A Low-Profile Unidirectional Antenna That Uses Folded Dipole With High Input Impedance as the Radiator

Abstract

By studying the impedance characteristics of the horizontal half-wave dipole near the metal reflector, we find that the antenna input impedance decreases when the distance between the dipole and the reflector is reduced. Therefore, a folded dipole with high input impedance is considered to replace the half-wave dipole to reduce the antenna profile. The folded dipole developed here occupies two connected dipoles with different widths, which is conducive to obtaining a higher input resistance, in other words, to obtaining a lower profile. In the rectangular area enclosed by the folded dipole, a coupling branch is added to extend the bandwidth. The antenna can cover the 2.3–2.62 GHz band, and provide stable directional radiation. Its size is only 0.35 × 0.2 × 0.08λ³ (λ is the free space wavelength corresponding to the center frequency of the dipole operating band). We made a prototype of the antenna and tested it. Both the measured and simulated results verify the effectiveness of the method of using such a dipole with high input impedance to reduce the antenna profile. The proposed antenna can be used in base station system.