Yesterday, I completed the antenna tower project I've been working on since December. Around noon, I cranked the tower over to start doing the other odd jobs it needs, and those will take much less effort than everything up to this point. Antenna maintenance is a regular, annual task.
Coax Seal compound I had used (known in the biz as "monkey poop"). Everything was acceptable, but the seal was getting a bit oxidized and ratty looking. I'm pretty sure that there would have been problems if they were up for another monsoon season and I was doing this next December.
The house bracket (the dark horizontal line above the base of the tower) was partially inaccessible until the tower was down, so I didn't know if it would need work or not. Thankfully, it didn't. There was a little rust on the parts of the bracket I could see and knew about already, and all of that cleaned up pretty easily with a wire brush. After wire brushing the entire bracket with a rotary brush in my cordless drill I sprayed the whole thing with a rust converter. This needs 24 hours to cure (although I suspect our 87 degree afternoon will reduce that a bit). Then I'll paint it white to match the house trim. Meanwhile, I'll build a replacement for the two by four that clamps the tower to the house bracket. By the time the white paint has cured, all the other antenna and ground wire maintenance will be done so I'll put the house bracket clamp on, crank up the tower and we'll be ready to rock and/or roll!
Serious question here, as I'm considering putting a hinge on a 30' section of Rohn 25G. I see that your cable attachment is only about 7' above the hinge, which puts serious bending stress there when the tower is nearly horizontal. How much can these take? To put it another way (but your an engineer, so I don't really have to), if you fixed a section horizontally, how far could you hang out to the side before it bent or broke? I looked for this info on Rohm's site but had no luck. You've worked with these, so you must have a feel for it...ReplyDelete
Mine will be against the gable end of a house, and I'm planning to put the top pulley about 60% of the way up the tower.
(My gosh, I must be getting old -- I mean "you're", not "your". I hate it when I make that mistake, because people don't believe it's just a typo.)Delete
Actually, all I went with was a gut feel it would work because I was never able to get numbers for that. Trusses are pretty strong, but the main reason I was pretty sure it would work is that Aluma sells (used to sell?) a mounting pole that allowed the fully loaded tower to be cranked over from about 7' up, so it will support the entire load when held horizontally 7' from the end. Here's a picture.Delete
A friend of mine used to have 55' crank-up Rohn that he built a mounting pole just like that for.
Sounds pretty stiff! I tried to get numbers, but all the specs are given in terms of wind loading conditions against cross-sectional areas of antennas, but those are relative to so many aerodynamic variables that's it's hopeless to get simple bending moment out of them.Delete
I don't think I'll worry about it for my setup, given that my cable attachment point will be so high. Thanks for the feedback.
Now if I can just manage to not pull off the end of my house. Five inch lagbolts into several 2x6 studs should be adequate. Famous last words, of course.
Houses are pretty heavy so you shouldn't pull it off its foundation. :D Actually, I wished I had a gable and could attach this at 15' or so. Aluma does recommend a house bracket on all their non-guyed towers, and they accept that many will be at 8'.Delete
Just some thinking out loud here, and perhaps Mark Matis will chime in. With bolts in general (so I have to assume lag bolts) a bigger diameter is always stronger, right? That says to get to get the largest diameter bolt even if it's a little shorter, right? Is it possible to through-bolt, with big hardware? I would think that would strongest.
Mine is lag bolted into the roof trusses and I don't even remember how big those bolts are.
Through bolts are always better (put big fender washers on the inner end to engage the broadest width of the stud). I will connect the bracket to the gable studs rather than the fascia because the latter is usually barely connected to the flying rafters. Of course, that means building a bracket that extends out past the fascia, no biggie.Delete
As far as lags are concerned, yep it's better to have large diameters, because you are engaging more wood. I'll either go with 1/2" lags or 3/8" through bolts and washers. I'm hanging a wind generator on top rather than an antenna, so I can count on a lot more side force in a windstorm (you are supposed to count the full swept diameter of the blades as the "area" being pushed -- much larger than even the most complex antenna).
The ultimate test of course is if it blows over ;-) -- if so, it just needs to have more screws, heh. It wouldn't hit my neighbor, so I'm not going to worry about it.
GAWD, I hate that stuff!
I hope you at least put down a layer of good quality plastic tape before you put that stuff on....
Oh, it's nasty stuff! Hence the nickname.Delete
But yes, I did.
I used to use the gray putty tape you can get for plumbing repairs, and that's widely used by RV owners. It's even nastier than Coax Seal.
Congratulations on a job well done!ReplyDelete
Looks like a first class job. Well done.ReplyDelete
Did you happen to measure the pole deflection when you started lifting the mast?
Not exactly sure why you would need to measure that, but it would be an interesting number to have.
I actually did measure that, when I put everything back up today. Measuring a few hundreds of an inch over 7' isn't that straightforward, so I had to come up with a way of doing it.Delete
I have a digital angle gauge like this one Rockler sells. Before I put the pole under load, it read 89.05. When I had it loaded, it went past 90 to 89.15. That's 1.8 degree (.95 on one side, .85 on the other). 1.8 degrees at 7' up is 0.22 inch. That's roughly half of what I "budgeted" for when I did the design. But the number I got doing the design was for a load with a safety factor, over twice what the actual load is. So it sounds like it's about what I expected.
I last used the digital angle gauge to set a gas block parallel with a flat top receiver. I used the three magnets on the bottom of the gauge to hold the gauge to an machinist parallel I figured if I got the angle the same that would be more accurate than using a bubble level. Same trick for leveling optics or red dots.Delete