1. Nicolas Fagnoni – Cosmology on Safari – 14th February 2017
The “Hydrogen Epoch of Reionization Array” (HERA) - Simulation of the chromatic effects of the antenna and impact on the detection of the EoR power spectrum
Nicolas Fagnoni – Cosmology on Safari – 14th February 2017
– Department of Physics, University of Cambridge, UK

2. Summary 1. Detection of the EoR signal with HERA
2. The problematic of chromatic effects
3. Electromagnetic and electrical co-simulation of HERA
4. Design improvement

3. Detection of the redshifted hydrogen 21cm signal with HERA

4. Detection of the redshifted hydrogen 21cm signal with HERA
EoR signal contaminated by the foreground signal
Galactic synchrotron emission + extra-galactic radio sources
Foreground ~ 105 more intense than the EoR signal
Detection of the signal:
“Foreground subtraction” method
Complicated, requires an excellent knowledge of the foreground properties and chromatic effects induced by the telescope
“Foreground avoidance” method
“Smooth” foreground spectrum vs varying EoR spectrum
Study of a specific region of the EoR signal not contaminated by the foreground

5. Power delay spectrum Delay spectrum Power delay spectrum
Components of the wave vector k

6. EoR window and “wedge” Credit: Liu, A. et al. (2014)
Delay power spectrum averaged in a cylindrical cosmic volume
k⊥ = parameter associated with the spatial scale of the observed region in the plane of the sky
Small k⊥ ⇒ large region probed
Proportional to the baseline
k∥ = parameter associated with the time scale of the Reionization
Depends on the redshift and baseline delay of the received signal
Credit: Liu, A. et al. (2014)

7. The problematic of chromatic effects

8. The problematic of chromatic effects
PRISim
Foreground sky model + EoR model (21cmFAST)
Convolved with antenna beam models
Achromatic beam
Foreground contamination limited by the maximum signal delay associated with the baseline (i.e the “horizon delay limit”)
Chromatic beam
Foreground leakage at high k-modes
Contamination of the EoR window
Thyagarajan, N., et al. (2016)

9. Sources of chromatic effects
Mutual coupling
Feed-vertex reflections
Crucial to understand, model and limit these chromatic effects

10. Electromagnetic and electrical co-simulation

11. Antenna model with CST

12. RF-front end model
RF front-end: active balun + transmission cables + analogue receiver
Electrical circuits simulated with Genesys
Generation of the S-parameters
PICTURE OF THE BALUN

13. Electromagnetic / electrical co-simulation
Antenna model + front-end S-parameters
Simulation excited by a plane wave coming from the zenith
Gaussian pulse centred on 150 MHz and with a bandwidth of 100 MHz
Transient solver (time domain simulation => solution for all freq. in 1 run)
“Finite Integration Technique”
Hexahedral mesh (18 million cells)
Simulation time: 1000 ns with a time step of ns
Simulation of the output voltage after the receiver

14. Antenna output signal Main signal Dish-feed Cable reflections

15. Antenna voltage response

16. Effect of the reflections on the EoR signal
Constraints on the attenuation level of the reflected signal as a function of the delay of the reflections
White area  the foreground spillover should not impact the EoR detection
Ideal scenario: signal attenuated by 60 dB after 60 ns
k∥-mode non-detectable up to 0.2 h/Mpc because of reflections in cables, otherwise h/Mpc
Credit:Thyagarajan, N., et al. (2016)

17. Impedance mismatch
Reflections caused by a problem of impedance mismatch between the balun and the antenna termination
Balun impedance: close to 65 – 30i ohm
BUT the antenna impedance varies a lot
Historical reason: RF front-end optimised for PAPER
Time signal + S-parameters + comparison imp ant vs balun

18. Are simulations reliable?
100-ohm termination

19. Coupling in HERA 19 Antennas excited by a plane wave Antenna response
Central antenna excited
S-parameters and beam model

20. Antenna response
100-ohm termination

21. Beam With coupling Slighlty higher sidelobes and lower gain
Sidelobes not smooth at all
Ohter effects under investigation
At 150 MHz

22. Design improvement

23. Matching network
New electrical matching circuit to be inserted between the antenna and receiver
Smooth the impedance transition
Made up of 10 lumped elements (inductors + capacitors)
Decrease the reflections by ~ 10 dB (k// modes above 0.1h/Mpc may be detectable, if reflections in cable avoided)
BUT additional losses (between 0.2 and 0.9 dB)
Noise figure of the amplifiers modified

24. Central parabolic cone
Central cone
Flatten the antenna impedance
Impedance matching easier (reflections decreased by 20 dB)
BUT increase the sidelobe level by 5 – 10 dB

25. Development of a new Vivaldi feed
Larger bandwidth: 50 – 250 MHz (z = 4.7 – 27.4)
115 million and 1.3 billion years after the Big Bang
Null experiment at high freq.
Probe the Cosmic Down at low freq.
“Travelling-wave” antenna
Impedance and beam more stable
over a large band

26. Conclusion

27. Conclusion
The study of the EoR signal is the key to understand the birth and evolution of the first galaxies and stars
Astrophysical results are limited by the hardware
Essential to properly understand and limit the impacts of the instrument on the data
Now it is possible to reach a good level of precision using end-to- end computer simulation
Same method applied to SKA-LA

28. Thank you for your attention. Questions?
Credit: SKA