SKA-LOW at Imperial

2017 is the 10th anniversary of the European Research Council, who fund my FIRSTDAWN group at Imperial. For our celebratory festival, I thought it would be fun to display an SKA dipole. They’re eye catching and a good prop to explain radio astronomy and the SKA. My colleagues at Cambridge, where they were designed, kindly provided me with one. It arrived in flat pack, with some assembly required. Cue: Team assemble!

I’m fortunate to have a fairly large group – Claude and Tom are PhD students, Suman and Emma postdocs (Catherine who’ll be joining in July was off in Pisa). And a team bonding moment was definitely needed to cast off mid term blues.

We all got to work with a claw hammer to get into the box. Claude immediately had to go off with a nose bleed, prompting calls for a post-facto health and safety assessment. “It’ll be fine, but please watch the nails”.

A low frequency antenna looks pretty much like an old style TV antenna, just bigger. The size of the antenna needs to be matched to the wavelength of radio wave that you hope to detect. For SKA-LOW these are around 2m, so the antenna needs to be about a metre or so in size. Apart from that it’s really just metal wire and feels very low tech.

We collected all the bits, but couldn’t find the paper instructions with the little cartoon man (maybe that’s only IKEA?).

Most of the construction was straightforward. The four arms screw to the top and to the base. The plastic support was more fiddly (and needed one more 8mm socket wrench than we had).

There were some nice design features – slanted brackets so we couldn’t get the orientation of the tray wrong.

We got to this stage and spent a while wondering what the correct alignment of the arms was. These need to be fixed relative to one another and that alignment helps determine things like the response to different polarisation of light. Something seemed funny.

And it was, we’d missed a crucial step. The plastic tray both strengthens the frame AND holds the arms in the correct alignment. Yay, there was much rejoicing.

Tightening everything up and fending off enquiries from intrigued astronomers. The electronics would slot into the top plastic module with a plug that screws in to hold them in place. Then a cable feeds out to take off to more electronics for beam forming. 

Five happy theorists all turned experimentalist for an hour. At our rate of assembly, the 130,000 dipoles needed for SKA-LOW would take us about 12 years. Working without sleep in the heat of the Australian outback. Not sure any of us would accept those working conditions.

Well deserved rest. And already our dipole has detected the CMB!

All in all a great afternoon. Very interesting to see the design and construction. This is a SKALA2 antenna, there’s a scaled up SKALA3 version currently to be tested. The advantage of this fractal like structure is that it gives a very wide bandwidth (50-350 MHz). But that comes from having numerous resonances across the band – basically one per strut on the arms I think – which makes for a complicated frequency response. The SKALA2 has a sharp resonance at about 60MHz that would make EoR science difficult. By scaling everything up that resonance gets moved down in frequency and out of tthenobserving band. A simple fix, but it still remains to see if it’s sufficient. 

Imagine thousands of these grouped into 35m diameter stations of 256 dipoles and scattered across several km of the red dirtbof the Australian desert is quite a thing. There’s one test station already – the AAVS1. 

But for now, we’ll stick with one at Imperial. For outreach, for discussion, and as a scientifically themed Christmas tree.

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