High-Performance Dual-Circularly Polarized Reflector Antenna Feed Joo-Young Lim, Jargalsaikhan Nyambayar, Je-Young Yun, Dong-Hyun Kim, Tae-Hyung Kim, Bierng-Chearl Ahn, and Jae-Hoon Bang This paper presents a high-performance dual-circularly polarized feed employing a dielectric-filled circular waveguide. Novel features are incorporated in the proposed feed, such as a dielectric rod radiator for high gain and good impedance matching; dual quarter-wave chokes for low axial ratio over wide angles and for low back radiation; an integrated septum polarizer; and two end-launch-type coaxial-to-waveguide transitions. The proposed feed shows excellent performance at. GHz to.2 GHz. Keywords: Reflector antenna feed, dual-circular polarization, low axial ratio. I. Introduction Dual-circularly polarized reflector antennas are widely used for telemetry, tracking, and command (TT&C) of air and space vehicles such as aircrafts, UAVs, rockets, artillery projectiles, missiles, satellites, and spacecrafts [1]. Since it is difficult and impractical to realize vehicular antennas with good circular polarization at all angles, dual-circularly polarized antennas are normally employed at the ground station for TT&C [2]. The performance of a reflector antenna is critically dependent on the quality of its feed. A dual-polarized feed can be realized in many forms, such as in a printed element [3], a crossed dipole or slot [4], or a waveguide open end or horn []. Among them, the waveguide or horn-type feed having a length of several wavelengths is more conveniently realized by employing an integrated septum polarizer [6]. In addition it is easier to increase the bandwidth and to enhance the feed performance. In this paper, a dual-circularly polarized feed based on a dielectric-filled circular waveguide is proposed. The proposed feed is developed for use in C-band (. GHz to.2 GHz) telemetry applications. The detailed design concept of each component and the combined structure are presented. The performance of the proposed feed is verified with measurements. Manuscript received Feb. 2, 214; revised June 28, 214; accepted July 28, 214. Joo-Young Lim (dlawndud22@naver.com), Jargalsaikhan Nyambayar (nyamba @yahoo.com), Je-Young Yun (zeros73@gmail.com), Bierng-Chearl Ahn (bician@ cbu.ac.kr), and Jae-Hoon Bang (corresponding author, jhbang@cbnu.ac.kr) are with the Department of Radio and Communication Engineering, Chungbuk National University, Cheongju, Rep. of Korea. Dong-Hyun Kim (mattew@kari.re.kr) and Tae-Hyung Kim (thkim@kari.re.kr) are with the Naro Space Center, Korea Aerospace Research Institute, Daejeon, Rep. of Korea. II. Feed Design Figure 1 shows the proposed feed operating at. GHz to.2 GHz. The feed consists of a dielectric-filled circular waveguide having an inner diameter of 2 mm and an outer diameter of 39 mm for size reduction; a dielectric rod radiator ETRI Journal, Volume 36, Number 6, December 214 214 Joo-Young Lim et al. 889 http://dx.doi.org/1.4218/etrij.14.114.118
Reflection coefficient (db) 1 1 2 2 w/ grooves w/o grooves 3 4... 6. Fig. 2. Impedance matching. Dielectric rod Choke Tunning groove Septum polarizer Coaxial probe (port 1) 1 Fig. 1. Proposed dual-circularly polarized feed. Coaxial probe (port 2) with an impedance-matching hollow region; three grooves for further impedance matching; dual chokes for low axial ratio over a wide angle and for low back-lobe radiation; a septum polarizer for dual polarization generation and separation; and two coaxial-to-waveguide transitions. The septum polarizer acts both as a polarizer and as an orthomode transducer generating right and left circularlypolarized waves. Its operating principles are well known [7] and are not repeated in this paper. The proposed feed is