Single-fed tunable electrically small antennas with circular polarization based on miniaturized Schiffman phase shifters

This paper advances a compact circularly polarized (CP) electrically small antenna (ESA) employing near-field resonant parasitic (NFRP) technology with frequency-tuning capability in the VHF band. In the proposed NFRP configuration, a capacitively loaded loop (CLL) is utilized to enhance the antenna’s characteristic impedance. The primary radiator comprises top-loaded crossed dipoles, interconnected via double loops, and printed meander lines. Frequency tuning is achieved by adjusting the spacing between the parasitic CLL elements. Circular polarization is realized without the use of a conventional external phase shifter. Instead, the 90° phase difference between orthogonal dipoles is generated within the antenna structure itself, employing a combination of the double loop technique and a Schiffman-inspired phase-shifting mechanism based on meandered lines. The antenna is fed through a single port, ensuring structural simplicity and compactness. At the center frequency of 205 MHz, the overall dimensions of the antenna are 0.13 λ×0.13 λ×0.07 λ, corresponding to a ka value of 0.59. The measured gain at 255 MHz is 3.95 dBi, and the axial ratio (AR) remains below 3 dB across the entire tuning frequency range. Experimental results show close agreement with simulations, achieving a 49% fractional tuning bandwidth (FTBW).