A Triple Frequency Miniaturized Monopole Antenna Applied to VHF Band

In this paper, a triple frequency miniaturized monopole antenna is proposed. The antenna consists of a monopole mounted vertically on the ground, a sleeve and a corresponding loading structure. The available space size of the antenna is 55mm*25mm*150mm. The addition of the sleeve, loading structure, and impedance matching network enables the monopole to operate at three frequencies, 121.5MHz, 156.5MHz, and 162MHz. In this way, monopole's height (150mm) is about 0.06 times the corresponding wavelength of the lowest frequency (121.5MHz), far less than 0.25 times the wavelength of a single monopole antenna. The loading structure of the monopole is divided into two parts: top loaded and bottom series inductor, both of which play the role of increasing the effective length of the antenna and miniaturizing it; At the same time, the sleeve of the bottom series inductor is added to expand the impedance bandwidth, which provides the conditions for impedance matching. The impedance matching network is composed by series-parallel connection of multiple capacitors and inductors, and the optimal solution is obtained by differential evolution algorithm, so that the VSWR (Voltage Standing Wave Ratio) of antenna is 1 at the three frequency points of operation. Finally, the real object is made to verify the feasibility of the overall scheme.