I’ve been pondering if I should put this on the forum or not but I’ve noticed other Amateur radio hobbyists on here so I’m looking for comments good and bad on an idea I’ve been thinking about doing.
I considering on building a 100ft HF tower for a multiband Mosley antenna. The weight on top should be around 290 pounds with the rotor.
I’m thinking of using 1-½ inch electoral steel conduit into a tripod or quad spread out three to four feet apart. I would use the same piping as my cross sections welded in a two foot increments Z pattern with a cross bar at the bottom of every Z. I would use an exhaust pipe expander to spread the pipe just enough to fit the pipe inside it’s self at least five inches and then mig weld the seam.
I’ve been looking at a lot of other towers in the area and none are using piping as big as this but then they are not supporting the weight I will be.
The other part is I will need to be able to hinge it so I can lower and raise as needed to work on the antenna system when needed. I was thinking of welding three feet of heavy angle iron to the legs to use as the hinge at the concret footer. If this design will work I’d like to do the same thing for a planned M2 brand 80 meter antenna.
Question is,,,Do you guys think it will work?
Just a few thoughts off the top of my head:
1. The wall thickness of your tubing is just as important for strength as the diameter. You need to see what thickness of what material your friend's antenna towers used.
2. Wind loads are not trivial. What is the projected area of the antenna and tower? What are the expected winds in your area? Your local FAA or FCC office might be able to provide guidance on this.
3. Hinging the tower to be able to raise and lower it is a great idea for maintenance, but, stresses on the tubing and the rest of the structure may be greater when lifting and lowering than in use. Be sure to consider that when working out your loads and sizing your materials. This is not a good time to be :backyard engineering".
4. Would a high pivot on the tower work? If it would, you the tower might be balanced and that might reduce stresses over a ground level hinge.
PS: I'm a highway engineer and this sort of design work is outside my comfort zone. It should be doable, just be sure it is safe.
Oh, I don't like this at all. I used to do this kind of work for a living, but am not qualified as a tower/structural engineer. I don't know if they are still in business or not, but try checking with Rohn Towers, I know they had an office in Reno NV among others. They used to supply us with engineering details when we needed them. I have installed towers over 100 feet in height and worked on towers over 200 feet. We did commercial systems and had to build to a higher standard than home type.
I have never seen a hinged base on anything over 40 feet. Lifting it would be like picking up a bowling ball with a fishing pole.
Generally speaking, anything over about 60 feet high would have to be guyed with heavy gauge guy wire and properly anchored (this is to survive wind loading, and that assumes a light antenna load). That pretty much lets out a hinged base. The exception to this is a broad base that tapers up to a standard smaller tower about 50 feet from the top. You should expect about a ten foot broad base for a hundred foot tower. THERE ARE commercial towers done this way that are hinged and can be lowered for service. It takes a great deal of power to raise and lower them, often, a large crane is brought in for the day. The over 200 foot tower I worked on a number of times was a free-standing broad base. No guy wires, no hinges. I went well up that tower a number of times, but never had to go clear to the top.
The Tallest one I climbed a lot was 110 feet. I often ran calls out there and found the trouble was up topside. We took very good care of our belts, and didn't haul them around in the trucks unless we were expecting to use them. I would climb to the top of the tower without a belt, hook my elbow through the tower to reach out to fix or replace a part.
I still have the tools, belt, gym pole and can probably still get about a mile of heavy duty guy wire. I still have the "spade" and "spoon" to dig the hole for the base.
I don't know about where you are, but around Califunny, good used tower sections are readily available. After the switch to digital broadcast, thousands of small private commercial systems went dead. I have had several offers to get a tower for free if I would take it down. But that is a lot of hard work for something I probably couldn't sell for more than a few hours wages. Taking down a used one is usually a lot harder than putting up a new one. Too many installers over-tighten the bolts and crush the joint. They don't want to come apart. A few times I had to hack saw legs to get them down.
Drive carefully, and enjoy the New Year! W2
Hire a registered engineer to review your design! I don't see any mention of foundation? Caisson vs spread footer? Setback/Fall Zone? Local ordinances? Tower design is very site specific and typically has a 150% to 200% safety factor with an ice load of 1/2" radial (norhtern states). Wind load must include both the tower and coax cables in the structure. Grounding is another serious issue. You will have 110v to control the antenna rotator? I don't think you will get that strength from thinwall conduit. A 300# load a 100' in the air is substantial and could be dangerous if improperly done.
Like my old boss, K6WS, used to say about the early automated data and text messaging systems in airliners: "Just pick up the mike."
The cost and maintenance of that thing would buy a pretty good brass T, I bet.
Just pick up the phone.
A 100' monopole tower designed and inplace is around $80,000. I think that would be a couple of brass T's.
KD8GYL formerly W2BDH