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A technique to realize stub-loaded monopoles with enhanced gain performance is introduced in this paper. The omnidirectional high gain is achieved by combining an electrically long monopole with stubs attached at optimal locations along the antenna. In general, the lengths of the stub-loaded monopoles presented in this paper are significantly longer than conventional quarter-wave monopoles. Long wires alone do not provide the desired gain and VSWR performance; the stubs connected to the long monopole are vital in realizing simultaneous high gain and low VSWR at the intended operating frequency. A genetic algorithm (GA) is employed to optimize the locations and lengths of each of the stubs required to meet the design objectives. The results of this work show that between 11 and 12 dBi of gain can be obtained when the stub-loaded monopoles are placed over an infinite ground plane, whereas 8 dBi is obtained for one example when sited over a finite ground plane. A prototype of an optimized stub-loaded monopole was fabricated and tested. Excellent agreement was achieved between the measured and simulated results.
Date of Publication: March 2008