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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

The Forefront of Space Science

New Antennas to Support a Variety of Scientific Observation Missions:Key Communication Equipment Capable of Withstanding a Harsh Environment
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Wafer-like light antenna with high heat resistance

Because planetary explorers perform far-distant communication over 1AU, a high-gain antenna by narrowing beam is required. In the past, parabolic antennas were used since they can converge radiowaves to a focal point on the antenna. For inner planetary explorers to visit Mercury or Venus, however, focal-structure antennas are unsuitable because they converge strong sunlight as well. We considered how we could develop and install planar antennas, completely different from traditional types, for such missions. Even if the antenna itself is small, there is a way to increase gain. If we array many antennas appropriately on the plane and align each one’s feeder power and phase, the antenna gain would increase in proportion to the number of antennas. Even if the gain of one is only 9dBi, it is possible to obtain a high gain of 36.4dBi by arraying 546 elements. One more key factor in creating highly efficient antenna is how to reduce loss in circuits to feed power to individual elements. In typical array antennas, loss in feed circuits is high making them inefficient.

The antenna we developed employs a feeding system with a wave-guide tube structure as shown in Fig. 3. The radio wave is emitted into the wave-guide tubes from the feeder pin in the center of the antenna’s reverse side and is fed to each antenna element by electromagnetic coupling. Therefore, loss becomes unboundedly zero and, by unifying feed distribution among the elements, we were able to realize a very high aperture-efficiency antenna.

Figure 3 High-gain antenna for Mercury explorer
Figure 3. High-gain antenna for Mercury explorer
Short-turn helical antennas with 2.5 turns are used for antenna elements. There are 546 elements in the array. In order to enhance coupling intensity, as shown in the figure, the length of helical pins inserted in the wave-guide tubes becomes longer as they are positioned further from the center. The feeding amplitude of each element can be unified by this arrangement. Furthermore, a design is possible that unifies phase distribution by changing the direction of each helical element.

Figure 3
Figure 3a. Engineering model
Figure 3b. Illustration of antenna structure
Figure 3c. Helical element

Fig. 3 shows the engineering model of the high-gain antenna for the Mercury explorer, built in house by our development staff, Mr. Kousuke Kawahara and Mr. Tomohiko Sakai. Since the highest temperature of the antenna surface is anticipated to reach 350 deg. C. under an 11-solar environment, a variety of ideas are introduced. Helical antennas are used for elements and high thermal-resistant ceramic is used for the fixing bobbin. An antenna of about 0.8m diameter provides about 36dBi, realizing a high aperture-efficiency of about 80%. This value almost equals the 37dB delivered by HAYABUSA’s parabola antenna of 1.6m diameter (6.8kg in weight). The excellence of our antenna’s performance is obvious. The antenna shown in Fig. 4 has the same feeding structure but its wave-guide-tube portion is made of honeycomb structure. To realize an ultra-lightweight, thin structure, a number of slots are carved on the metal surface to make antenna elements. Over 2,000 small slots are carved into the metal surface. This antenna is being developed for the Venus explorer PLANET-C. The antenna resembles a wafer cake about 6mm in thickness, 0.9m in diameter and 1kg in weight. The antennas introduced here will make their world premieres when the above planetary missions are realized.

Figure 4	High-gain antenna for Venus explorer
Figure 4a. Engineering model
Image of pattern measurement in the radio anechoic room where radio waves are not reflected.
Figure 4b. Partially enlarged photo
2,226 element slots (antenna elements) in 13 lines are carved on the copper foil surface. Two T-shape elements form one pair that produces a circular-polarized wave.
Figure 4c. Honeycomb structure

Further research and development

The antenna is an electrical circuit but an unusual one. It has a 3D-spatial structure and its property is largely affected by its shape. In this regard, research and development in collaboration with rapidly advancing structural and material research is highly anticipated. In addition, if we achieve downsizing as well as lightweight, high-reliability and low-loss feed circuits by introducing the latest remarkable advances in microwave circuit research, it may be possible for us to install active phased-array antennas on explorers, of which direction and beam width can be freely and flexibly changed by control of the feeding amplitude and phase. A variety of antennas suitable for project-specific environments must be proposed in the future. One feature of the antenna is its diversity, so it is very possible that new antenna technologies will be created that respond to and suit newly emerging space projects.


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