1 Design considerations for a low-frequency Vivaldi array element A. Tibaldi 1, G. Virone, F. Perini, J. Monari, M. Z. Farooqui 1, M. Lumia, O. A. Peverini, G. Addamo, R. Tascone, and R. Orta 1 1 Dipartimento di Elettronica e Telecomunicazioni (DET), Politecnico di Torino, Corso Duca degli Abruzzi 4, 119, Torino, Italy Istituto di Elettronica e di Ingegneria dell Informazione e delle Telecomunicazioni (IEIIT), Consiglio Nazionale delle Ricerche (CNR) c/o Politecnico di Torino, Corso Duca degli Abruzzi 4, 119, Turin, Italy Istituto di Radioastronomia (IRA), Istituto Nazionale di Astrofisica (INAF), Via Fiorentina 1, 49, Medicina (BO), Italy Abstract A cavity-backed Vivaldi antenna is suggested as dual-polarization array element for the low-frequency instrument of the Square Kilometer Array (SKA) project. A design strategy aimed at maximizing the sensitivity for such an array element is described. As an example, an antenna was obtained, with a sensitivity higher than 1 cm /K in the operative bandwidth and in the 4 sky coverage angle, for each polarization. 1. INTRODUCTION The Square Kilometer Array (SKA) represents one of the most interesting new-generation radio-telescopes owing to its extreme sensitivity performance [1]. One of the main SKA subsystems is the low-frequency Aperture Array (AA-low), which has to operate in the [7, 4] MHz band []. Several wide-band antenna systems have been studied within this framework: the spiral antenna [], the BLU-antenna [], the logperiodic antenna [4]. Recently, a dual-polarization Vivaldi array element has been proposed as a potential candidate for AA-low []. The main advantages of this configuration are a single ended Ω matching and a low cross-polarization in the principal planes, owing to the symmetry of the antenna. Furthermore, the antenna is self-standing, does not require either bulky dielectric parts or ground planes and can be manufactured in a gridded version []. VivaldiStub VivaldiGNDPlane CavityBackedVivaldi Front to Back Ratio FBR (db) 1 1..1.1.....4.4 frequency (GHz) Figure 1: Front-to-back ratio of Vivaldi with three back structures.. DEVELOPMENT OF A CAVITY-BACKED VIVALDI CONFIGURATION The first item of this work is the selection of a suitable geometry for the rear part of the antenna, in order to enhance its performance in terms of front-to-back ratio (FBR). The starting structure is the Vivaldi antenna with a circular back stub adopted in []; its FBR is reported in Figure 1, as a black dashed line. This parameter can not be improved by introducing a ground plane behind the antenna, because this would introduce an additional ripple in the curve, without significant improvements (red dash-dotted line). On the contrary, recalling the TEM horn structures [7], the cavity-backed configuration reported in Figure has been conceived. The feed point of the antenna is located at the junction of the Vivaldi wings with the cavity. The design parameters of this structure are: the front width of the non-blended antenna
(a) Lateral view. (b) Front view. Figure : Cavity-backed Vivaldi antenna. W, the aperture width A, the antenna length L, the blending radius C, the base width W b, the back cavity length L b and the back cavity width A b. In Figure 1 it is possible to observe that the FBR curve of a non-optimized version of this structure is almost monotone with improved values at higher frequencies. This structure has been used as starting guess for the following design procedure.. DESIGN PROCEDURE The AA-low instrument is a sparse random array, where the average embedded element pattern is, as a first approximation, similar to the pattern of the single (isolated) element [8], [9]. In this regard, the element design can be performed focusing on the sensitivity enhancement of the single element []. The maximization of the worst-case sensitivity of the array element corresponds to the minimization of the number of antennas needed to satisfy the SKA sensitivity specifications, leading to a reduction of manufacturing costs. The sensitivity S is defined as the ratio of the element effective area to its noise temperature : S(ˆr, f) (ˆr, f) (f) ( ) m where ˆr indicates the observation direction and f is the frequency. is calculated from the radiation patterns, which are obtained using a full-wave simulator. For what concerns the denominator, can be calculated as the sum of three contributions: K (f) = (f) + (f) + (f) () (f) and (f) quantify the noise contributions coming from the sky and from the ground. These two quantities are evaluated by means of the Cortes model [1]. The measured receiver noise temperature (f) is approximately K in the whole band. The structure should be designed in order to maximize the sensitivity in the 7 4 MHz band within the 4 sky coverage from zenith (SC), for each polarization. This goal is achieved by exploiting a synthetic representation of the sensitivity as a function of the geometrical parameters. It is useful to define the goal function S: S(f) = min S(ˆr, f) () ˆr SC As far as frequency is concerned, it should be noted that the operative conditions in the AA-low band are not homogeneous. Indeed, in the lower part of this band, the sky noise contribution is dominant. On the other hand, the most significant high-frequency noise contributions