than –41.3dBm/MHz. Satisfactory radiation properties and gain over the entire frequency range is also a very important issue. Being compact to be easily integrated in the circuitry of UWB systems is an add–on asset to any design.
One of the main operational challenges facing UWB systems is their coexistence with legacy standards, many of which are often situated within the UWB allowed spectrum such as, IEEE 802.16 standard for worldwide interoperability for microwave access (WiMAX) system (3.3–3.9 GHz) and IEEE 802.11a standard for wireless local area network (WLAN) system (5.15–5.825 GHz), and European HIPERLAN/2 (5.15–5.725 GHz). Thus, it is a very important demand to design the antenna in a way that accommodates one or several rejection bands.
This thesis focuses on UWB printed antenna design and analysis. Studies have been undertaken covering the aspects of UWB fundamentals and antenna theory. Extensive investigations were also carried out on five different types of printed UWB antennas. Moreover, the equivalent circuit models for the proposed antennas are established using Vector Fitting (VF) technique to help system designers to predict and consider the effect of the Radio Frequency (RF) components such as UWB antennas in the communication system with the whole system simulation.
The first type of the proposed UWB antennas studied in this thesis is a miniaturized UWB printed monopole antenna that employs planar electromagnetic band gap (EBG) structures to obtain and control a band–notched performance in the 5.0–6.0 GHz band that corresponding to WLAN and HIPERLAN/2 standards for UWB applications. The design process involves two phases; the first phase is to design a new shape of UWB printed monopole antenna. The second phase is to incorporate the designed antenna with either a slotted patch electromagnetic band gap (spEBG) structure or a parasitic strip electromagnetic bang gap (psEBG) structure to achieve the required frequency band–notched characteristic for the UWB applications. The simulated results along with the equivalent circuit results and the measured data elucidate that the EBG structures exhibit well–behaved band stop characteristics required for UWB applications. The results appear that the proposed antenna with spEBG element can cover the frequency band from 3.7 to beyond 10.6 GHz with a sharp band rejection in the 5–6 GHz band. The radiation parameters of the proposed antenna show almost omni–directional field patterns and stable gain over the whole frequency band excluding the rejected band.
The second type of proposed UWB antennas is a design of a circular slot antenna with band–notched characteristics for UWB applications. The frequency–notching characteristics is obtained by inserting a parasitic metallic–type split ring resonator (MTSRR) element coplanar to the slot and backed to a circular patch into the opposite side of the antenna substrate. The designed antenna satisfies the voltage standing wave ratio (VSWR) requirement of less than 2.0 in the frequency between 2.85 GHz and 10.98 GHz, which covers the UWB frequency band as approved by FCC. The proposed antenna maintains a band–stop in the frequency band of 5.30 GHz to 5.89 GHz which defines higher range of frequency bands of the WLAN in USA and HIPERLAN/2 in Europe. Parametric studies are presented to investigate the tuning effects of the geometrical parameters on the impedance matching. The antenna is fabricated and the measured return loss versus frequency shows very good agreement with the simulated results and also with the equivalent circuit results. The radiation parameters of the proposed antenna show almost omnidirectional patterns and stable gain over the whole frequency band excluding the rejected band.
