Multi-Input Multi-Output Compact Antenna with One-Way Circular Polarization for WiMAX 3.5 and ITU-R Applications

In this research, a compact multi-layer microstrip antenna for Multiple-Input Multiple-Output (MIMO) systems with dual-band circular polarization capability has been designed, simulated, and fabricated. The antenna structure employs separate coupling networks for each frequency band, each comprising a hairpin resonator, a straight slot, and a U-shaped groove. This configuration enables the establishment of two signal paths with the requisite phase difference to feed the patch. By applying a ±90° phase shift, the antenna achieves dual-band circular polarization operation. The principal innovation of this work is the implementation of a multi-layer feeding network that ensures an axial ratio below 3 dB, achieved through the combined action of the straight slot and U-shaped groove to excite a circular polarization resonance mode. The antenna exhibits right-hand and left-hand circular polarization characteristics in different frequency bands. Simulation and experimental results validate the antenna’s performance, demonstrating impedance bandwidths of 3.0–4.2 GHz and 5.5–6.6 GHz, with near-complete circular polarization coverage across the first band. The gain response increases linearly with frequency, achieving an average value of 5 dB.