A Complex 'Array' in Shepparton

17 Mar 2009

If a cable breaks on a regular transmission site, it's a problem, but nothing compared to when a fibre core steel catenary rope suspending an entire collinear broadside array (also called a curtain array antenna) breaks, as was the case late last year at Broadcast Australia's Shepparton site in Victoria.

The facilities at the Shepparton site transmit high frequency broadcasts to Irian Jaya, Ambon, Maluku, Sulawesi, Southern Philippines, Mindanao, South East China and East Timor.

The Japanese 9MHz and 11 MHz antennas are 69 metre high and 182 metre wide curtain arrays that look more like a spider's web than a traditional antenna, and repairing the damaged fibre core steel rope involves more than simply sending out a single technician to site for repairs. Before the antenna could be repaired, a team of experts investigated the issue and developed a remedial plan.

Given the height of the antenna, there were two options to access it by:

1. lowering the entire antenna structure to the ground; or
2. utilising a large crane to undertake repairs "where it stands".

It was agreed that the second option would be the most efficient way to restore services quickly because lowering the antenna would need to be done very delicately to prevent damage to other components.

With the Christmas break looming, the team hoped it would be feasible to restore services quickly. Numerous riggers, engineers, field service technicians and antenna infrastructure staff worked tirelessly to fix the antenna and were faced with many challenges in the repair process.

The first challenge was the immense height of the antenna. There were no local cranes suitable for the job so a larger crane with a fly jib extension was sourced from Melbourne. The second problem was the windy conditions which made the repair of the steel rope on the antenna difficult.

For this task, riggers needed to be lifted above the antenna array in a cage - a slow process when Mother Nature has her say. For safety, the crane is not able to work when wind gusts exceed 23km an hour. This can make the process very slow and particularly difficult for the team trying to make repairs from a cage in windy conditions.

The works were eventually finished on the afternoon of 24 December, just in time for Christmas with additional fine tuning completed shortly after the New Year.