PIERS ONLINE, VOL. 4, NO. 2, 28 176 The Effect of Cooling Systems on HTS Microstrip Antennas S. F. Liu 1 and S. D. Liu 2 1 Xidian University, Xi an 7171, China 2 Xi an Institute of Space Radio Technology, Xi an 71, China Abstract High-temperature superconducting (HTS) antennas must work at a temperature below the critical temperature of the HTS material, so a cooling system is needed and the antenna should be placed inside it. Would the cooling system influence the properties of the HTS antenna? And if any, how does it? To answer these questions, a cryocooler is simulated with a 3-D model and its effect on HTS microstrip antennas is studied. It is found that when the vacuum chamber of the cryocooler has a suitable dimension, the gain of the antenna inside it can be improved greatly. 1. INTRODUCTION Since the surface resistance of high-temperature superconductor (HTS) is much lower than that of the normal conductors [1], HTS has found wide applications in various fields [2 6]. These HTS devices should work at temperatures below the critical temperature of HTS materials, which is usually over the boiling point of liquid nitrogen. A cryocooler is often used to cool these devices because of its easy handing and convenience compared with liquid N 2. In a cooling system, the HTS antenna to be cooled is packaged in a vacuum chamber for thermal isolation. However, a vacuum chamber is generally made of metallic or dielectric materials. If the distance between an antenna and a chamber is within a wavelength, interference of the chamber on the antenna properties strongly appears. Therefore, it is important to investigate the effect of cooling systems on the HTS antenna. K. Ehata and his fellows [7] have first presented an experimental radiation property of a HTS antenna in a cryocooler. In their experiment, the gain of the Cu antenna in the cryocooler is 11.2 db higher than that in free space, while a HTS microstrip antenna at the temperature of 6 K can obtain a gain of 2.6 db higher than that of its Cu counterpart. That is, the gain of a HTS antenna at 6 K is 13.8 db higher than that of the same Cu antenna in free space. In a paper in 21 [8], they gave a rough theoretical explain to this phenomena, but no thorough analysis was made. In this work, a 3-dimensional structure is constructed to model the vacuum chamber of the cryocooler and the radiation characteristics of a HTS microstrip antenna inside it is studied in detail. 2. THE COOLING SYSTEM FOR HTS ANTENNAS The cooling system consists of a compressor, a cold head, a vacuum chamber and other related parts. Among these, the vacuum chamber is the part which influences the properties of HTS antenna most. Figure 1 shows a schematic diagram of the vacuum chamber [7, 8]. The vacuum chamber consists of a cylindrical stainless jacket, a stainless flange with a cold head, and a dielectric window. The dielectric window is employed for microwave propagation between the vacuum environment and the atmosphere. The antenna to be cooled is mounted on the sample stage and a Cu-Ni coaxial cable is used to feed it. Since the thermal conductivity of Cu-Ni is 1/2 comparing with that of Cu, heat coming from room temperature to the cooled environment can be reduced. 3. PROPERTIES OF HTS ANTENNAS IN A COOLING SYSTEM For comparison, the same antenna as in [8] is analyzed. It was cooled in a vacuum chamber with a quartz glass window which had a permittivity of 3.8 and thickness of 8. mm. The antenna was placed 11 mm apart from the window as shown in Figure 1. According to the structure shown in Figure 1, a cylindrical cavity of perfect conductor is used to model the cylindrical stainless jacket of the vacuum chamber as well as the stainless flange at the bottom. The dielectric window of vacuum chamber is assumed as a circle dielectric plate of thickness t, which has the same radius as that of the cylinder and serves as a cover of the
PIERS ONLINE, VOL. 4, NO. 2, 28 177 Figure 1: Schematic diagram of the vacuum chamber. cylindrical cavity (shown in Figure 2). The height and radius of the cylinder is denoted as H and R respectively, and the distance between the antenna and the underside of the dielectric plate is denoted as d. Figure 2: Schematic diagram of the 3-D model. R=4 mm R=6 mm R=8 mm 33 1 3 R=4 mm R=6 mm R=8 mm 33 1 3 3-1 -2 6 3-1 -2 6-3 -3 27-4 9 27-4 9-3 -3 24-2 -1 12 24-2 -1 12 21 1 18 15 21 1 18 15 Figure 3: The gain patterns for various radius of the dielectric plate.
