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An omnidirectional microstrip antenna using a parasitic cylinder is presented. A rectangular patch is formed on a dielectric substrate and it's completely covered with an aluminum cylinder which is somewhat shorter than a half of free space wavelength. Under such configuration the aluminum cylinder works as a parasitic element. This antenna can provides uniform omnidirectional radiation patterns and a broad frequency bandwidth. In this paper an experimental method for designing such an element is described. Measured input impedance characteristics, current distribution around the surface of the cylinder and patterns are also shown. By properly adjusting the coupling intensity between the patch and the parasitic cylinder a broad bandwidth antenna element can be realized. Some methods to adjust the coupling intensity are shown. A wide bandwidth element up to 14% for VSWR<1.5 is obtained. Arranging many patches lengthways on a substrate and placing metallic cylinders around each patches, we can realize a high-gain and broad bandwidth collinear antenna.