How big? It weighs 1,300 pounds, call it 600 kg. It's sixty feet tall and about a yard in diameter.
Since it was mounted about nine hundred feet up the tower, there's no lifting it down with a crane; instead, the tower is "rigged" with temporary "blocks" (think of a pulley on steroids) in interesting and complicated ways, a heavy cable is string up through them, the antenna is connected to the cable with a multi-part sling that hooks into several lifting eyes built into it, and a hoist sitting on the ground chugs away. A "tagline" connected to the antenna and coming off at an angle to the hoist cable serves to steer the antenna as it is lowered.
Once the hoist has taken most of the weight of the antenna -- but no more! -- men working on the tower remove the multiple huge bolts that have been holding it in place, and if everything works out (there's a lot of radio chatter and specialized gesturing), the antenna is "floating" as the last bolt is loosened.
Ours jumped a little when the final bolt was taken out -- less than a foot -- and the riggers were able to carefully push it away from the tower. The first steps of the lowering process proceed with painful slowness; a mistake here could cost fingers, hands, limbs or lives.
As the antenna begins to move down, strain is applied to the tagline. Riggers keep pushing the antenna clear of other structures on the tower and if the angle of the tag and hoist lines was correct, by the time it's too far away from them to reach, the antenna is clear of everything on the tower.
Don't forget, the antenna has to kept clear of the heavy guy cables, too! This is trickier while it's close the tower; as it is lowered, there's more room. But you can't move it too far out -- towers are designed to withstand huge vertical forces; the lateral loads one them are much lower, largely the result of wind. It's much more than you might think,. but not so much you can hang a 1,300-pound antenna hundreds of feet away from the tower while raising or lowering.
It all calls for a lot of planning and well-informed, experienced judgement before the work ever begins. Mistakes here make headlines.
|Click to enlarge. You can see the antenna hanging to the right of the tower. The angle isn't a mistake -- that's how the manufacturer specifies it is to be suspended for hoisting operations.|
Unrigging the antenna and securing the cables for next week's work only took about 45 minutes. Taking care of some four thousand feet of cable, strung through blocks 950 feet above ground, is not quite like running out a clothesline!