In this paper, we present a low-profile, compact, ultra-wideband antenna that uses a set of closely coupled radiators. The system of two coupled radiators has two different linearly independent modes of operation with complementary frequency bands of operation. These include the differential mode and the common mode of operation. When the antenna is excited in the common mode of operation, it acts as an ultra-wideband (UWB) antenna covering a broad frequency band. When excited in the differential mode, the antenna operates as a wideband dipole in a frequency range below that of the common mode. Thus, by appropriately combining the two modes using a suitably designed feed network, the bandwidth of the antenna can be extended and its lowest frequency of operation is reduced. Mode combining is achieved with a feed network that employs a frequency-dependent phase shifter. Using this feed network, the two modes of the antenna are combined and a single-port broadband device is achieved that has a bandwidth larger than that of either the common or the differential mode individually. A prototype of the antenna is fabricated and experimentally characterized.
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