Microstrip vehicular antenna for mobile-satellite communications

Abstract

Circularly polarized antennas are used in mobile-satellite or MSAT communication systems. This reduces the polarization mismatch due to the rotational orientation between the transmitter and the receiver and it also minimizes the interference from multipath reflections. Circularly polarized microstrip antennas are commonly used in these systems because of compactness, the light-weight and the low-profile. It eases the installation on a vehicular body or on the surface ofa satellite. This paper presents a circularly polarized microstrip circular patch antenna designed for vehicular use for MSAT communications at a frequency of around I.6GHz. It has right-hand circular polarization (RHCP) and near hemispherical coverage needed by the user. The antenna was first simulated using MatLab software for TM110 resonance mode and fabricated on a substrate of epoxy glass fiber. The antenna was tested on a curved ground plane, which was similar to the surface of a vehicle. This low profile microstrip antenna is suitable for vehicle rooftop mounting. A combination of the resonance cavity model and the transmission-line model was used to analyze the antenna, which consists of a single circular patch as the driven clement. In order to obtain circular polarization, perturbation segments to the perimeter of the patch were inserted with a single probe feed on the 45° axis with respect to the perturbation segments, known as mode-de- tuning. As such no external phasing network is required to obtain the 90° time phase difference required for circular polarization. It was fed with a 50Ω coaxial feed line which was soldered to the inset probe. By adjusting the area of the perturbation segments circular polarization and a good axial ratio was obtained. Half power beam widths of the antenna are 115° in the H plane and 105° in the E plane. The antenna has a gain of 5.7dBi and a bandwidth of 286MHz which is approximately 17% of the centre frequency. The design criteria and performance of the circularly polarized microstrip antenna, which is suitable for MSAT communications, are presented.