Application Article Compact Circular/Linear Polarization Dual-Band Prime-Focus Feed for Space Communication

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1 Hindawi Publishing Corporation Volume 212, Article ID 86951, 7 pages doi:1.1155/212/86951 Application Article Compact Circular/Linear Polariation Dual-Band Prime-Focus Feed for Space Communication Rastislav Galuscak, Pavel Hadra, and Milos Maanek Department of Electromagnetic Field, Facult of Electrical Engineering, Cech Technical Universit in Prague, Technicka 2, Prague, Cech Republic Correspondence should be addressed to Pavel Hadra, hadrap@fel.cvut.c Received 2 Januar 212; Revised 7 April 212; Accepted 1 April 212 Academic Editor: Hamsakutt Vettikalladi Copright 212 Rastislav Galuscak et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in an medium, provided the original work is properl cited. We propose a novel, compact, prime-focus antenna feed for space communication. The feed requires full-wave simulator optimiation for a given parabolic reflector and is designed to operate simultaneousl on two bands, offering LHC/RHC polariations for the 13 cm band and V/H polariations for the 7 cm band. With performance results confirmed b measurement, it has been verified in practice that this compact feed is suitable for use in a low-noise Earth-Moon-Earth communication link. 1. Introduction Amateur radio Moon Bounce, EME (Earth-Moon-Earth), communication is a ver technicall demanding discipline [1]. Usuall, high-gain reflector antennas are used, but the require appropriate prime-focus feeds designed mainl for low-noise operation [1]. EME communications do not require simultaneous multiband operation; so, to change the working band, a station operator has enough time to echange prime-focus feeds. Additionall, the antenna gain depends on the diameter of the dish reflector, which is ver often limited b space and operating cost requirements. For users, it is ver important to squeee ever possible tenth of a db from their antennas, recogniing the inherent lack of adequate gain. This requirement is in contrast to the effectiveness of multiband reflector antennas whose efficienc is usuall lower than that of monoband antennas due to the compromised designs of their feeds. Dual-band or multiband prime-focus feeds are used onl b stations equipped with electricall large and especiall phsicall large-sie reflectors, where replacement of the prime feed is mechanicall ver demanding, epensive, or difficult to implement. A description of a 1 m dish antenna s dual-band primar feed for the 7/23 cm bands was published b Doug McArthur [2]. McArthur s feed consisted of a dual-mode waveguide feed for 23 cm surrounded b four dipoles for 7cm. See Figure 1. Since the dipoles impedance is about 15 ohms, the must be impedance-matched b emploing feed lines of λ/4 length with 72.5 ohm impedance. This design also requires isolating the dipoles from their ground plane, see Figure 2. This solution introduces unwanted and unpredictable impedance behavior, especiall when the feed is mounted onto a dish. However this design was successfull fabricated b several operators worldwide [2]. Unfortunatel, measured radiation patterns were not disclosed b the author. Similar solution consisting of waveguide and dipoles has been recentl reported for X/Ka bands [3]. The proposed design of a dual-band prime-focus feed utiliing butterfl-shaped dipoles and a ground plane subreflector equipped with a choke can be regarded as a completel novel approach for coverage of EME 7/13 cm bands. This paper describes a compact, dual-band feed (7/ 13 cm 432/232 MH) intended for a parabolic reflector with a diameter of 6.4 m and f/d ratio of.4. Horiontal and vertical polariations are available on the 432 MH band. The

