Bidirectional Gain Enhancement of Folded Dipole Antenna Using Midstrips Array

Abstract

A novel low-profile method to improve the bidirectional gain of a dual-element folded dipole antenna (DFDA) is presented. The proposed technique employs surface current distribution conditioning using integrated parasitic elements while preserving the radiators. We introduce a middle layer consisting of a 1-D array of metal strips embedded between the upper and lower surfaces of the dipole, called midstrips. The midstrips are designed as parallel resonators and play the key role of current polarizing of the dipole currents. At the presence of the midstrips, the cross currents on the dipole surface are polarized out of phase and cancel each other, improving radiation-polarization purity consequently. The co- components are changed from a distributed form to a triple current form, which leads to the excitation of a new resonance. The geometric-progression-based array factor is extracted to model the gain enhancement mathematically. Two DFDA prototypes with and without midstrips are fabricated with identical thickness ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.014\lambda _{0}$ </tex-math></inline-formula> at 2.7 GHz) and 10 dB-S11 bandwidth of over 18.5%, which shows a bidirectional-gain enhancement of more than 2.9 dB and an efficiency of over 84% at the presence of the midstrips.