Input Impedance of Radiation Efficiency Deterioration State

The factors of radiation efficiency and impedance matching were investigated for a variable reactor-loaded antenna. It was found that when the conductivity decreases, there are reactance regions where the input power increases, and the radiation efficiency deteriorates in the regions. These reactance regions overlap with the regions of low input resistance, but in low frequency they overlap with the regions of low input reactance. This is thought to be because its absolute value becomes large in wide reactance ranges in the low frequency and the input reactance becomes dominant. Since impedance matching is improved when the input reactance is near 0, controlling the reactance value to achieve matching in low frequency leads to reducing radiation efficiency. Therefore, it is difficult to achieve high antenna efficiency for electrically small antennas.