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 fields 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 MIMO array designs. By loading decoupling meander lines between closely spaced elements (<0.02λ₀), port isolations are increased by over 9.6 dB. Moreover, the distorted radiation patterns are significantly improved. A prototype of the 1 × 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. © 2025 IEEE.