A Cavity-Backed Crossed-Slot Antenna Element for an S-Band Circular Polarization Spherical Coverage Satellite Antenna System

We present here the results of a preliminary study on a circularly polarized, S-band satellite antenna element consisting of a cavity-backed, crossed-slot radiating element and a microstrip feeding network. This study, involving extensive calculations and engineering model measurements, has verified the feasibility of the antenna element and will constitute the outset for the further development of the antenna system. 1. Introduction A forthcoming mission within the Danish Small Satellite Programme [1], with the two-fold science objective of measuring oscillations in nearby stars and searching for gamma-ray bursts, calls for a circularly polarized, S-band telecommand/telemetry antenna system with a spherical coverage gain of -10dBi. The up- and down-link frequencies are fixed at a 241/220 ratio in the 2.0-2.3GHz frequency band, and the resulting 12% bandwidth thus allows for a single system. In order to fully exploit the avaiable space in the launch vehicle fairing and minimize the risk of mechanical damage, this antenna system must be flushmounted with the solar panels covering the box-shaped satellite. The spherical coverage can be obtained by employing a space diversity principle with two hemispherical coverage antenna elements mounted on opposite faces of the satellite. Helix antennas [2] as well as turnstile antennas [3] have previously been employed for similar purposes but are disregarded here because of their protusion from the satellite body. We have instead chosen to employ a flush-mounted antenna element consisting of a cavity-backed, crossed-slot radiating element excited through a microstrip feeding network. This type of antenna element has mainly been used as aircraft antennas [4] due to their wide beamwidth and low profile. In the past decade mobile satellite services have motivated further developments [5, 6] and crossed-slot antennas have been introduced as radiating elements in phased array applications. The present paper reports the results of a preliminary study to verify the feasibility of this antenna element. The study, which involved extensive calculations with an integral equation-based computer code and impedance as well as radiation measurements on an engineering model, has shown that the cavity-backed crossed-slot antenna element offers good hemispherical radiation characteristics and a VSWR below 1.5 over at least a 12% bandwidth. The antenna suggested in this paper is similar to that of [5] but a larger bandwidth is obtained by using two wide slots. 2. Radiating element The geometry of the cavity-backed, crossed-slot antenna is shown in Figure 1. The two slots are endloaded to increase the effective length of the slot. The slots are backed by a square air-filled cavity which is approximately one tenth of a wavelength deep and the element is excited by four probes to obtain circular polarization and a rotationally symmetric radiation pattern. The probes 1-2-3-4 are sequentially phased - - - , penetrate the cavity from the back, and are connected to the top plate at 4 feed points. This feeding arrangement helps eliminating higher order modes and excites two orthogonal modes in phase quadrature resulting in left hand circularly polarized (LHCP) radiation. The resonance frequency, the antenna input impedance, and the impedance bandwidth are functions of several geometrical parameters. In particular, the resonance frequency decreases by increasing the width or the depth of the cavity, by increasing the probe displacement from the cavity center, by extending the length of