are the remaining two. Therefore, S(f) has been parametrized by means of its minimum values in three sub-bands: B 1 = 7 MHz, which is the sky-noise dominated band; B = MHz, which is a transition band; B = 4 MHz band, which is dominated by the receiver noise. This representation of the goal function is very convenient, for the evaluation of the effects of the geometrical parameters; for example, S(f) is reported in Figure as a function of the length of the Vivaldi antenna L and of its aperture width A. The white star markers identify the optimal values for the B and B bands. From the figure it is also possible to observe that the design procedure, in this case, is mainly driven by the higher band, where the sensitivity is generally low. Moreover, the sensitivity in B 1 appears to be almost independent of A and L. The same procedure must be performed varying other couples of parameters, in order to complete the design of the structure. (1)
8 min(aeff/tsys) (cm /K), f B 1 8 min(aeff/tsys) (cm /K), f B 8 min(aeff/tsys) (cm /K), f B 1 7 7 4 4 8 9 1 11 1 7 7 4.4 8.1 4 8 9 1 11 1 7 7 4 4.8.1.4 4 8 9 1 11 1 9 8 7 4 1 Figure : Minima of S(f) in the three sub-bands. 4 S( ) S(4 ) H S(4 ) E solid: θ= ; dashed : θ=4, H plane; dash dotted : θ=4, E plane S(θ,f) (cm /K) 1 1 1 1 4 4 Figure 4: Sensitivity goal function of the designed structure. 4. DESIGN RESULTS The performance of a significant design example is discussed in this section. The main dimensions of the designed antenna are 1. 1. m footprint and 1. m height. Figure 4 shows the sensitivity function S(ˆr, f) for ϑ = and ϑ = 4 in the H-plane and in the E-plane; it is possible to observe that S(ˆr, f) is higher than 1 cm /K in most situations. The effective area of the designed antenna and the three noise temperature contributions are reported in Figure and in Figure. In particular, Figure confirms that the main noise contribution for lower frequencies is the sky one, while the receiver noise is dominant for higher frequencies. Moreover, the new configuration exhibits a good symmetry of the pattern, which leads to high IXR values [11]. In Figure 7 it is possible to observe that the Ω reflection coefficient is below -1 db above 17 MHz.
4 4. x 14 ( ) 7 ( ) ( ) 4 (4 ) H (4 ) E (4 ) H (4 ) E 4 (4 ) H (4 ) E. 4. 4 1. 1 1 1. 1 1 1 4 4 Figure : (ϑ, f) of the structure. 4 4 4 1 1 1 1 1 1 1 1 4 4 Figure : (f) contributions of the designed structure.
1 S 11 (db) 1 1 1 4 4 Figure 7: Reflection coefficient of the designed structure.. CONCLUSION A cavity-backed Vivaldi antenna has been proposed as array element for the SKA AA-low project. A design procedure based on a full-wave simulator and the Cortes noise temperature model has been developed and described. An effective organization of the sensitivity parametric analyses has been proposed in order to define a suitable and exhaustive design strategy; it should be noted that the same design procedure can be applied to other antenna structures. In conclusion, the results obtained demonstrated that good performance can be achieved with the Vivaldi element. REFERENCES 1. http://www.skatelescope.org/. De Vaate, J.G., Witvers, R.H. Low Frequency Aperture Array Developments for Phase 1 SKA, General Assembly and Scientific Symposium, 11 XXXth URSI, pp. 1-4, Istanbul, 1- August 11. De Lera Acedo, E., Razavi-Ghods, N., Garca, E., Duffett-Smith, P. and Alexander, P. Ultra Wide- Band Aperture Array Element Design for Low Frequency Radio Astronomy, Antennas and Propagation Society International Symposium, 9. APSURSI 9. IEEE, pp. 1-4, 1- June 9 4. De Lera Acedo, E., SKALA: A log-periodic antenna for the SKA, Electromagnetics in Advanced Applications (ICEAA), 1 International Conference on, pp. -, -7 Sept. 1. Virone, G., Addamo, G., Peverini, O. A. and Tascone, R. Broadband Array Element for the SKA Low- Frequency Aperture Array, Electromagnetics in Advanced Applications (ICEAA), 11 International Conference on, pp. -1, 1-1 Sept. 11. Virone, G., Sarkis, R., Craeye, C., Addamo, G. and Peverini, O. A. Gridded Vivaldi Antenna Feed System for the Northern Cross Radio Telescope, Antennas and Propagation, IEEE Transactions on, vol. 9, no., pp. 19-1971, June 11 7. Malherbe, J.A.G., Hybrid elliptic TEM horn with internal fins, Antennas and Propagation in Wireless Communications (APWC), 1 IEEE-APS Topical Conference on, pp.19-111, -7 Sept. 1 8. Gonzalez-Ovejero, D., de Lera Acedo, E., Razavi-Ghods, N., Craeye, C. and Garcia Munoz, L.E., Nonperiodic arrays for radio-astronomy applications, Antennas and Propagation (APSURSI), 11 IEEE International Symposium on, pp. 1717, -8 July 11. 9. De Lera Acedo, E., Razavi-Ghods, N. et alii SKA AA-low Front-End Developments (At Cambridge University), Antennas and Propagation (EUCAP), 1 th European Conference on, pp.1-, Prague, - March 1 1. Bolli, P., Perini, F., Montebugnoli, S., Pelosi, G. and Poppi, S. Basic Element for Square Kilometer Array Training (BEST): Evaluation of the Antenna Noise Temperature, Antennas and Propagation Magazine, IEEE, vol., no., pp.8-, April 8 11. Fiorelli, B., Arts, M., Virone, G., de Lera Acedo, E., van Cappellen, W.A. Polarization Analysis and Evaluation for Radio Astronomy Aperture Array Antennas Antennas and Propagation (EUCAP), 1 7th European Conference on, pp. 4-47, April 1.