Low-Profile Log-Periodic Monopole Array

This paper presents a low-profile log-periodic monopole array antenna with end-fire radiation and vertical polarization. It consists of 15 monopole elements and a conductor-backed coplanar strip line as the feeding line. The monopoles are of the same height but loaded with top hats of different sizes to achieve the required resonant frequencies. A wideband transition from coaxial line to conductor-backed coplanar strip line is designed for the antenna feed. A prototype is fabricated and tested. Measured results show that an impedance bandwidth from 1.5 to 6.8 GHz (4.53:1) for VSWR <;2.3 and a gain better than 4.5 dBi over the same operating frequency band are obtained while the antenna has a very low profile of only 0.047 λ_L (λ_L is the free-space wavelength at the lowest operating frequency).