Compact Wideband Circularly Polarized MIMO Antennas for IoT Applications

The rapid increase of Internet of Things (IoT) devices has intensified the demand for compact, flexible, and high-performance antenna systems capable of reliable operation across diverse and dynamic environments. This thesis focuses on the development of circularly polarized (CP) multiple-input multiple-output (MIMO) antennas tailored for IoT applications, emphasizing wide bandwidth, high isolation, and mechanical flexibility. A single-element CP antenna is first designed using a C-shaped coplanar waveguide (CPW)-fed structure on a flexible polyethylene terephthalate (PET) substrate, fabricated via inkjet printing. This antenna achieves a wide 3-dB axial ratio bandwidth (ARBW) of 5.66 GHz (79.94%) from 4.25 GHz to 9.91 GHz, along with a 10-dB impedance bandwidth (IBW) of 7.67 GHz (99.55%) from 3.87 GHz to 11.54 GHz, making it highly suitable for wearable and conformal IoT devices. Building on this, a two-element CP MIMO configuration is developed with a diamond-shaped decoupling structure, achieving over 20 dB isolation and a remarkable ARBW of 6 GHz (4.7–10.7 GHz). The mirrored arrangement of antenna elements enhances polarization diversity and ensures stable performance under bending—critical for integration into flexible and body-worn IoT systems. For fixed IoT installations requiring robust and compact solutions, a four-element CP MIMO antenna is proposed on a Rogers TMM4 substrate. Featuring a plus-shaped decoupling structure (PSDS), the design achieves an ARBW of 3.29 GHz (5.02–8.31 GHz) and over 25 dB isolation, without increasing the antenna footprint. The proposed antenna systems demonstrate significant advancements in bandwidth, isolation, and mechanical adaptability, offering versatile solutions for next-generation IoT communication platforms.