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LOFAR first DIRECT lightning strike in LBA array

Submitter: Menno Norden and Jaap Bregman
Description: On the 5th of June, after a thunderstorm, we discovered a tripped earth leakage circuit at remote LOFAR station RS305. Superficially, everything seemed to work normally after powering up, but inspection of the LBA array showed something unusual. The top of an LBA antenna head had been blown six meters away from the support pole. This was clearly caused by a direct lightning strike! This offered an opportunity for verifying our theoretical models. Five years ago, we presented a paper on this subject at the EMC2010 in Wraclaw.

The LBA damage was somewhat worse than we expected, probably due to a more complex lightning discharge pattern than we assumed in our models. In total, we had to replace 54 LBA (antennas) and 4 RCU boards. For the LBA we found open, short or clipping circuits in the X or Y polarisation. For LBA-33 (direct impact) we found that the PCB track before the Gas Discharge Tube had been vaporized on the connected RCUs. Two other RCUs had a broken transistor switch for selecting LBL or LBH input. On the positive side, none of the 288 coax cables was damaged, and this is very good news.

Intriguingly, the signal delay communication for three HBA tiles had also failed. It was found that the modem program in the EEPROM had been erased due to high induction voltage. After re-progamming, these three RCUs worked as before!

We will continue to study the vulnerabilities in the LBA design, also for the benefit of future radio telescopes. Part of our new knowledge and technology is already included in SKA-LFAA prototype designs.
Copyright: Astron
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