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

In this paper, two new methods are proposed for miniaturizing planar ultra-wideband horizontally-polarized omnidirectional Vivaldi antenna arrays. In the first method, the order of a power divider is decreased by means of feeding two neighboring Vivaldi antenna elements in series as a sub-array, 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, non-uniform 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 non-uniform 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. Measured results show that the final design has advantages of a large operating bandwidth, a small footprint of π × (0.284λ_max)² and a high gain. Besides, a clear design flowchart is given. 

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