A Study of Microstrip Antenna Elements and Arrays.
ILLINOIS UNIV AT URBANA-CHAMPAIGN ELECTROMAGNETICS LAB
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Thin microstrip antennas have many unique and attractive properties, but their operation is much limited in bandwidth. In this report an attempt is made to develop a procedure for array design, in particular, to determine whether or not the band of operation could be broadened by using a small array of two elements of slightly different dimensions. By broadband, it is meant that the antenna must have over the bandwidth 1 an input impedance variation within a certain prescribed SWR say 31 2 a stable pattern in particular a stable beam-pointing direction 3 a small variation in gain or efficiency and 4 in some applications, small variations in beamwidth and sidelobe level. In this work only the first two fundamental performances are considered. Insofar as the impedance is considered, the array design is essentially a circuit problem. Our theory based on the cavity model has previously shown that the element impedance can adequately be described by a Foster network representation, namely, a series circuit of infinitely many parallel resonant circuits, each corresponding to a mode of the cavity. For frequencies in the vicinity of a certain mode, the circuit can further be reduced to a single parallel resonant circuit of that mode in series with an inductance. Our earlier investigation has also shown that, except for very close spacings, the mutual coupling effect between elements can be ignored.
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