In this paper, a method to miniaturize a stepped open-slot antenna (SOSA) for ultra-wideband (UWB) applications is proposed. The antenna consists of a stepped open slot, J-shaped slots, and two strip directors. A broadband microstrip-to-slotline transition with circular stubs is applied to feed the antenna. J-shaped slots are inserted on the ground plane of the antenna to create a new resonance in the low-frequency region, thereby miniaturizing the size of the antenna. Finally, two strip directors are appended above the stepped open slot in order to increase the gain in the middle- and high-frequency regions, as well as to enhance input impedance matching in the high-frequency region. A prototype of the miniaturized SOSA is fabricated on an FR4 substrate with dimensions of 30 mm×32 mm. It shows a measured frequency band of 2.99-10.87 GHz for a voltage standing wave ratio < 2, which ensures UWB operation, and the measured gain range is 3.2-7.3 dBi in the band with a front-to-back ratio > 8.7 dB.
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