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Title: Design and analysis of a helical wire antenna with a parasitic element
Other Titles: She ji yu fen xi fu you ji sheng yuan jian de luo xuan xing tian xian
Authors: Chow, Yan-wai (周仁偉)
Department: Dept. of Electronic Engineering
Degree: Master of Philosophy
Issue Date: 2001
Publisher: City University of Hong Kong
Subjects: Antennas (Electronics)
Notes: CityU Call Number: TK7871.6.C47 2001
Includes bibliographical references (leaves 123-129)
Thesis (M.Phil.)--City University of Hong Kong, 2001
x, 130 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: Nowadays, mobile and satellite communications are popular. In particular, the industry of mobile phones is exorbitantly successful as its rate of penetration in many metropolitan areas is much higher than its wired counterpart. Compared with the development of other electronic devices, such as audio and visual products as well as household electrical appliances, its maturity is much faster. Although there are many competing standards when the second generation phones were first introduced several years ago, the majority of them are now operating under the platform of the Global System for Mobile communications (GSM for short), Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). Aforementioned the several platforms in the second generation, different systems are standardized in many countries and metropolitan area, such as GSM system for European countries, and GSM 90011900MHz in United States, Personal Handy System (PHs) in Japan and GSM 90011800MHz in Hong Kong. Due to the inconvenience of the accommodation in different standards, a roaming service and multiple band system take advantage for frequent travelers. However the difficulty of globalization in the mobile standard still not be solved. Meanwhile, manufacturers change their targets to support the advanced functions in mobile phones, such as a voice call and short message service (SMS) message and therefore the second and a half generation is developed. General Packet Radio Service (GPRS) is an enhancement to existing GSM and TDMA networks that supports a high-speed packet data transmission for email, internet access and other services. Moreover, the Wireless Application Protocol (WAP) is another global standard that accesses the internet services through handheld devices. They assist today's businesses adapt to the new economy and create cost-effective ways to communicate with customers. Furthermore, these services are the steps towards the third generation 3G. Due to the saturation in the existing mobile communications, 3G supports a wide range of mobile internet services with greater bandwidth operation. This new generation could be standardized throughout the world and is the best technology for success breeds demands. In order to support the third generation, the wideband operation and saving battery feature play the important roles in the design of mobile phones. The wideband operation provides the advance of frequency hopping. However the battery is drained in the system connection due to a high loss and low receiver sensitivity or called G/T ratio. Recently, the design focuses on the minimization of the RF circuit but there is not much improvement on saving power. A large LCD display is needed for 3G mobile phone but it drains so much power. Moreover, power is consumed so much for the network connection in the terrain. Up to now, no engineer pays much attention to the design of antenna because it works well in an urban area. After all, it is an engineering problem. Due to the efficiency of output power in mobile phone, the antennas play the key role to work effectively on battery and provide a greater wideband operation. Some manufacturers began to introduce better antennas, such as the patch and dielectric resonator antennas. These antennas which are low profile and convenient to be embedded inside the mobile phone, are the step forward design. Notwithstanding, the high loss in low Q dielectric substrate and relatively expensive in material cost are their drawbacks compared with the wire antennas. Primitive short antennas are mostly wire antennas, such as monopole and normal mode helical antennas but they are deficient to provide a wideband and high gain. Having said that, they are not the only alternatives of small antennas. The monopole antenna coupled with a parasitic normal mode helix was designed to support an attractive wide bandwidth and a sufficient power gain to mobile phone. A simple but efficient designing concept puts forward this antenna to replace the deficient monopole and helical antennas. In addition, its light in weight and low material cost are conducive to the installation in mobile handsets compared with the patch and dielectric resonator antennas. In some cities, the demands on mobile phones are so severe that the regulatory agency has to free the frequency band for the services. To this end, many dual frequency mobile phones are available in the market. In fact, these phones help the mobile users to bypass the traffic congestion during peak hours. However the drawbacks of the narrow bandwidth, low gain and the drainage on battery are retained in the conventional dual band antennas. For these reasons, the monopolehelical antenna coupled with a parasitic normal mode helix was proposed to enhance bandwidth and gain in dual band system. Furthermore, the receiving signal in either GSM 900/1800MHz or 900/1900MHz can be adaptively controlled by the mechanism of adjusting the parasitic helical height. In other words, this dual frequency antenna is accessible to roam the different standards in Asian, European countries and United States. For the prospect of accommodating multiple standards, a stand-alone antenna is externally adoptive to mount onto the mobile phone. A preferable circularly polarized antenna can be replaceable to support the mobile satellite communications in the same mobile phone. This type of antenna is insensitive to ionospheric polarization rotation and thus is widely used in the handheld systems and ground earth terminals of Very Small Aperture Terminal (VSAT), Inmarsat satellite phones and Low Earth Orbit (LEO). Recently the conventional quadrifilar helix antennas (QHA) are versatile to the satellite and terrestrial networks, but they only provide a near hemispherical coverage and a narrow axial ratio bandwidth. Therefore, a simple improvement is launching the parasitic helical strips on QHA itself. This antenna supports a wide radiation pattern because of a good isolation between right hand circular polarization (RHCP) and left hand circular polarization (LHCP). For a user in a remote area, it facilitates one user to communicate with the rest of world via the satellite. The better axial ratio makes antenna receive no loss in the CP wave via the Faraday rotation in the ionosphere. Hence, this compact antenna is efficient to the mobile terminals for the satellite communications systems like Global Positioning System (GPS). In our research, the key analysis on the bandwidth enhancement by the effect of a parasitic element is experimentally and theoretically studied in our design methodology. The impedance bandwidths and radiation patterns of the antennas were investigated by precisely trimming the copper wire during experiments. During the measurement procedure, the optimization of their characteristics was explicitly found via the network analyzer and compact range anechoic chamber. In the theoretical study, the electric field integral equations of our antennas are calculated by the Method of Moments (MOM) and Mini Numerical Electromagnetic Code (MININEC). Good agreements on input impedance and far field characteristics between the theoretical and experimental results are shown in our parametric study. To summarize the results of our study, a parasitic helix mounted onto the conventional wire antenna provides a significant bandwidth enhancenient compared with the unaided antenna. In our first two designs, wide bandwidths in frequency ranges of 900MHz, 1800MHz and 1900MHz are obtained while the far field characteristics are radiated in broadside pattern. For the quadrifilar helical antenna with parasitic helical strips at S band operation, a wide beam elevation coverage, good circular polarization and axial ratio are improved in the comparison to the conventional quadrifilar helix antenna. The optimization of a coupling effect in these antennas was entirely investigated. These antennas provide a simple design, ease on construction, low cost, wide bandwidth and low power consumption for applications on the third generation 3G and satellite communications systems. Further improvement on size reduction and efficiency is needed. Through the study and obtained results, some knowledge may be provided for engineering designs of mobile handset antennas.
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