Unidirectional Small Antenna Design Using Optimal Currents Based on Characteristic Modes

A method for designing unidirectional electrically small antenna is proposed by analyzing the achievable directivity using characteristic modes. By examining the optimal currents of the achievable directivity within the design space, the fundamental antenna structure can be established. Structural parameters are then optimized to achieve the antenna with high gain, a high front-to-back ratio, and good impedance matching. The proposed method is validated through the simulated design of a unidirectional electrically small antenna with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\boldsymbol{ka}=\mathbf{0.7}$</tex>, achieving a gain that closely approaches the achievable gain of the initial design space, along with a good FTBR. This method provides valuable design guidance for configuring high-performance unidirectional antennas within constrained spaces.