A Light-Activated and Dual-Polarized Harmonic Transponder for IoT Applications

This article presents a photo-activated, dual-polarized harmonic transponder based on a photodiode, which enhances harmonic signal strength and improves structural integration. Based on an equivalent model of the photodiode, the relationship between key device parameters and energy conversion efficiency is analyzed, from which a selection criterion is derived. A novel circuit architecture is designed in which the photodiode provides bias to the nonlinear device, enabling operation at its optimal bias point and thereby increasing harmonic output power. To further improve integration, a dual-polarized, dual-band antenna with a cross-shaped topology is designed. This antenna supports shared transmit/receive functionality with high port isolation, simplifying the overall transponder system. Measurement results of the frequency doubler circuit show that when the input power is -25 dBm, the proposed transponder achieves a conversion loss (CL) of only 11.9 dB, which is significantly lower than that of existing harmonic transponder designs. Under identical test conditions, the complete prototype demonstrates a significantly greater detection range than previous designs. The proposed design is well suited for future low-power Internet of Things (IoT) applications.