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It is well known that a monopole antenna above a ground plane radiates an omni-directional beam around its axis. The height of this monopole can be reduced by using top-loading techniques. One such technique involves attaching a patch to the top of a monopole for use as a microwave antenna. In this case the structure is recognized as a low-profile, center-fed patch antenna. When the antenna height of this center-fed patch is extremely small, the reactance component of the input impedance is large and matching to a 50-ohm feed line is difficult. However, it has been found that this large reactance can be reduced by cutting a slot into the center-fed patch. This slotted patch constitutes two regions: an inner patch island and an outer conducting ring. It has also been found that, when the outer conducting ring is shorted to the ground plane with conducting pins, this structure (abbreviated as a PSP antenna, because the structure is patch with a slot and pins) has a wide bandwidth. Recently, a modified version of the PSP antenna structure has been reported, where the inner patch island above the ground plane is replaced with a conducting body of revolution (BOR). This new structure is designated as the BOR-SPR (BOR above a ground plane, surrounded by a shorted parasitic ring). So far, the VSWR of the BOR-SPR has been analyzed; however, the effect of the size of the ground plane on the radiation characteristics has yet to be revealed. This paper presents the input impedance (VSWR), radiation pattern, and gain of the BOR-SPR when the diameter of the ground plane is varied.