designed in the following steps. First, the feed is designed without the septum polarizer and coaxial transition. Second, the septum polarizer and coaxial transition are designed separately and then combined. Finally, the overall structure is constructed and optimized. The impedancematching grooves and coaxial transition are adjusted for better impedance matching. The widely used Microwave Studio TM v. 212 by CST has been employed in the design. The circular waveguide is filled with low-loss polycarbonate material having dielectric constant of 2.76 and loss tangent of. at GHz to reduce the waveguide diameter. The dielectric rod radiator [8] is realized by extending the polycarbonate material filling the waveguide. A hollow cylinder is formed at the rod s end for impedance matching. The gain and beamwidth of the feed are controlled by adjusting the diameter and length of the rod. Three grooves on the dielectric rod inside the waveguide, together with the hollow region at the rod s end, improve the impedance matching. Groove regions are filled with air. Groove dimensions and locations were iteratively optimized Gain (db) 1 1 w/ choke & dielectric 2 w/o choke w/o dielectric 2 18 12 6 6 12 18 Fig. 3. Effect of choke and dielectric rod on antenna gain pattern. 2 1 1 w/ choke w/o choke 18 12 6 6 12 18 Fig. 4. Effect of choke on axial ratio. for good impedance matching at. GHz to.2 GHz. Figure 2 shows the impedance matching performances with and without the three grooves. The reflection coefficient is reduced from 11 db to 21 db at.1 GHz. Dual chokes improve the axial ratio at wide angles and suppress the back lobe. The initial depths of the dual chokes are set to.2 λ and. λ, and their optimum values are obtained by a parameter sweep. Figures 3 and 4 show the effects of the dielectric rod and dual chokes on the antenna performance. The dielectric rod increases the gain by about 3 db. The dual chokes suppress the back radiation by about 9 db and 89 Joo-Young Lim et al. ETRI Journal, Volume 36, Number 6, December 214 http://dx.doi.org/1.4218/etrij.14.114.118
L 1 L 2 L 3 S 11 2. D 2 D 1 A 1 S 21 Axial ratio 1. L ε r1 T w H H 4 H 3 H t S-parameters (db) 2 3 1.. H 2 C C 2 C 1 T 1 T 2 B 1 B 2 B 3 B 4 B B t H 1 4. 4... 6. Fig. 6. Reflection, isolation, and axial ratio of septum polarizer. G 1 G 2 H p L p a A 1 A 2 A 3 T p Q 1 Q 2 Q 3 b S 1 S 2 ε r2 Fig.. Dimensional parameters of feed. Table 1. Dimensions (mm) of feed. L 1. B 1 18.37 H 1 3.9 Q 1 2.18 L 1 4. B 2 11.8 H 2 4.7 Q 2 2.38 L 2. B 3 7.19 H 3 4.71 Q 3 2.31 L 3 113. B 4 1.6 H 4 7.9 H p 4.63 D 1 1. B 33. H.3 L p 27. D 2 29. B t 71.74 H t 2. T p 1. A 2. C. G 1 3.33 a 1.26 A 1 1.9 C 1 17. G 2 2.84 b 4.1 A 2 17.6 C 2 28. S 1 2. T 1 1. A 3 18.46 T w 7. S 2 2. T 2 1. significantly lower the axial ratio at angles beyond ±3. Feed components are separately designed and combined to form the final structure, which is then further optimized to account for interaction between components. Dimensions of the optimally designed feed are shown in Figure and Table 1. A septum polarizer with five steps is employed for the generation of a dual-circularly polarized wave. Step dimensions are determined by parameter sweeps for good impedance matching, high isolation, and low axial ratio. Figure 6 shows the performance of the designed septum polarizer. Over. GHz to.2 GHz, the reflection coefficient is less than 3 db, the isolation between the two input S-parameters (db) 1 1 2 2 Fig. 7. Fabricated feed and its components. Simul. (S 11 ) Simul. (S 21 ) 3 Meas. (S 11 ) Meas. (S 21 ) 3 4. 4... 6. Frequency (Ghz) Fig. 8. Measured reflection and isolation of fabricated feed. semicircular waveguides is greater than 29 db, and the axial ratio is less than.2 db. An end-launcher-type coaxial-to-circular waveguide transition [9] having a reflection coefficient less than 23 db at. GHz to.2 GHz is designed to connect the feed to the receiver ETRI Journal, Volume 36, Number 6, December 214 Joo-Young Lim et al. 891 http://dx.doi.org/1.4218/etrij.14.114.118
