The pattern nulls of low profile ultrawideband (UWB) omnidirectional antennas in the main beam region cause the performance degradation of UWB wireless communication system in the null directions. In order to achieve ultrawide pattern bandwidth, a method to improve the radiation pattern of low profile UWB omnidirectional antennas is proposed in this letter. The mathematical operation of improving the radiation pattern is derived. A low profile UWB discone antenna is designed as a representative of the low profile UWB omnidirectional antennas, and the pattern characteristic is investigated. Then, the effects of the sleeves and modified cone on the antenna pattern and input impedance are analyzed using microwave simulation software. The results show that the pattern bandwidth is increased from 6.5:1 to 11.4:1. In the operating band, the pattern nulls along the entire phi plane are eliminated, and the minimum value of the pattern nulls is optimized from -35.0 to -12.4 dB. The corresponding gain exceeds 0.2 dBi in the main beam region of the theta direction 30° to 90°. The optimization is based on a constant volume of 0.774 ka. Finally, an antenna prototype is fabricated and the measurement results are consistent with the simulation results, verifying the validity of the method

In this letter, a compact frequency reconfigurable beam-switching antenna based on a single-layer frequency selective face (FSS) is proposed. The proposed antenna consists of a dual-band omnidirectional antenna and a hexagonal single-layer FSS. The dual-band omnidirectional antenna is designed as a radiation source and surrounded by the FSS. Each FSS unit consists of two crossed patches and diodes. Frequency reconfigurable characteristics at 2.4 or 5 GHz are achieved by switching the states of the diodes on the single-layer FSS. With specific combinations of FSS units, the omnidirectional radiation patterns can be transformed into directional ones scanning along the entire azimuth plane. To validate the design, a prototype is fabricated and measured. The results show that the proposed single-layer FSS antenna can perform a 12-step scan along the azimuth plane at 2.4 and 5 GHz, respectively