Radiation improvement from a very narrow slotline using a short-ended double spur-line

In this letter, a simple design to improve an antenna radiation performance by adding a matching network consisting of short-ended double spur-line is proposed. By inserting a short-ended double spur-line in the slotline and connecting it with a ground plane, a parallel resonance with a bandpass characteristic occurs, and the characteristic impedance of the very narrow slotline is close to the impedance of free space. To verify this characteristic, an equivalent circuit model is presented to offer a detailed explanation, and the comparisons between the simulated and measured results are demonstrated.