PIERS ONLINE, VOL. 4, NO. 2, 28 178 First, we studied the variation of the antenna s radiation performance with the vacuum chamber s radius. Figure 3 shows the simulated gain patterns when R = 4, 6 and 8 mm with t = 8. mm and d = 11. mm. It can be seen from the two figures that when R = 6 mm the gain in the forward direction is the highest. t=7mm t=8mm t=9mm 3 33 1-1 -2 3 6 t=7mm t=8mm t=9mm 3 33 1-1 -2 3 6-3 -3 27-4 9 27-4 9-3 -3 24-2 -1 12 24-2 -1 12 21 1 18 15 21 1 18 15 Figure 4: The gain patterns for various thickness of the dielectric plate. To demonstrate whether the thickness of the dielectric plate affects the antenna gain, the gain patterns with t = 7, 8, 9 mm and R = 6 mm is calculated and shown in Figure 4. It shows that the gain is highest when t = 7 mm, but no remarkable enhancement is observed compared with the other cases. According to the above analysis, we suppose that the rather high gain enhancement in [8] is due to the whole structure of the vacuum chamber. So the antenna is simulated for various structure parameters of the chamber. The patterns for various H when R = 4, 6, and 8 mm are shown in Figures 5, 6 and 7, respectively. From these figures we can see that the cooling system will severely affect the radiation characteristics of HTS antennas inside it, which leads to a large distortion of the radiation patterns. But it can be found from Figure 6 that when R = 6 mm and H = 3 mm a remarkably enhanced gain of 13.45 db can be obtained. And Figure 7 shows that the antenna can even achieve a gain of 14.15 db when R = 8 mm and H = 3 mm. H=3mm 33 1 3 H= 15mm H= 3mm H= 45mm 33 1 3 3-1 6 3-1 6-2 -2-3 -3 27-4 9 27-4 9-3 -3-2 -2 24-1 12 24-1 12 21 1 18 15 21 1 18 15 Figure 5: The gain patterns for various height of the cylinder (R = 4 mm).
PIERS ONLINE, VOL. 4, NO. 2, 28 179 H=3mm 33 1 3 H=3mm 33 1 3 3-1 6 3-1 6-2 -2-3 -3 27-4 9 27-4 9-3 -3-2 -2 24-1 12 24-1 12 21 1 18 15 21 1 18 15 Figure 6: The gain patterns for various height of the cylinder (R = 6 mm). H=3mm 33 1 3 H=3mm 33 1 3 3-1 6 3-1 6-2 -2-3 -3 27-4 9 27-4 9-3 -3-2 -2 24-1 12 24-1 12 21 1 18 15 21 1 18 15 Figure 7: The gain patterns for various height of the cylinder (R = 8 mm). 4. CONCLUSIONS To study the effect of the cooling system on HTS antennas, a cylindrical structure is constructed to model the vacuum chamber of the cooling system. Analysis of the antenna s properties is made for various structure parameters of the 3-D model, which shows that the existence of the cooling system will cause a great distortion of the antenna s radiation patterns, but when the vacuum chamber has an appropriate dimension the gain of the antenna can achieve a remarkable enhancement. REFERENCES 1. Wu, C. J., C. M. Fu, and T. J. Yang, Microwave surface impedance of a nearly ferroelectric superconductor, Progress In Electromagnetics Research, PIER 73, 39 47, 27. 2. Bourne, L. C., R. B. Hammond, et al., Low-loss microstrip delay line in Tl 2 Ba 2 CaCu 2 O 8, Appl. Phys. Lett., Vol. 56, No. 23, 2333 2335, 199. 3. Bonetti, R. R. and A. E. Williams, Preliminary design steps for thin-film superconducting filters, IEEE MTT-S Int. Microwave Symp. Digest, Vol. 1, 273 275, 199. 4. Awan, S. A. and S. Sali, Self-field ac power dissipation in high-tc superconducting tapes and a prototype cable, Progress In Electromagnetics Research, PIER 36, 81 1, 22. 5. Tchernyi, V. V. and E. V. Chensky, Electromagnetic background for possible magnetic levitation of the superconducting rings of saturn, Journal of Electromagnetic Waves and Applications, Vol. 19, No. 7, 987 995, 25.
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