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INTRODUCTION
The demand for wireless communications services is growing at an extensive rate all over the world. The intensive development and wide application of new generations of personal communication systems and wireless local systems, have increased the need for new antenna designs [1]. As the wireless communications networks expand, the number of both unwanted directional interferences and strong nearby sources increase, which degrade system performance. Optimal antenna arrays play an important role in the improvement of communications systems by providing increased coverage through antenna gain control and interference rejection [2, 3]. A system consisting of an array and a processor can perform filtering in both space and frequency to reduce the sensitivity of the system to interfering directional sources.
There are many requirements that new communications systems pose on antennas. The most common are low cost, low weight and compact designs with high-performance radiation and impedance characteristics. Therefore uplink and downlink antennas undergo a lot of modifications and performance improvement [1]. In many wireless communications systems, the signal-to-interference ratio (SIR) is improved by using multiple nulls in the directions of the interferences while maintaining omnidirectional coverage in the direction of the network users. The nulls in the radiation pattern may be used to reduce the interference from strong nearby sources as well as co-channel interference by introducing nulls in the directions of surrounding communication sys- tems.
For the communication system considered in this thesis, the interferences are static and their directions of arrival are known. A non-adaptive antenna array is needed to provide the spatial filtering in a static wireless environment. Multi-path scenarios and space-time filtering via an adaptive array are therefore not addressed.
Omnidirectional arrays, such as cylindrical arrays, are the most suitable to provide the omnidirectional coverage and are capable of suppressing interferences when nulls are inserted in the radiation pattern. Some examples of cylindrical arrays are shown in Figure 1.1.
Microstrip antennas, which the work in this thesis will focus on, are popular components in modern systems, since they are low in profile, light in weight and well suited for integration with microwave integrated circuits. They can also be made conformal to non-planar surfaces such as cylinders to form cylindrical arrays as shown in Figure 1.1(c).
This type of cylindrical array is also classified under conformal arrays of which the analysis and design can be divided into three areas [4]:
1. the analysis of the antenna element radiation pattern,
2. the array pattern analysis and synthesis to obtain the element excitations, and
3. the design of the radiators and the feed network to obtain the desired excitations.
The analysis of cylindrical microstrip patch antennas and arrays have been discussed in many publications [5–31]. The cavity model theory, method of moments, geometric theory of diffraction (GTD) and finite-element method have all been applied to obtain the resonant frequency, impedance behaviour and/or radiation pattern of a rectangular microstrip patch on a conducting cylinder. For electrically thin substrates, the cavity model has been shown to be sufficient to compute the characteristics of the patch antenna [23,24,29]. Hybrid modes are excited for electrically thick patches which require more accurate analysis methods [14]. When the radius of the cylinder is much larger than the operating wavelength, the effect of the curvature on the characteristics of the patch antennas may be neglected. For radii comparable to the operating wavelength, the input impedance, resonant frequency and radiation pattern of the patch antenna are effected by the curvature. The cavity model [13] and a commercial finite difference ime domain (FDTD) analysis program have been used during the design of the radiating lement to incorporate the influences of the curvature [30]. The FDTD analysis program can also be utilised where electrically thick patches are considered to improve bandwidth and gain performance.
1 INTRODUCTION
1.1 Background
1.3 Methodology
1.4 Layout of thesis
2 BACKGROUND
2.1 Characteristics of a cylindrical microstrip patch
2.1.1 Cavity model for cylindrical microstrip patches
2.1.2 Radiated fields
2.1.3 Axial polarisation
2.1.4 Circumferential polarisatio
2.1.5 Characteristics of the radiation patterns
2.2 Cylindrical array pattern
2.2.1 Equally spaced cylindrical arrays
2.3 Null synthesis techniques
2.3.1 Definitions of parameters in null synthesis
2.3.2 Superposition of sequence excitations
2.3.3 Fourier approximation of an ideal pattern
2.3.4 Orthogonal projection method
2.3.5 Pattern synthesis with null constraints
2.4 Mutual coupling compensation
2.4.1 Minimising the mutual coupling effects
2.4.2 Compensation using coupling and impedance matrixes
2.4.3 Modification of the driving impedances
2.5 Summary
3 NULL SYNTHESIS
3.1 Orthogonal projection method
3.1.1 Modification of the orthogonal base
3.1.2 Results of the projection method
3.2 Objective weighting method
3.2.1 Performance function and Pareto optimality
3.2.2 Results of the objective weighting method
3.3 Constrained optimisation
3.3.1 Results of the constrained optimisation
3.4 Comparison of null synthesis methods
3.5 Multiple null synthesis
3.6 Influences of the antenna element characteristics
3.7 Results for various null positions
3.8 Summary
4 IMPLEMENTATION OF CYLINDRICAL MICROSTRIP PATCH ARRAYS
4.1 Design of cylindrical microstrip patch element
4.2 Effect of mutual coupling
4.2.1 Effect of mutual coupling for linear patch arrays
4.2.2 Effect of mutual coupling on the amplitude pattern of cylindrical patch arrays
4.3 Mutual coupling compensation
4.3.1 Mutual coupling compensation for linear patch arrays
4.3.2 Mutual coupling compensation for cylindrical arrays
4.4 Test cases
4.4.1 Linear patch array test case
4.4.2 Cylindrical patch array test case
4.5 Summary
5 CONCLUSIONS
5.1 Null synthesis using cylindrical microstrip patch arrays
5.2 Implementation of cylindrical microstrip patch arrays
REFERENCES
