TOP > Report & Column > The Forefront of Space Science > 2008 > New Antennas to Support a Variety of Scientific Observation Missions:Key Communication Equipment Capable of Withstanding a Harsh Environment
Broadband circular-polarized wave donut beam antenna
For spin-stabilized explorers such as the Mercury Magnetospheric Orbiter (MMO), an omnidirectional antenna to the spin plane (plane orthogonal to spin axis) is required. Pioneering Japanese planetary explorers such as SAKIGAKE and SUISEI were equipped with Collinear-array antennas, a pole-like antenna with three-tiered dipole antennas. Since its antenna element is the resonating-type, the bandwidth is narrow with a linear polarization wave. Moreover, its sidelobe (radiation to unrequested direction) is large and the mainlobe’s beam width (radiation to requested direction) is about 20 deg., with a maximum gain of around 5dBi. Of course, the MMO’s antenna has to satisfy the electrical property requirements of a Mercury explorer. Furthermore, since it approaches 0.3 AU from the sun, the thermal environment is severe. The explorer is exposed to solar energy 11 times (11 solars) that of earth-orbiting satellites. Thus, we had to develop a radical new antenna far beyond the conventional design.
The antenna developed is shown in Fig. 2. It is a dual-reflector structure antenna combining parabola and conical mirrors. It can radiate almost evenly broadband circular-polarized wave to the spin plane. Fig. 2a is the structural illustration and Fig. 2b is a photo of 1/1-scale engineering model. Fig. 2c shows its typical radiation property with almost no sidelobe, a maximum gain of 7dBi and a beam width of about 20 deg. The pattern on the spine plane is almost circular, so it has an ideal radiation property. We were also able to attain good circular-polarized wave property covering broadband. Since the antenna’s bandwidth is not narrow, unlike conventional resonating antennas, one of its advantages is the elimination of frequency drift due to minor thermal deformation on the antenna.