Gain (db) 2 1 1 2 3 4 18 12 6 6 12 18 Simul. (LHCP) Simul. (RHCP) Meas. (LHCP) Meas. (RHCP) Fig. 9. Measured gain pattern of fabricated feed. shown in Fig. 8. Figure 9 shows the measured co- and cross-polarized gain patterns in the plane normal to the septum surface. The antenna s measured gain is 9.8 dbic, and the half beamwidth at the 1 db taper level is 9.6. The cross-polarization level is less than 19 db over 6 θ +6. Figure 1 shows the measured axial ratio, which is. db at θ = and 1.8 db at θ = ±6. Figure 11 shows the phase center versus the frequency relative to the rod s end. At. GHz to.2 GHz, the phase center varies from 27. mm to 28. mm. The fabricated feed s performance agrees well with the simulation. III. Conclusion 2 1 1 Simulation Measurement 18 12 6 6 12 18 This paper presented a dual-circularly polarized feed employing a dielectric-filled circular waveguide. The proposed feed incorporated a circular-waveguide-fed dielectric rod antenna made of low-loss polycarbonate material, impedancematching grooves, dual quarter-wave chokes, a septum polarizer, and two coaxial transitions. The proposed feed showed excellent characteristics, such as compactness, low axial ratio over wide angles, and low back radiation. The concept proposed in this paper can be applied to the design of high-performance feeds for prime-focus tracking reflector antennas with a wide range of F/D (front-to-back) ratios by properly adjusting the dielectric rod length. Fig. 1. Measured axial ratio pattern of fabricated feed. Phase center locations (mm) 2 26 27 28 29 3 4... 6. Fig. 11. Measured phase center vs. frequency of fabricated feed. circuit. Figure 7 shows the fabricated feed and its components. The performance of the fabricated feed was measured and is shown in Figures 8 11. At. GHz to.2 GHz, the measured reflection coefficient is less than 21 db, and the isolation between the RHCP and LHCP ports is greater than 2 db, as References [1] R.C. Baker, A Circularly Polarized Feed for an Automatic Tracking Telemetry Antenna, IRE Trans. Space Electron. Telemetry, vol., no. 3, Sept. 199, pp. 13 11. [2] P. Kumar et al., High Performance Dual Circularly Polarized S- Band Feed, Proc. URSI General Assembly, New Delhi, India, Oct. 2, pp. 66 68. [3] Z.A. Pour and L. Shafai, A Novel Dual-Mode Dual-Polarized Circular Waveguide Feed Excited by Concentrically Shorted Ring Patch, IEEE Trans. Antennas Propag., vol. 61, no. 1, Oct. 213, pp. 4917 492. [4] S.-Y. Eom, I.-P. Hong, and J.-M. Kim, Broadband Printed Cross- Dipole Element with Four Polarization Reconfigurations for Mobile Base Station Array Antenna Applications, Int. J. Antennas Propag., 211, pp. 1 1. [] G.H. Schennum and T.M. Skiver, Antenna Feed Element for Low Circular Cross-Polarization, Proc. IEEE Aerospace Conf., Snowmass, CO, USA, vol. 3, Feb. 1 8, 1997, pp. 13 1. [6] M.J. Franco, A High-Performance Dual-Mode Feed Horn for Parabolic Reflectors with a Stepped-Septum Polarizer in a Circular Waveguide, IEEE Antennas Propag. Mag., vol. 3, no. 892 Joo-Young Lim et al. ETRI Journal, Volume 36, Number 6, December 214 http://dx.doi.org/1.4218/etrij.14.114.118
