The SKA AA-lo array; E.M. simulation and design

1. The SKA AA-lo array; E.M. simulation and design Eloy de Lera Acedo (UCAM) Nima Razavi-Ghods (UCAM) David Gonzalez-Ovejero (UCL) Luis Enrique Garcia (UC3M) Christophe Craeye (UCL) Peter J. Duffett-Smith (UCAM) Paul Alexander (UCAM)

2. Overview • Towards the SKA: The SKA-AAlo • The BLU antenna • The BLU antenna in a regular array • Simulating large random finite arrays… • Numerical results • Current state specifications • Future work • Conclusions

3. Towards the SKA: The SKA-AAlo • It is part of Prep-SKA. • Frequency range from 70 MHz to 450 MHz and +/- 45o scan range: • Irregular vs Regular / Dense vs Sparse (Memo 87). • Sky noise limited? • At 100 MHz: 4000 m2/K. • Up to at least 10000 elements per station. • For more specifications: Memo 111 (R. Bolton et al). 10 70 100 200 300 400 450 MHz BLU antenna

4. Overview • Towards the SKA: The SKA-AAlo • The BLU antenna • The BLU antenna in a regular array • Simulating large random finite arrays… • Numerical results • Current state specifications • Future work • Conclusions

5. The BLU antenna • Bow-tie Low-frequency Ultra-wideband antenna. • Low profile and differential feeding. • Wide beam width. • Good matching at the high end of the band where the sky noise does not limit the performance. • Small and cheap. BLU antenna

6. Overview • Towards the SKA: The SKA-AAlo • The BLU antenna • The BLU antenna in a regular array • Simulating large random finite arrays… • Numerical results • Current state specifications • Future work • Conclusions

7. L L The BLU antenna in a regular array • Infinite array simulations were carried out to analyze the sensitivity of a unit cell containing a BLU antenna versus the inter-element spacing, the antenna size and the tilt angle of the arms. + Info - GND @ λ/4 @ the highest freq. - No dielectric. Z = 200Ω Z = 200Ω BLU antenna d

8. Physical size of the unit cell The BLU antenna in a regular array • The sky will dominate a large part of the band. Furthermore, grating lobes will show up in the band. • + Info • Sky noise • Receiver noise • System noise Table 1: Idealize LNA parameters. Table 1: Idealize LNA parameters. System noise Receiver noise BLU antenna

9. The BLU antenna in a regular array • In the infinite array the sensitivity of a unit cell in the regular array improves as sparser is the array in the dense regime while it saturates for the sparse regime. The transition band is the key. Sensitivity for different Inter-element spacing distances BLU antenna

10. The BLU antenna in a regular array • Larger antennas bring a multi-lobulation issue at high frequencies. Sensitivity for different antenna sizes. BLU antenna

11. The BLU antenna in a regular array • An optimum tilt angle can be found for the antenna arms. The impedance match improves and so does the receiver noise. This is of special interest for the region in between the dense and sparse regions. Trec for different antenna angles α BLU antenna

12. Overview • Towards the SKA: The SKA-AAlo • The BLU antenna • The BLU antenna in a regular array • Simulating large random finite arrays… • Numerical results • Current state specifications • Future work • Conclusions

13. Simulating large random finite arrays… • Based on Method of Moments + MBFs (CBFs) and the interpolation technique presented in [1], where the computation of interactions between MBFs is carried out by interpolating exact data obtained on a simple grid. [1] D. Gonzalez-Ovejero and C. Craeye, “Fast computation of Macro Basis Functions interactions in non-uniform arrays,” in Proc. IEEE AP-S Soc. Int. Symp., San Diego, CA, Jul. 2008.

14. The interactions between MBFs are computed only within that region reducing drastically the number of unknowns. A so-called “radius of influence” is defined for every antenna in the array. The system is solved within each region of influence for each antenna. Simulating large random finite arrays… 30λ

15. Overview • Towards the SKA: The SKA-AAlo • The BLU antenna • The BLU antenna in a regular array • Simulating large random finite arrays… • Numerical results • Current state specifications • Future work • Conclusions

16. λ0 λ0 Numerical results - Array radius = 30λ0. - Number of elements = 1000. - Minimum average separation = 1.5λ0. - Distance to ground plane = λ0/4. - No dielectric. Z = 200Ω

17. *Embedded Element Pattern for radius of influence = 10λ Numerical results λ = 3λ0

18. Numerical results EEP- SEP = 0.379 dB in broadside EEP- SEP = 0.2 dB in broadside

19. Numerical results

20. Current state specifications For previous simulation: • Computation time: • Preprocess (computing data to interpolate): 37 min • Computing 1000 EEP: 1000•36 s ~ 10h. • Memory: • Only 17.8 MB of contiguous memory required. • Speed • 10h per simulation: excessive. • Sparse matrices (initial results): • Simulation time reduced to 1h. • Requires more contiguous memory: 1.77 GB

21. Future work • Full EM simulation of a real size SKA station. • Optimization: • Use of different NFFT (Non-uniform Fast Fourier Transform) schemes for the array pattern calculation. • Paralelization of the code in a cluster of computers. • Full migration to C code. • Use of sparse matrices solvers. • Validation of the code with a scaled prototype of an SKA AA-lo station. • Analysis of the mutual coupling effects in irregular arrays (random sparse vs dense regular ?). Station+antenna design. • Work towards the telescope calibration… • Scaled and small SKA AA-lo station. • 400 elements. • Scale: 30:1 of the BLU antenna. • 1m2. • UWB baluns. • Band: 2.1 GHz to 13.5 GHz.

22. Conclusions • MoM/MBF based method to simulate large finite irregular arrays. • Objective: Full fast EM characterization of a SKA station. Analysis and design (randomness, sparseness, etc.). Can we help? • Analysis of an infinite regular array of BLU antennas proving its suitability for the SKA AAlo. BLU antenna

23. End Thank you! BLU antenna Eloy de Lera Acedo eloy@mrao.cam.ac.uk