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The Tower Project
DISCLAIMER: This is a description of how I built my tower. It is provided in the hope that it will be useful but without any warranty of any kind. Make sure to get the necessary permits for your area, if applicable and if you have any doubt, talk to a professional civil engineer even if not required by local regulations.
Even though a permit was not required in my county and subdivision (not required for ham towers under 75'), I thought about getting one for the peace of mind. However, in my county, the tower and antennas would have to be rated 140 mph to get a permit, something few ham radio budgets can afford, and something unnecessary in my case since the tower folds. Further, I had access to persons knowledgeable about building codes and permits and the final design takes into consideration the advice I got informally from these qualified individuals. While this is not as good as "money in the bank", or a building plan certified by a professional engineer, I felt it was sufficient for my purpose. Your mileage may vary.
If you intend to use information on these pages to build a permanent (non-folding) tower, make sure you understand the differences and protect yourself accordingly, both from safety and legal stanpoints.
Useful tower building resources:
- N1LO's home page contains a tremendous amount of information on tower building and anything related.
- K7NV's page on wind load calculations (this page has not proven to be reliably available, so you may want to check the links below).
Here is a link to his YagiStress software (download the demo version).
- WB6WUW's very informative page, which was featured in July 2003 QST.
- Champion Radio's 10 Most Common Tower Building Mistakes.
- Check Kari, SM0HRP's page about his own tower base project.
This page would not be complete without the deepest thanks and appreciation for the members of the Playground Amateur Radio Club PARC, W4ZBB who came in droves to help with the tower raising party.
My Tower Project
I bought this 50' folding Rohn 25 tower used from a local ham who had it attached to the side of his house. For extra safety (we are on the Florida coast, and we do have hurricanes and strong winds), he also had installed guy wires.
The guy wires were going from just below the hinge (about 22' from the ground) to the top of 7' high, 3" diameter schedule 40 steel tubes (guy posts) instead of directly to the ground because there was not much room where his tower was located and the guy wires would have been too steep to be effective.
I thought this would be a neat idea to reduce the risk of someone (like a kid) running into the guy wires. With the normal mounting scheme, guy wires leave the ground around 45 degree angle and that means that shrubs or a fence have to be installed to prevent accidents, and I certainly did not want to loose that much real estate. I also wanted to put the tower as close to the fence as possible. With the guy posts, I loose much less real estate and the guy wires take off from the tower closer to horizontal by about 15 degrees, so they should do a good job. The guy posts are at about 25 feet from the base of the tower.
So I did the same thing, except that in my case, the tower is detached from the house and squarely in the middle of my backyard.
To stiffen the guy posts, another EHS guy cable runs from the top of the guy post to a J bolt about 1.5' from the base of the post. This does little for ultimate strength but does stiffen the tubes and reduces flexing in the wind.
The tubes are also filled with concrete all the way to the top to make sure they do not collapse under extreme stress. The concrete makes them stronger and prevents moisture and water from getting inside the tube and causing corrosion. The concrete also dampens the tubes, so they don't sing like a bell when something hits them.
Because it is a folding tower (it folds at about 22' from the ground, approximately in the middle, a design no longer offered by Rohn unfortunately), there are 4 guy wires instead of the usual 3 so that the fulcrum that allows to fold and raise up the tower has clearance.
Note: I read that Rohn stopped making the folding type tower because hams were overloading them and having problems. The tower is certainly very vulnerable when it is being cranked up and down, particularly when it is at a 90 degree angle. I wanted to take it down during moderate winds at one time, and the effect of the wind on the structure when the tower is folded is significant. You do not want to leave it like that very long. However, when the tower is straight up and properly rigged, it is as strong as a normal Rohn 25 tower.
The guy posts (tubes) are actually 12' long 3" diameter schedule 40 steel tubes (structural grade tubing, not just "pipes", pipes are designed to transport liquid and are not rated for strength) with 5' in the ground, in concrete. The guy post holes are therefore 5'6" deep, and 1.5' x 2.5'and filled with 3000 lbs rated strength, fiberglass reinforced concrete. The post base also holds the J bolt for the post guy. Place a brick at the bottom of the hole so the post is at least 3" off the dirt. I poured concrete up to about 3" under the ground surface, so that if and when I move out of the house, I just have to cut the tubes flush with the concrete and cover the hole with dirt and grass.
The tower concrete base also is filled with 3000 lbs rated, fiberglass reinforced concrete. I selected the fiberglass reinforcement because the tower base is bolted down in the concrete using four J bolts, instead of just burying a tower element as I have seen it done often. The fiberglass gives the concrete an extra measure of strength around the J bolts and only adds about $5.00 per cubic yard to the cost of the concrete. In total, I used 5 cubic yards of concrete, in addition to what I used to fill the posts (I used regular Quickrette for that). 5 yards was the minimum quantity for which there was no delivery charge from all the vendors in the area.
Please note that fiberglass reinforcement does not allow to get rid of the rebar cage unfortunately.
To make sure the concrete hardens and does not just dry, keep it covered from the sun and keep it moist. I used an old piece of carpet laid over the concrete pad, and covered with a large piece of cardboard. I waited until the next day in the morning, when the concrete surface was already somewhat hard. Then twice a day I would hose the carpet and cover it with the cardboard so it did not dry too quickly. I took carpet and cardboard off after about 4 days of that regiment. The concrete looks pretty hard to me. I pounded on it with a hammer and could not chip it.
I let the concrete cure for two weeks before putting up the tower. While it was far from having achieved its full strength (that takes 4 to 6 weeks), the concrete was plenty hard enough by then and we had absolutely no problem, even though it was interesting when two of us were hooked up to the tower at about 20' up in the air before we had the guy wires attached (please note we had rope guys for safety while we were doing that, but these don't do much to stabilize the tower to the point I felt I needed for peace of mind.)
Pictures taken during construction.
The finished tower and antennas.