Discussion in 'Amateur Radio Amplifiers' started by K7JCN, Jul 12, 2014.
Good to know. I'll check that out.
Nah, I don't think you'd find that. But 1-1/2" NPS threaded galvanized pipe Schedule 30 is 1.900" O.D. with a .125" thick wall and is very strong. You join 10' sections using threaded couplers. All at Home Depot.
It's much "heavier" than "TV mast" but also very considerably stronger.
Getting off the topic of amplifiers here, but I'm enjoying the feedback. I've been told that threads eventually fail on the pipes so I've not really explored it but at one point I was looking for some but didn't find them either. I'll look more closely and see next time I go in there.
Those places won't sell you NPS galvanised SCH 30. You can get galvanised SCH 40, NPT. Every such hardware or 'big box' store carries that. I wouldn't use the threads; I'd sleeve the pipes. Find two sizes that fit one inside the other, and cross-bolt them.
I have a 30' mast like that, and it works great. But I wouldn't do it again. I could have got much better tubing cheaper from a local steel supplier. I only did it because I wanted to see if I could successfully build a good mast from hardware store parts.
The threaded couplings are incredibly strong: I've never actually seen one fail.
I installed 40' of pipe this way (four ten foot sections, three threaded couplings, all 1.900" O.D. stuff) back in 1977 in NJ, using a house bracket to support it at 22' above ground and a few bags of Quickrete and gravel at the bottom, and it's been supporting a repeater antenna for 37 years through storms and ice. It's still up, just fine. The repeater antenna it's supporting is a Stationmaster 220, which is 20' long and weighs about 25 lbs. When it's ice covered, followed by winds, its equivalent weight is several hundred pounds.
I tighten the couplings using pipe wrenches, and that's that.
My local Home Depot has NPS galvanized SCH 30, I bought some about three years ago at the H-D in Canoga Park.
The problem with using sizes that fit inside each other is that means the upper pipe is likely too small to clamp a Stationmaster to, since that won't clamp on anything smaller than 1.9" O.D. and barely does that (really designed for 2.0" O.D.). So if you telescope four sections with .125" walls, to make that work the bottom section would need to be 2.65" O.D., which isn't quite a standard and would also be much heavier and more expensive. I've found just using 1.9" O.D. all the way up, with the threaded couplings, works very well for a mast as long as it's well supported at its base plus a house bracket midway up.
"TV antenna mast" which is often 18 gauge (or HD stuff is 16 gauge) and only 1-1/4" O.D. and painted steel, is a really bad choice for any kind of permanent antenna installation unless all it's supporting is maybe the center of a balanced and lightweight wire inverted vee or similar. That would never last here with a 2m omni collinear on it after 2-3 windstorms. I've had to replace lots of those for locals over the years, and they all wish they never used that.
Water or gas pipe couplers engage only the first few threads and makes a very weak joint. Using electrical conduit couplers engages ALL the threads and is MUCH stronger. I use them on my homebrew tilt-up masts. My 40' tall FD mast is 1-1/4" Schedule 40 steel and a 35' tall mast at home is 2" Schedule 40 6061 aluminum. I found the aluminum electrical conduit couplings at McMaster-Carr. The FD mast is guyed and uses a falling derrick tilting gizmo. The aluminum mast isn't guyed, and gives a bit w/ the wire off the top to one side.
I've never seen threaded couplers fail either, but I see SCH 40 NPT threaded pipe ends fail all the time. I can demonstrate it happening with a coupler in a vise with a pipe screwed into it. It is actually very easy to break the threaded part.
You used NPS pipe. That should be stronger than NPT pipe. I haven't played around much with that. Also, some metals will obviously perform better than SCH 40 galvanised plumbing pipe.
So you only have one coupling above the bracket. That's important, as there is almost no side load on the couplings below the house bracket. The only coupling that's seriously affected is the topmost one, 8' above the house bracket. It probably doesn't see a lot of side load there. With proper guying and bracing (which needs to be done in any case), you can get away with using threaded couplers. But, sleeved pipes are still much stronger (though heavier), and will still hold up even when the pipes are mostly rotted away. They even survive if the guying should fail, as the pipe will likely bend somewhere other than the sleeved part anyway, if it does bend. I would happily sleeve two or more sections of pipe above the topmost guys. I wouldn't dare try that with NPT couplers.
Pulling or pushing a threaded pipe coupling is not a problem. Side loading is what matters. In fact, extra weight should strengthen the couplings some.
The ones around here don't. Maybe this is a regional thing. In fact, I don't know if I could buy SCH 30 anything around here without ordering it.
Not necessarily. The topmost section of my 30' mast, which I think uses 5 or 6 sections has an OD of 1.900". I could increase that with another sleeve. When sleeving pipes to make a mast, you don't have to go large to small to smaller like a traditional telescoping pole. You can go large-small-large-small-large. You can even go small-large-small-large-larger, if you want. My mast goes 1-1/2" - 1-1/4" - 1-1/2" - 1-1/4" - 1-1/2", I think. Either way, the top section is 1.900" OD, which is the biggest OD of any of the sections.
Good comments, and I basically agree with all of them!
When I discussed the "equivalent weight of several hundred pounds," I wasn't speaking of vertical (compression, dead-load) weight: I was speaking of equivalent side-loading, greatly increased by both the added weight and added wind loading of the antenna in the presence of strong winds.
In zero wind, weight wouldn't matter at all.
In a 60 mph wind, it does, since as the wind displaces the top of the antenna (vertical VHF collinear) to make it off-center, the load on the mast below is greatly increased. The more weight and the more ice coating (extra wind surface area) it has, the worse this gets. This kind of condition bends "TV masts" pretty quickly, but not the threaded galvanized .125" wall galvanized tubing.
@KC9UDX, so you're using 1.900" electrical conduit and sleeves? My house is a single story so I'd have to guy it anyway, and am looking at 40 feet of mast that I want to try. I'm putting up a GP 15 vertical, not real heavy. If I thought I could get away with putting two verticals on it I would but don't want to press my luck.
This is all great input! Thanks!
1.75" ID structural threaded tubing which is indeed 1.9" O.D. is available everywhere and not expensive. 4x stronger than "TV antenna mast." Go for it.
I wouldn't use 1-1/4" "TV antenna mast" to support anything.