Tuesday, 10 July 2012

LBA pegs

Every LOFAR station needs pegs to hold the LBA ground planes (the metal base grid) to the ground. Because of the varying soil conditions, these pegs vary greatly from site to site. The Dutch stations have curved aluminium sheet pegs which are great for the soft Dutch market-garden loam soils. Other stations however, need steel pegs due to clay and rocky soil. And these tend to vary from station to station.

Ideally, each peg needs two spurs. One is to hold the ground plane down. The other is to allow the guy-wire of the aerial to be attached. (There will be a web log post about this later!) Although in principle a single spur can do this, from a practical point of view, it makes maintenance easier if the guy wires can be detached without interfering with the peg or grid.

The design that was chosen for the KAIRA build is a two-spur model. However, unlike other stations, the spurs are opposing. This means that the grid itself provides support to the peg and stops it loosening in the wind. That, in turn can result in LBAs being blown down in storm conditions (EXAMPLE).

Best peg yet? Simple. Efficient. And comes with a free smart phone...
no, wait! The smart phone is just for scale. Pity. Photograph by Arttu Jutila.

There are four corner pegs per aerial.They are set just in from the corner of each grid. The aerial itself only comes about half-way down the guys. The following diagram shows the placement.

Top-down schematic of the LBA aerial grid and peg layout. This shows the locations of the survey pegs
and anchor pegs. The diagonal grey lines show where the guys go that support the aerial post.

Once in place, the two survey pegs are removed. There is one at the north and one at the south. Note that the grid is hard against the north peg. This defines the position of the grid. The south survey peg is hard against one wire of the grid, but is otherwise in the middle of a "square". The reason for this is that the survey pegs are only accurate to about 1 centimetre. The south peg, therefore is only to define the rotation, not the position, of the aerial. Thus, to avoid it fouling on an east-west wire, it is moved up slightly.

Construction of the LBA will start very soon now.

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