MODERN ANTENNA DESIGN USING MODE ANALYSIS TECHNIQUES

In this paper, the modal theory of antennas is re-visited, believing that it brings invaluable information towards facilitating the design of multi-feed multi-band antennas.First, some subtle changes are proposed to enhance the applicability of the theory.Next, using some efficient computational techniques, the proposed formulations are shown to predict, to a very high accuracy, the input impedance of any antenna under study.This greatly simplifies the antenna problem and focuses design efforts on finding the appropriate complex resonance frequency to cover a required band.Finding the appropriate feed location is then a matter of extracting the corresponding impedance map for this antenna through simple field manipulations. BACKGROUNDModal analysis among the circuit and filter communities is widely spread.A close look at the design steps of many of the available modern filters reveals the utilization of modal techniques as a key design component [1].Notably, modal analysis of antennas has long been known in the antenna community.Lo and Richards contributed significantly to the advancement of this theory in the late 1970s and early 1980s [2].However, their approach was limited in its accuracy due to the utilization of some theoretical assumptions of the complex resonant frequencies of the antennas they studied.These assumptions imposed significant limitations on the antennas' structures that could be analyzed, along with limitations on the achievable Q values.To date, modal analysis in antenna design has not evolved at the same rate it did in circuit/filter applications.There exists a plurality of work on antenna analysis using modal expansion techniques.The work of Harrington and Mautz is one example [3], and the work of Shen and MacPhie is another [4].Remarkably, the analysis and design of printed antennas is widely documented in numerous research papers and assembled in several books (see for example [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]).In its simplest form, a planar printed antenna is modeled as a transmission line model with its radiating edges treated as slots.A more rigorous analysis came in 1977 by Professor Lo [2] who treated the planar printed antenna as a cavity.His work was amended by multiple refinements to the modal approach for antenna design [14,15].However, the research following his approach essentially diminished since the early 1990s with rare subsequent occurrences in publications.This is primarily due to the limited accuracy of the approach when dealing with practical antenna designs.The limited accuracy was mainly attributed to the approximate calculation of the complex resonant frequency.This means that if such a frequency were calculated to a higher accuracy, then the technique would result in much better results.This will be the main focus of this chapter.Although the transmission-line model is easy to use, it suffers from numerous disadvantages [11].For instance, it is only useful for patches of rectangular shape, the fringe factor must be empirically determined, it ignores field variations along the radiating edge, and it is not adaptable to inclusion of

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