Radiation Pattern Reshaping of a Narrow Slot Antenna for Bandwidth Enhancement and Stable Pattern Using Characteristic Modes Analysis

A novel design concept to reshape radiated fields of a slot antenna for improved bandwidth and stable radiation pattern is proposed. Initially, the radiated fields of the traditional slot antenna are theoretically studied using characteristic modes (CMs). The results demonstrate that its CM1, CM2, and CM3 are resonated around 2.39, 4.78, and 7.17 GHz, respectively. Both CM1 and CM3 maintain the bidirectional radiation patterns, whereas CM2 generates a radiation null in the broadside direction. Then, in order to utilize the nearby CM1 and CM2 for wide bandwidth, the linear slot is properly folded so as to transform the nonbidirectional pattern of CM2 into the bidirectional one. After that, the narrow slot is reformed as the stepped scheme, thus reallocating these dual modes in proximity to each other. With these arrangements, the impedance bandwidth of the antenna is dramatically widened with two attenuation poles, while keeping a compact size and stable radiation pattern. Finally, the proposed antenna is fabricated and tested. Measured results show that the antenna without any extra feeding network has gained a wide bandwidth (
$\vert S_{11}\vert < -10$
dB) of around 31.4% ranging from 3.22 to 4.42 GHz, which is about 2.24 times wider than its traditional counterpart (14%). Besides, the overall size of the slot radiator is kept as small as about
$0.42\lambda _{0} \times 0.06\lambda _{0}$
(
$\lambda _{0}$
is the free-space wavelength). Particularly, the bidirectional radiation pattern, stable gain of around 5 dBi, and low cross-polarization of below −12.3 dB are all satisfactorily generated during the operating band.