Design of Tunable Electrically Small Antennas With Circular Polarization

This paper presents a frequency tunable electrically small antenna with circular polarization. The circular polarization is obtained through the use of crossed-dipole antenna elements, and the miniaturization is achieved by introducing parasitic elements in the near-field region of the driven elements. Additionally, top-loading technique is applied to the crossed-dipole antenna structure to reduce its size further. In order to tune the operating frequency, the antenna employs mechanically adjustable capacitances in the parasitic elements. Without loss of generality, the antenna operating frequency can be adjusted within the range of 105-350 MHz, as demonstrated through both design and construction. The overall dimensions of the antenna are 0.14λ × 0.14λ × 0.08λ, and its electrical size is characterized by <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">kr</i> < 0.63 at the center of the tunable frequency range (220 MHz). The antenna achieved a forward-direction gain of 3.8 dBi at 220 MHz. In addition, the axial ratio (AR) remains below 3 dB across the tunable frequency range of 105–350 MHz. The measurement results within this band are in good agreement with the simulation results.