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 nf323@mrao.cam.ac.uk – Department of Physics, University of Cambridge, UK

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

Detection of the redshifted hydrogen 21cm signal with HERA

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

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

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)

The problematic of chromatic effects

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)

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

Electromagnetic and electrical co-simulation

Antenna model with CST

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

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 0.003 ns Simulation of the output voltage after the receiver

Antenna output signal Main signal Dish-feed Cable reflections

Antenna voltage response

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 0.15 h/Mpc Credit:Thyagarajan, N., et al. (2016)

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

Are simulations reliable? 100-ohm termination

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

Antenna response 100-ohm termination

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

Design improvement

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

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

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

Conclusion

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

Thank you for your attention. Questions? Credit: SKA