VK6RCT antenna damage


This spectacular example of lightning damage was discovered by Glen VK6IQ the then site manager for Cataby. The repeater was off air and a site visit found the cause even before reaching the site, no antenna at the top of the 60' mast. So what had happened...? A possible explanation below...
Antenna damage

The antenna as above is up side down. The top is speared into the rock around the base of the shed. Note also the antenna has punched a hole in the fibro Sun roof. The antenna must have broken off from the tower and fallen vertically, spearing its way through the Sun roof.

This means it could not have been very strong wind damage, as once the antenna had broken away from the mast the strong winds would have carried the antenna away from the shed. And these fiber glass antennas just don't break in strong winds, they are very strong.

So what caused this unusual damage...?

VK6RCT hole in roof

Antenna punching hole through Sun roof

The antenna was removed from its unusual landing position and examined. There was no indication as to what had caused the antenna to break away and fall.

VK6RCT antenna damage

It was not until the mast was climbed it became obvious that a lightning strike had been the primary cause of the damaged antenna, all be it difficult to work out how. The antenna showed no evidence of a lightning strike. There were no burn marks on the antenna, just shredded fiber glass. Also the inner metal parts of the antenna showed no signs of lightning such as burn marks or melted metal.

The metal base mounting section of the antenna was still attached to the tower but the short connection lead from the coax run up the mast to the antenna was badly burnt with much of the plastic outer and inner dialetric melted.

So it was now obvious that a lightning strike on the mast was the cause, but why no burn marks on the antenna, despite such a shredded fiber glass antenna...?

This is one possible explanation...

Lightning is un-predictable for starters. It does not always strike the highest object around. The metal parts of the antenna are inside the long fiber glass tube and fiber glass is a very good insulator. You might think that a few milli metres of fiber glass is little impediment to millions of volts from a lightning strike, but lightning has to create a path to strike, in stages. The air is gradually ionised as the voltage builds and this ionisation is at low voltages. The few milli metres of fiber glass provide sufficient insulation that there is no ionisation into the air from the fiber glass antenna, but from the top of the mast.

lightning at eiffel tower

Lightning does not always strike the top of a tower.....Eiffel Tower Paris

I have been almost at the top of the Roleystone mast when there was no lightning in the area but there was a small dark cloud directly above the mast. As I came within 50' of the top of the 300' mast I could hear sparking from the top of the tower. A hissing crackling sound that was easily heard from 50' below. This is the initial stage of lightning finding a path to ground. I waited for the dark cloud to move and the crackling to stop, then continued the climb to the top.

So at Cataby, the thin fiber glass prevented ionisation from the highest point on the mast. Rather the ionisation came from the metal points at the base of the antenna. When the lightning struck this point on the tower a bit of guess work is needed to explain the shredded antenna.

It could have been the explosion occurring almost parallel to the antenna. Lightning heats the air so rapidly, that a shock wave explodes outwards, followed by the reverse as the air rushed back in to fill the cooling vacuum. This could well be the explanation of such complete damage to the fiber glass outer of the antenna, a large explosion right next to the antenna.

VK6RCT Lightning strike

Lightning striking mast at Cataby but not the antenna...!

This explanation is the most likely, particularly as the inside working parts of the antenna were undamaged, so much so that they were retrieved and with a new fiber glass support, reinstalled at Cataby and working as of today.....!

VK6RCT refurbished antenna

The refurbished antenna at Cataby with a strong Easterly blowing.