Best of the ISP-Lists

Fixed Wireless Equipment

The Rules of Rohn

It's okay to buy a cheap tower if it's a brand name tower, but it's not okay to cut installation costs.

[August 3, 2004]

On the ISP-Wireless list in July, CB, responding to JN's weather story (see The Guys You Depend On) asked:

Just wondering. We have a Rohn 25 at our backbone. It is a 50ft-er with a small 18"x18"x4" concrete pad at the bottom and a house bracket at close to 18 foot level. All we have on it is a 1 foot square dish (no lightning rod). The company that put it up said we need no guy wires. But I do not wish to get a phonecall like the gentleman did. Do I have reason to worry?

SC had a detailed series of suggestions:

"1) You really don't have much in the way of a foundation. The purpose of the foundation is two-fold. First to keep the tower from sinking and second to keep the tower from raising. The Rohn site appears to be down right now and I don't have the catalog with me...but I am almost certain they recommend more than an 18"x18"x4" pad.

2) The building brackets are nice but only if they are secured properly to the building. I have seen some shoddy installation of these brackets. Are the bolts through or anchored? If anchored, how?

3) Your tower is extending 30 feet above the highest bracket. This is about 20 feet more than we would ever do without a guy wire. Preferably, you guy within 5 feet of the highest section on a 25G because they are so flimsy.

4) A 1 foot dish is not too bad...however, there is still wind load there. I am not sure if you have ever climbed that on a windy day but it probably sways 12" or so in either direction. That can cause havoc on your link.

5) Lighting protection and grounding is the most commonly neglected part of towers. There should be a grounding rod inserted in the ground specifically for the tower. In order to know how many grounding rods you need you should have had a soil sample done. These should be a minimum of 5/8" x 8' and they should be driven into the ground so the top is approximately 2 feet below ground level. There should also be an inspection tube there so you can inspect the potential of the ground. A lightning rod should at least be on the top of the mounting pipe of the dish with a #2/0 stranded down wire. All connections should have exothermic welds (CAD welds).

So to sum this up, I would recommend upgrades before I ever climbed it. My company would not be willing to maintain it in its current condition. If you have them put in a guy wire make sure that they use a guy bracket and don't wrap the guy wire around the legs, they use a real anchor point (if a concrete deadman cannot be used then at least a bolt through roof bracket), all guy wires are on azimuth with the legs, guy wires are grounded, and there is a proper safety loop in the anchor assembly. And make sure they use hot-dipped galvanized parts or stainless steel. Do not let them use steel wire from home depot or radio shack, or aluminum turnbuckles, etc. If you have any more specific questions I would be happy to talk with you offline. "

[FC added] "With no lightning rod, I hope your dish is not the highest thing on the tower!"

[GM disagreed] "With respect, there are just too many generalizations going around in this discussion with regards to tower loading and guying. First, you need to know what wind speed zone the site you want to erect your tower is in. Call you local weather office to find out. Once you know that, you can work out what's necessary for guying etc. We operate in a 70 Mph area, so can run 25' above a roof line with no problem at all. Those in 110 Mph areas shouldn't dare to do that trick."

[SC replied] "With all due respect, I was not making a structural determination here, simply a safety one. I have been on 25G towers that were of similar construction and they are not safe.

Regardless of that, I am also certain that this is out of spec from the manufacturer. Rohn says it should be guyed at 23 feet and 45 feet for a 70mph wind zone. I think the eave bracket would count as the first guy level but there should still be something higher. In addition, Rohn says the foundation for a bracketed 25G is as follows: If a round drilled pier is used (caisson) then it need to be 2 feet diameter and 4 feet deep. It should rest on a minimum 6 inch bed on compacted sand and gravel for drainage. It should also be at least 6 inches lower than the frost level. For instance, in Michigan the frost level is approximately 48 inches so the caisson would need to be 54 inches deep and there should be 6 inches of compacted sand and gravel under that for drainage. That means the hole needs to be 60 inches deep.

These are the manufacturer specifications, not mine. The grounding and lightning protection is outlined in the EIA/TIA 222 F standard."

On a friendlier note, GM replied:

"We have a two story house with eaves at 22' (tall first story). We attach once to the eaves, and possibly once to the wall of the house. We then put a Trango 900 SU at 47' (40' of tower, 7' mast).

We will cheerfully maintain that configuration assuming it was in good shape and installed in our area. But we generally have 70' trees surrounding a residence, so residential wind loading isn't quite as critical as it is in the Great Plains, for example.

SC, this isn't aimed at you specifically or even generally. It's an observation that tower design, even for little towers, has several determining factors that should be considered before doing anything else. A lot of the discussion so far has glossed over accurate determination of wind load and completely ignored wind zone issues. Not good. Refer this back to the original poster—what wind zone is he in? Anyone know?

I'm not advocating light guying. We have around 20 towers with APs on them currently that are roughly equivalent to Rohn 25. They are guyed every 20' to 100', then guys every 10 feet to 120'. We use 4400lb galvanized wire with 12 ton(WL) turnbuckles. All fittings are hot dipped.

I agree with you completely on foundations and grounding. Get the foundation wrong, you can jack a tower right out of the ground due to frost. Watch out for drainage or leakage water or internal condensations as well. You can split a tower base easily if you don't. Also, many people completely ignore resistivity tests on grounding systems."

—End