Compact Dipolar Patch UHF RFID Tag Antenna Using Interdigital Capacitor for Metal Platforms

A compact dipolar patch antenna is proposed for on-metal UHF RFID tag applications. The design employs two centrally located shorting stubs to establish a direct ground path, effectively increasing the antenna’s electrical length and lowering its resonant frequency. To enhance impedance matching and further reduce size, multiple inductive slots are etched into the top patch, introducing distributed inductance. The key innovation lies in the integration of dual interdigital capacitors (IDCs) within the radiating patch, which serve as compact, tunable capacitive elements. By carefully adjusting finger dimensions and spacing, the IDC capacitance is analytically modeled and optimized. These capacitive elements introduce additional reactance, improving impedance matching, increasing antenna gain, and shift the antenna’s operating band into the UHF RFID frequency range (860–960 MHz). Despite its compact dimensions of 50 mm $\times 25$ mm $\times 3.32$ mm ( $0.153\lambda $ $\times 0.077$ $\lambda $ $\times 0.010$ $\lambda $ ), the proposed tag achieves a maximum read range of 13 meters under 4W effective isotropic radiated power (EIRP), outperforming many existing on-metal RFID tags of similar size. The entire antenna structure is fabricated on a polyimide and foam substrate, optimizing both mechanical flexibility and performance on metal surfaces. The antenna demonstrates strong potential for space-constrained and metal-mountable RFID applications, offering a robust and efficient solution for industrial and logistics environments.