3, June 211, pp. 142 146. [7] M. Chen and G. Tsandoulas, A Wide-Band Square-Waveguide Array Polarizer, IEEE Trans. Antennas Propag., vol. 21, no., May 1973, pp. 389 391. [8] C. Kumar et al., Design of Short Axial Length High Gain Dielectric Rod Antenna, IEEE Trans. Antennas Propag., vol. 8, no. 12, Dec. 21, pp. 466 469. [9] B.N. Das and G.S. Sanyal, Coaxial to Waveguide Transition (End Launcher Type), Proc. Institution Elec. Eng., London, UK, vol. 123, no. 1, Oct. 1976, pp. 984 986. Dong-Hyun Kim received his BS and MS degrees in radio communication engineering from Korea Maritime and Ocean University, Busan, Rep. of Korea, in 1997 and 1999, respectively. From 2 to 2, he served as a captain in the Republic of Korea Air Force. Since 2, he has been with the Korea Aerospace Research Institute, Daejeon, Rep. of Korea, where he is currently a senior researcher at the Naro Space Center. His research interests include telemetry engineering and antennas. Joo-Young Lim received his BS degree in computer software engineering from Korean Bible University, Seoul, Rep. of Korea, in 213. Since 213, he has been working toward his MS degree in radio and communications engineering at Chungbuk National University, Cheongju, Rep. of Korea. His research interests include waveguide structure design and antenna engineering. Jargalsaikhan Nyamabayar received his BS and MS degrees in information measuring electronics and telecommunications from the School of Information and Communication Technology, Mongolian University of Science and Technology, Ulaanbaatar, Mongolia, in 211 and 213, respectively. Since 213, he has been working toward his doctoral degree at the School of Electrical and Computer Engineering, Chungbuk National University, Cheongju, Rep. of Korea. His research interests include antenna engineering, active device design, and microwave engineering. Je-Young Yun is currently working toward his BS degree in information and communications engineering at Chungbuk National University, Cheongju, Rep. of Korea. His research interests include antennas and microwave circuits. Tae-Hyung Kim received his BS and MS degrees in electronics engineering from Kyungpook National University, Daegu, Rep. of Korea, in 1989 and 1991, respectively. He has worked at the Agency for Defense Development, Daejeon, Rep. of Korea, from 1991 to 1996 and at DACOM R&D Center, Daejeon, Rep. of Korea, from 1997 to 22. In 22, he joined the Korea Aerospace Research Institute, Daejeon, Rep. of Korea, where he is currently the head of the Launch Operations Department, Naro Space Center. His research interests include tracking systems and antennas. Bierng-Chearl Ahn received his BS degree in electrical engineering from Seoul National University, Seoul, Rep. of Korea, in 1981, his MS degree in electrical engineering from KAIST, Daejeon, Rep. of Korea, in 1983, and his PhD degree in electrical engineering from the University of Mississippi, MS, USA, in 1992. From 1983 to 1986, he worked for the Goldstar Precision Company, Anyang, Rep. of Korea, as a research engineer, and from 1993 to 1994, he worked at the Agency for Defense Development, Daejeon, Rep. of Korea. Since 199, he has been with Chungbuk National University, Cheongju, Rep. of Korea, where he is currently a professor at the School of Electrical and Computer Engineering. His research interests include applied electromagnetics and antennas. Jae-Hoon Bang received his BS and MS degrees in radio and communications engineering and his PhD degree in information and communications engineering from Chungbuk National University, Cheongju, Rep. of Korea, in 1997, 1999, and 23, respectively. From 23 to 27, he worked as a research engineer at Kukdong Telecommunications Company, Nonsan, Rep. of Korea. He is currently working at the BK21 Chungbuk Information Technology Center, Chungbuk University, as a visiting professor. His research interests include numerical techniques in electromagnetics, military radar systems, and antenna engineering. ETRI Journal, Volume 36, Number 6, December 214 Joo-Young Lim et al. 893 http://dx.doi.org/1.4218/etrij.14.114.118