Friday, January 9, 2009

Putting Up a Tower - Antennas, Etc.

With the tower up in our previous installment, it's now time to add the antennas, feedlines and everything else that makes the tower system work.

A tall tower is a great target for lightning, so every tower system needs a protective ground. Mine has four 10 foot ground rods -- one next to the tower base, and three others fifteen feet away, all tied together with 6 gauge copper wire. 

Getting forty feet of ground rod into the ground might be a lot of work, except I built a special tool. It's a 12" steel pipe nipple with couplers on each end and a plug on one end. fifteen pounds of dumbell weights are added to the other end. Basically, it's a 15 lb sledgehammer that can't miss the end of the ground rod. I've been using this tool for years, and it works really great. 

So long as the soil has a reasonable amount of moisture, you can pound in a ground rod within a foot of the ground in less than five minutes. The last foot is taken care of with an ordinary eight pound sledgehammer. During the drought in the late 1980's, the Georgia red clay was particularly difficult to put ground rods in, but this tool could handle it.

With the ground portion in hand, let's look at preparing the antennas. I purchased a Cushcraft A3S years ago. It's one of the last tribanders designed without computer modeling. It's rugged yet lightweight, and offers reasonable performance. For a small (14 foot boom) tribander, it's probably one of the better designs available. 

It was up at my old QTH for about six years, then in my basement for five years, then up at Mike, W1YM's place while he was waiting for his Butternut Skyhawk for almost two years. At this point, it needed some help. 

I ordered replacement trap end caps from Cushcraft -- all 24 of them. With the end caps off, I blew all the debris out of the traps. I also tightened up all the sheet metal screws which secure the ends of the traps to the elements. I wrapped electrical tape around the driven-element insulator, in order to protect it from the Sun. 

As I re-assembled the elements, I chose to set the beam half-way between the CW and MID settings. I believe this is the best compromise to cover the entire band with a reasonable SWR. I got the idea from the recommendations that Steve, K7LXC made about the Cushcraft 40-2CD.

It's always a good idea to test an antenna on the ground before getting on the tower -- where it will be much harder to make adjustments. This can be done by pointing the antenna skyward with the reflector 3 or 4 feet of the ground. I used my back deck to support my antenna on a short piece of mast lashed to the deck rail.

At this time, I also added the feedline pigtail for the antenna, and the balun. Cushcraft recommends making a coil of coax feedline and taping it to the boom. While this will work, it may not be the best choice. WA2SRQ wrote a posting to the TowerTalk reflector several years ago that showed other balun designs would work much better. 

Originally, I planned to add a piece of 4 inch PVC mounted to the boom with eight turns of coax. However, I happen to stumble on some very large ferrite beads that would fit entirely over RG-213 coax. They appear to be of type 77 or similar material -- excellent for a choke balun. 

So the balun consists of ten of these beads, strung on the coax pigtail, held in position to the coax with wire ties. The coax is suspended below the boom with twisted 12 gauge solid copper wire. 

A quick test with the antenna analyzer showed the antenna SWR curves were right where the should be -- about 50-100 kHz low because of the near effects of the ground. Now it's time to put it on the tower.

Remember, this was my first tower work. While I had done all the climbing to stack the sections, I wasn' t really comfortable with the tippy top of the tower. Once you are 20 feet above the upper bracket, there' s a certain amount of sway thats rather unnerving. Fortunately, my friend Gary K9AY was close by. Gary has much more tower experience than I, and he was kind enough to install the antenna. I was on the haul line for the gin pole, but I did managed to snap this picture as the antenna goes in place. 

With the antenna on the mast, there's still much work to be done. Loosening the mast set screws the gin pole is used to move the bottom of the mast well above the rotator shelf. And then the rotator was installed. Be sure to position the antenna North to match where we left the rotator.

I routed my feedlines an rotator cable up the inside of the tower. This is much harder than just taping it to the outside of the tower, but it prevents complications if you later want to shunt-feed your tower. The cables are held to the tower alternately with electrical tape and twisted 12 gauge wire. The twisted wire is better than a wire tie, because it can be reused. It's also really cheap to salvage bits of wire out of household romex. 

Remember my rotator had a defective motor cap. One really odd thing about the Ham series of rotators is that the cap is in the control box, instead of in the rotator. I opted to place the cap near the rotator by installing it in a small NEMA box that is U-bolted to a tower leg. To allow the rotator to be removed, I used two 4-pin trailer connector sets. These are inexpensive, weatherproof and very rugged. To avoid cross connecting, I used the mostly male and mostly female ends  on each side. The trailer connectors join with the rotator cable inside the NEMA box.

So, in early September, 2001, my tower was fully operational.

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