2 2 A B D2 D1 B A Figure 1: Doug McArthur s feed for 7/23 cm bands. Figure 3: Laout of the dual-band feed, overall view. L = 216 PTFE Figure 2: Front view of Doug McArthur s feed. 232 MH band feed can operate with both right and lefthand circular polariation. Hence, this compact feed offers dual polariation operation at both bands. 2. Requirements for Dual-Band Prime-Focus Feed Above considerations dictate the following requirements for the dual-band feed: (1) high gain in antenna assemblminimal blockage of reflector; (2) low noiseedge taper about 15 db; (3) low backward radiation; (4) RHC- and LHC-polariation capabilit with good aial ratio for 232 MH (13 cm band); (5) vertical and horiontal polariation for 432 MH (7 cm) band; (6) able to handle high power, up to 3 kw with ver good impedance match; (7) low weight <7 Kg (in order to avoid mechanicall distorting the shape of the parabolic reflector). It should be emphasied that the feed needs not to be broadband since, for the EME purposes, the following W = 16 5Ω coaial line Figure 4: Detailed view of dipole feed. frequencies are used: MH ± 5 kh on 7 cm and two frequencies MH ± 5 kh / MH ± 5 kh on 13 cm band. 3. Design Concepts The proposed 6-port feed consists of two independent radiating sstems described belowsee Figure 3. Septum polarier design was performed b MICIAN [4] software. All other simulations were performed b CST Microwave Studio software [5] MH (7 cm) Band. The 432 MH feed is composed of two pairs of opposing bow-tie dipoles (A-A, BB ) responsible for vertical/horiontal polariation. These smmetrical dipoles are placed approimatel λ/4 above a circular ground plane. The dipoles are fed b 5 Ω coaial lines which are designed to mechanicall and electricall match to 7/16 DIN stle connectors, located on the bottom side of the ground plane, that are filled with PTFE to handle ver high power. No impedance transformation

3 3 HR HD H D3 TE11 L1 L2 Septum polarier L3 TE11 + TM11 C B A Figure 5: Laout of the dual-band feed, longitudinal cut (left) and internal view of the dual-mode horn equipped with five-step septum polarier (right) (a) (b) Figure 6: 3D radiation pattern at 432 MH; copolariation (a) and cross-polariation (b). between connectors and dipoles is required (Figure 4). This arrangement allows direct grounding of the connectors to the ground plane with four screw fasteners. Smmetr is maintained with the outer shell of carrier pipes, which function as a sleeve balun [6, 7]. For all polariation states, each pair of dipoles is ecited b an eternal 3 db Wilkinson power divider and 18 degree phase shifter, since dipole feed orientation is swapped in order to improve radiation pattern smmetr and minimie the suppl line lengths MH (13 cm) Band. The 232 MH dual-port (LHC/RHC) feed is based on our previous research work dealing with septum polariers [8 1], see also [11, 12]. Using CST software simulation, we found that conversion to circular polariation using a 5-step septum occurs at virtuall 1% efficienc [9]. To obtain good pattern smmetr, suppression of side lobes, and separation of crosspolariation, a dual-mode (HE11) Potter-stle horn [13 15] equipped with a septum polarier has been chosen [15] see Figure 5. The transition between coaial feed line and waveguide is achieved b using conical probes [9]. The circular ground plane functionall performs as a choke [16] having diameter D2 and height of rim HR. The choke effect is to slightl modif the Potter horn s radiation pattern causing a small decrease in the feed boresight, thus providing a slight performance improvement in our prime-focus feedconfigured antenna [1, 1]. The ground plane provides for placement of the 432 MH dipoles as well. Main design dimensions of the proposed feed are summaried in Table 1; λ432 and λ232 indicate wavelengths at center of respective band.

