Tuesday 6 August 2013

Fallen LBA aerial

Yesterday, we showed some subband statistics data that indicated that something had gone awry with RCU 034 and 035. Of course a LOFAR expert would recognise that these are adjacent RCUs, and thus pertain to the same antenna. Knowing the KAIRA system (or making an educated guess) you could tell that this was antenna 17. Furthermore the peak at around 60 MHz (and the fact that the frequency range was 0–100 MHz) meant that this was an LBA unit.

The next hint, was that RCU 035 took 3 seconds to fail but RCU 034 took only 1 second, starting a second after the first. This indicates a failure on the Y-polarisation first, and implies that this first failure on Y, led to the failure on X. Sure enough, that was indeed the cause.

Here is what we found on arrival at the site.

Luckily the wires were not damaged on the peg! (Photo: D. McKay-Bukowski)

As the data suggests, one of the guys broke causing the aerial to fall. The first second would have the northwest line (the Y-polarisation with elastic) drop down. At this instance the X-polarisation is still intact. Then, without tension on the northwest axis, the antenna begins to fall. As it reaches the ground the X-polarisation deteriorates rapidly. That is why there was a difference in timing of the failure. Also, from initial break, to hitting the ground, the fall took approx. 3 seconds. It was probably assisted by the wind.

The fallen LBA aerial. (Photo: D. McKay-Bukowski)

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