UWB planar monopole antenna with stable radiation pattern and low transient distortion

In this study, an ultra-wideband (UWB) microstrip-fed planar monopole antenna with stable radiation pattern is presented. The antenna structure consists of a circular disk as a radiator element and a rectangular ground plane. The antenna is fed by a microstrip line. Although small size and simplicity of planar antennas made them a good candidate for UWB application, they usually suffer from radiation pattern rotation in different frequencies. This effect can be alleviated using some compensating current distribution. To create the desired current distribution, a metallic staircase plane is placed in bottom layer of substrate. Results show that antenna has a stable broadside radiation pattern in 3.1-10 GHz frequency band. A new definition of pattern stability factor (PSFII) is proposed which is proper for both of directional and omnidirectional radiation patterns. Calculated PSFII is higher than 95% which shows a stable radiation pattern of proposed antenna. Furthermore, the fidelity factor of radiated pulse in the main beam direction is higher than 95% which indicate low transient distortion.