The third type of UWB antennas studied in this thesis is a proximity–fed annular slot antenna for UWB operation. Different techniques are introduced to impose the band rejection property to the proposed prototype antenna. The prototype antenna consists of a circular radiating patch placed non–concentrically inside a circular aperture in the ground plane. The patch is proximity–fed by a 50 Ω microstrip line on the other side of the substrate. This configuration provides an UWB performance in the frequency range of 2.85 to 7.95 GHz with relatively stable radiation parameters. Three different techniques to construct a resonant circuit have been investigated to assure the validity of the proposed antenna to achieve the band–notch property in the 5.0–6.0 GHz band that corresponding to the WLAN and HIPERLAN/2 networks. In the first, a single complementary split ring resonator (CSRR) and in the second a complementary spiral loop resonator (CSLR) is etched off the circular patch. whereas, a spurline is etched off the microstrip line in the third technique. The main idea is to adjust the resonant frequency of the etched slots with the required rejection band to provide the band notch property over the desired frequency range. The design encounters tuning of the resonant frequencies by adjusting the dimensions of the etched slots. These resonator slots provide the band–notch property without degradation of the UWB performance of the prototype antenna. Furthermore, the band–notched resonance frequency and the bandwidth can be easily controlled by adjusting the dimensions of the slots. The antenna is fabricated and the measured data of the return loss versus frequency show very good agreement with the simulated results. The equivalent circuit of the antenna is also obtained to understand the behavior of the antenna. The performance of the antenna is analyzed using parametric study to investigate the tuning effects of the resonator parameters on the impedance matching. The proposed antenna has a simple shape and provides almost omnidirectional patterns, relatively flat gain and high radiation efficiency over the entire UWB frequency band excluding the rejected band. Satisfactory antenna performance with a simple structure and small size makes the proposed proximity–feed antenna a good candidate for UWB communications.
The fourth type of UWB antenna is a simple annular slot antenna based on proximity–feeding technique with an arrangement of electromagnetic band gap (EBG) via holes having standard band–notched characteristic for UWB applications. The EBG via holes is used to enhance the impedance bandwidth of the proposed antenna to cover the whole UWB frequency band as approved by FCC. A band–notch at 5.51 GHz is achieved by inserting a split–ring parasitic element around the radiating patch of the antenna. The VSWR of the proposed antenna is less than 2.0 in the frequency band from 2.82 to 10.74 GHz, while showing a very sharp band–rejection performance at 5.51 GHz preserved for WLAN and HIPERLAN/2 services. The performance of the proposed antenna is confirmed by simulation results, equivalent circuit results and measurement data in which good agreement is obtained. The proposed antenna provides good gain flatness, high efficiency and almost omnidirectional field pattern over the whole frequency band excluding the rejected band. The results show that the proposed antenna is suitable for UWB applications.
Finally, the fifth type of UWB antenna is an annular slot antenna based on proximity–feeding technique with 3.6/5.5 GHz dual band–notched characteristics for UWB applications. By attaching a parasitic arc–strip to the microstrip feed line and etching an arc–slot in the ground plane of the antenna, dual frequency band–notch characteristics are achieved. The proposed antenna has an impedance bandwidth in a frequency range of 2.94–9.02 GHz for VSWR less than 2, with dual band–notch characteristics at 3.30–4.21 GHz and 4.93–6.01 GHz at which IEEE 802.16 standard for WiMAX system, and IEEE 802.11a standard for WLAN system are assigned, respectively. The antenna shows good impedance matching, stable gain, high radiation efficiency and nearly omnidirectional radiation patterns over the entire operated UWB frequency band excluding the two rejected bands. The simulated results of the antenna are assured by equivalent circuit results and measured data of the fabricated antenna.
The five types of the proposed UWB antennas are fabricated using photolithographic technique and their performances and characteristics are studied experimentally and numerically. The fabricated antennas were tested using Agilent HP8719 vector network analyzer. The numerical investigations are conducted using ready–made software, Zeland IE3D which uses Method of Moments (MoM) solver.
Vector Fitting (VF) technique is utilized to establish the SPICE lumped–elements equivalent circuit models of the simulated input admittance of the proposed antennas and the validity of the method is verified. The parametric studies for achieving optimal operation of the proposed antennas and to study the effect of geometrical parameters on antennas performance are also analyzed extensively in order to understand the antenna performance. Curve fitting formulations are obtained to describe the influences of notched structures on the notched frequencies by using first and second order polynomial. Good agreement between the simulated results, and the equivalent circuit results along with the measured data is obtained, which showed that the proposed antennas are good candidates for UWB applications.