Wednesday 13 July 2011

Cover up

Once each LOFAR HBA tile has been put on the frameset, it is necessary to apply the cover. This huge piece of UV-resistant polyurethane serves several purposes. Firstly, it keeps the water out of the antenna. Not so much for the antenna dipoles themselves, but for the electronics that are embedded between them. Secondly, it holds the entire antenna in place.

When the tile is first placed on the frameset, it is not connected in any way; it just sits there. However the fitted cover (like a fitted bed sheet) goes over the top and is locked down on all sides with strong elastic O-rings, thus securing the tile to its location.

A cover is laid out on the ground, ready for lifting.

Unlike conventional LOFAR installations, where the tile connects directly to the ground, for KAIRA the tiles connect to the framesets instead. There are typically eight connection points per tile. However, to provide additional strength, the outside edges of the array have even more connections, interleaving the standard ones. These are not standard on the covers, so additional connections rods and O-rings need to be applied.

Han Wessels attaches a new support rod to the cover edge.

Once ready, the cover is folded up over the tile. Care is taken not to walk on the inside of the cover and that the polystyrene lids are also kept free of grit. This helps improve the lifetime of the tile covers.

A cover is lifted up onto a tile. Note that the polystyrene
can easily take the weight of a person or two.

Once over the top of the tile, the O-rings are stretched down and are connected to screws on the wooden framesets. Because there is a very slight 'roof effect' built into the tiles, shortening knots are tied into the O-rings on the low edges but left off for those connection points at the ridgeline.

Some completed tiles. The white lines
are left over pieces of packing tape.

Once completed, the tiles are secure and will not move. The covers provide protection for the polystyrene and turn an otherwise delicate structure into one that is actually very robust (as our destructive tests showed!).

Photos: D. McKay-Bukowski

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