Omnidirectional antennas have been widely used in wireless communication devices, including the cellphone base station, Wi-fi routers and many portable devices. It is also adopted for omnidirectional imaging radar systems. Usually vertically polarized antennas are applied for these systems since they could easily achieve wide bandwidth and good omnidirectional radiation pattern. However, for more advanced commuinaction and radar applications where polarization diversity is considered, a horizontally polarized (HP) omnidirectional antenna is also required to obtain higher efficiency in communication and improved in imaging radar systems. Many of these systems operate with wide bandwidth. However, many of traditional HP antennas suffer from narrow bandwidth. There are several designs reported to increase the bandwidth omni-directional HP antennas recently, but none of these have bandwdith more than 50%.
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