Miniaturization of Ultrawideband Horizontally Polarized Omnidirectional Vivaldi Antenna Arrays Using Nonuniform Elements

In this article, two new methods are proposed for miniaturizing planar ultrawideband (UWB) horizontally polarized (HP) omnidirectional Vivaldi antenna arrays. In the first method, the order of a power divider (PD) is decreased by means of feeding two neighboring Vivaldi antenna elements in series as a subarray or termed as a double-Vivaldi antenna element, and thus, the layout area required for a feeding network is reduced. Therefore, the layout problem of a conventional circular Vivaldi antenna array with a confined size is solved. Meanwhile, nonuniform elements with different taper rates are used to solve the in-band peak gain variation that is caused by a disc monopole mode. In this way, a nonuniform series-fed double-Vivaldi antenna array is presented. In the second method, a folded loop structure is introduced for an electrical size reduction of the aforementioned array. As a result, a lower resonant frequency is obtained. The measured results show that the final design has the advantages of a large operating bandwidth, a small footprint of $\pi \times $ ( $0.284\lambda _{\max })^{2}$ , and a high gain. Besides, a clear design flowchart is given.