Omnidirectional Printed Filtering Antenna and MIMO Array Using Folded Structures

A printed omnidirectional filtering antenna is investigated without using any extra filtering circuits. The antenna consists of three folded structures: one narrow closed loop and two split loops. One of the split loops has a narrow width with its opening at the bottom, while the other split loop has a wider width with its opening at the top. The latter features folded PIFA-like structures. All these structures are connected in parallel and etched on a one-layer substrate. Due to the field’s cancellation, the design generates three radiation nulls near the passband, realizing filtering responses. The antenna achieves a measured −10-dB impedance bandwidth of 12.0% and exhibits omnidirectional radiation patterns. Its measured total efficiency within the passband is 87.4%, while the suppression level in the stopband exceeds 18.8 dB. This basic omnidirectional filtering design is extended to multiple input multiple output (MIMO) array designs. By loading decoupling meander lines between closely spaced elements ( $\lt 0.02\lambda _{0}$ ), port isolations are increased by over 9.6 dB. Moreover, the distorted radiation patterns are significantly improved. A prototype of the $1\times 4$ MIMO array was fabricated and tested to verify the idea. A reasonable agreement between measured and simulated results was obtained. This MIMO array has couplings lower than −19.8 dB with rectangular coefficients below 1.35. Compared to many reported omnidirectional studies, our design features advantages in frequency selectivity, stability of omnidirectional radiation pattern, and scalability of antenna element.