4 (a) (b) Figure 7: 3D radiation pattern at 232 MH; copolariation (a) and cross-polariation (b). (a) (b) Figure 8: Feed prototpe at the ERA anechoic chamber. Ver important is distance H = 123 mm, that is, phase center position of the 232 MH feed above the ground plane. It was designed to achieve flush positions for both phase centers. 4. Results 4.1. Simulated 3D Radiation Patterns. 3D radiation patterns from CST are shown in Figure 6 for 432 MH and Figure 7 for 232 MH. The smmetric patterns show low crosspolariation for both bands. Note the location of the phase centers (red points). For both the 432 MH and 232 MH bands, the calculated phase centers are located ±1 mm at the aperture plane (dimension HR). A feed prototpe (Figures 8 and 9) was fabricated from aluminum allo and was measured in the anechoic chamber at ERA compan [17]. The feed s weight is about 5 kg Measured Radiation Pattern for 432 MH. Since vertical and horiontal polariation pattern smmetr is ecellent, the measured radiation pattern cut of H plane horiontal polariation is identical to the cut of E plane vertical polariation and vice versa. See Figure 1 for both polariation states. Note the ver low cross-polariation level at the main lobe (suppression < db) Measured Radiation Pattern for 232 MH. Measurement of onl one polariation state (RHC) was performed with and without the 432 MH section acting as a choke (Figure 11). Its effect ma be seen as a slight decrease in the main lobe, reducing blockage effects and improving the overall efficienc. However, the choke subtl increases backward radiation. The simulated Aial Ratio (AR) parameter is better than.1 db in the boresight direction. No special measurement of AR has been performed, since our previous

5 5 Conical coaial-to-waveguide transitions Level (db) step septum polarier Figure 9: Feed prototpe at the ERA anechoic chamber, front view (note 232 MH section). Level (db) (deg) Horiontal polariation Horiontal X-polariation Vertical polariation Vertical X-polariation Figure 1: Measured pattern cut for horiontal (A dipoles) and vertical (B dipoles) polariation. results showed ver low AR for a similar feed [1]. Such feed with scaled dimensions operates at 1296 MH with a measured Aial Ratio better than.3 db. So the Aial Ratio <.3 db (no choke) and <.6 db (with choke) for the proposed 232 MH feed section in the boresight direction can be epected Return Loss for 432 MH Band. Return loss for this band was measured behind the Wilkinson power divider to eliminate its influence on the measurement. Two Mini- Circuits directional couplings, ZFDC1-2-S, connected to the appropriate dipole were used for this measurement. The feed was ecited b an Agilent E8257D signal generator. Output level was measured with a Wiltron 37347A spectrum (deg) RHCP, cut for = deg no choke RHCP, cut for = 9 deg no choke RHCP, cut for = deg with choke RHCP, cut for = 45 deg with choke RHCP, cut for = 9 deg with choke Figure 11: Measured pattern cut for RHC (RHC port ecited) polariation without and with 432 MH section, working as a choke. S11, S Frequenc (MH) CST simulation 18 deg phase shift Measured A-A dipoles with 18 deg phase shift CST simulation deg phase shift Measured A-A dipoles with deg phase shift Measured B-B dipoles with 18 deg phase shift Measered B-B dipoles with deg phase shift Figure 12: Measured and simulated return loss for 434 MH section. analer. Calculated and measured results are shown in Figure 12. For each case, one set of respective dipoles (A- A or B-B,seeFigure 3) was simultaneousl ecited with or 18 mutual phase-shift, realied b a different length of feeding cables Return Loss for 232 MH Band. A Wiltron 37347A vector analer was used for these measurements. Ver good impedance match throughout a wide bandwidth was observed (Figure 13); however, for EME, fractional bandwidth of onl about 1% around 232 MH is required. The differences between receiving (R) and transmitting (T)

