A compact circularly polarized wearable filtering antenna for off-body communications

This paper presents a compact circularly polarized (CP) wearable filtering antenna operating in the 2.4 GHz ISM band for off-body communications. The design is enabled by a mutual synthesis of a probe-fed CP patch antenna connected to a bandpass filter composed of coupled stripline open-loop resonators, which provides an integrated radiating and filtering module with a compact form factor. The integrated CP filtering antenna was fabricated and characterized, showing a strong correspondence between simulated and measured results. The resulting antenna achieves an S₁₁ <;-14 dB, an axial ratio of less than 3 dB and gain higher than 3.5 dBi in the targeted ISM band. With the integrated filtering functionality, the antenna exhibits good out-of-band rejection over an ultra-wide frequency range from 1 to 6 GHz. Further full-wave simulations and experiments were carried out, verifying that the proposed filtering antenna maintains these desirable properties even when mounted in close proximity to the human body, thus making it a promising candidate for incorporation into various wearable systems.