Electrically small loop antenna surrounded by a “shell” of concentric split loops

Electrically small antenna formed by planar loop with a “shell” of three concentric split loops with the relative outer size ka = 0.40 has been designed with complex input impedance Zin ~ 22 + j195 Ω to be used as UHF RFID tag antenna. The “shell” of concentric loops increases the total current path length so that it reaches the length about 1.08 λ₀ at 869 MHz. The measured radiation and antenna efficiency is thus improved and reaches the value about 51 and 33%, respectively.