6 6 5 1 S11 (db) f (GH) RX port TX port Figure 15: Prototpe of the feed mounted on antenna during Sun noise measurements. Figure 13: Measured return loss for the 232 MH section. 5. Practical Eperience Antenna R performance was tested using Sun versus cold sk comparisons on both bands, see Figures 14 and 15. On 432 MH, a LNA with noise figure of.2 db was used. On 232 MH, a LNA with noise figure of.45 db was used. A solar flu level of 112 sfu was present for all measurements. An average Sun noise to cold sk ratio of 14 db was achieved for the 432 MH band. A Sun noise to cold sk ratio of 17 db was achieved for the 232 MH band. There eists further opportunities for performance improvement, mainl b reducing cable lengths between the feed connectors and the LNAs. Figure 14: Prototpe of the feed installed at the focus of a 6.4 m dish. Table 1: Main design dimensions of the proposed prime-focus. Variable L W HD HR H D1 D2 D3 L1 L2 L3 Dimension [mm] Dimension [λ432 ] Dimension [λ232 ] ports are attributed to dissimilar connector tpes at the ports. While the R port uses an N stle connector, the T port emplos a 7/16 DIN connector because of its power handling capabilit. 6. Conclusion A compact, dual-band feed was designed for ver low-noise applications and was successfull tested for Earth-MoonEarth operation. Antenna efficienc was evaluated b the EME Performance Calculator [18]. Reasonable values of 47% (4.5 dbi) 232 MH and 55% (26.64 dbi) 432 MH based on Sun versus cold sk measurement were obtained. The lower efficienc on 232 MH is attributed to slight deviations from the parabolic shape of the reflector. With butterfl-shaped dipoles used for 432 MH band, ver good impedance matching was achieved over a wide frequenc range and also the required parameters for power handling were satisfied. Acknowledgments The authors would like to thank Robert Valenta for discussions. This paper has been created under the COST IC112 VISTA action and partiall supported b the COST LD 1255 AMTAS (Advanced Modeling and Technologies for Antennas and Sensors). References [1] R. Galuscak, Advanced design of reflector based antennas [Ph.D. thesis], 211, CTU-FEE.

7 7 [2] Doug McArthur-VK3UM, Dual 7 cm and 23 cm Dish Feed, DUBUS Magaine, 26, VK3UMweb.pdf. [3] Y. B. Karandikar and P. S. Kildal, X-Ka dual band prime focus feed for satellite earth terminals, in Proceedings of the 3rd European Conference on Antennas and Propagation (EuCAP 9), pp , March 29. [4] Online, [5] Online, [6] H. E. King and J. L. Wong, An eperimental stud of a balunfed open-sleeve dipole in front of a metallic reflector, IEEE Transactions on Antennas and Propagation, vol.2,no.2,pp , [7] T. A. Milligan, Modern Antenna Design, JohnWile&Sons, New York, NY, USA, 25. [8] R. Galuscak, Septum Feed Revisited, DUBUS, 24. [9] P. Hadra, R. Galuscak, and M. Maanek, Optimaliation of the septum polarier feed for GH EME station, in Proceedings of the European Conference on Antennas and Propagation (EuCAP 6), November 26. [1] R. Galuscak and P. Hadra, Prime-focus circular waveguide feed with septum polariation transformer, DUBUS, 27. [11] R. Behe and P. Brachat, Compact dupleer-polarier with semicircular waveguide, IEEE Transactions on Antennas and Propagation, vol. 39, no. 8, pp , [12] J. Bornemann, S. Amari, J. Uher, and R. Vahldieck, Analsis and design of circular ridged waveguide components, IEEE Transactions on Microwave Theor and Techniques, vol. 47, no. 3, pp , [13] W. V. T. Rusch and P. D. Potter, Analsis of Reflector Antennas, Academic Press, New York, NY, USA, 197. [14] Ch. Yao, C. Ai-Xin, and S. Dong-Lin, The optimiation design of the Pickett Potter horn antenna for ka band, in Proceedings of the Asia-Pacific Smposium on Electromagnetic Compatibilit and 19th International Zurich Smposium on Electromagnetic Compatibilit (APEMC 8), pp , Singapore, Ma 28. [15] M. J. Franco, A high-performance dual-mode feed horn for parabolic reflectors with a stepped-septum polarier in a circular waveuide, IEEE Antennas and Propagation Magaine, vol. 53, no. 3, pp , 211. [16] K. Miata and M. Suuki, Effects of choke-load position on radiation properties in double-choked small horn antennas, Electronics Letters, vol. 19, no. 9, pp , [17] Online, [18] Online,