Thursday, January 8, 2009

Putting Up a Tower - Erecting the Tower

In our last installment, a solid foundation for the tower had been successfully constructed. The next phase is the erection of the tower itself.

At this point, there's almost 20 feet of tower standing, but 4.5 feet of the bottom section is below the surface of the concrete, so the tower is a little more than 15 feet tall, bracketed at about 9 feet off the ground

In the picture, you can see the remaining tower sections lying on the ground, as well as the Gin pole loaned to me by Dan, W4EA.

Before erecting the tower, I had done some preliminary work. Since this was a used tower, I had fitted all the sections together on the ground to ensure they would mate properly. I also did some work on the top section.

I used the Rohn 25AG3 pointed top section. This section ends in a 2 1/4 inch inside diameter galvanized tube, which can be used to hold a mast. I wanted to make sure the rotator shelf and mast were properly aligned on the ground, so I came up with the following technique.

With the section horizontal, I fitted the mast through the top. I then mounted the rotator to the mast. The rotator is an ancient Ham-M that I found at a hamfest for $50. It needed a bit of cleanup and a new motor cap, but then it worked fine. I then mounted the AS25 rotator shelf first to the rotator, then to the tower and tightened everything down. This ensures all the parts are aligned with each other.

At this point, I used bungee cords to prop up the top section next to the standing sections and wired up the rotator for a test. The mast turned without any binding. I then removed the rotator and mast from the top section, leaving the shelf in place. Rotate to North before removing the rotator.

Before going any further, the next step is to drop the mast into the tower. This will make it much easier to later haul up inside the tower, rather than having to hoist it up and insert it down through the top section. My mast is 9 feet of 2 inch diameter, 1/4 inch aluminum tubing, donated to my project by Gary K9AY. 

Stacking sections is pretty straightforward. Hoist the gin pole near the top of the existing tower, then use the gin pole to hoist the next section in place. Bolt the sections together and repeat.

I planned for brackets at 17 feet and 25.5 feet. The top bracket is just underneath the eaves of the house. After putting the third section on, I mounted the 17 foot bracket. It is tied into the rim joist with lag bolts. This is not an optimal arrangement. The lag bolts could easily pull out of the wood -- but the 17 foot bracket is just there to help stabilize the rest of the tower from flexing, and to act as a backup should the top bracket fail.

These brackets are first mounted to the tower, then bolted to the house. This ensures proper alignment between the two. Just make sure the tower is plumb before drilling into the house.

After the 17 foot bracket was in place, I removed the bracket at the 9 foot level.  The next section is added, and it's time to mount the top bracket. 

The top bracket gets a lot better mounting treatment. I removed part of the siding so I could get directly to the top plate of the wall. I added a 2x4 to make up the missing depth. Fastening the bracket to the tower, I then used the bracket holes as a guide and drilled entirely through the top of the wall. 

Carriage bolts inserted from the inside of the wall through to the outside and secured with nuts. The ends of the bolts inside are hidden under a bit of crown moulding. 

The last section is the top section, with the rotator shelf in place. Once mounted, the gin pole is used to lift the mast to the top, and locked in place with the set-screws in the top section. Tbe tower is ready to receive antennas, so we'll get them ready next time.


  1. When water fills the tubes at the base you will have cracking in the winter. Are drain holes provided?

  2. The base tubes are open to 6" of pea gravel in the bottom of the hole for drainage. In the eleven years this tower has been up, no problem with condensation. I covered this in the previous installment.