Special Pages

Saturday, November 5, 2022

Tower Repair Finale

The tower is repaired and back up.  It's even more or less vertical and I didn't have to tear up the work I've done already to get it into place.  I just had to walk back and forth between the east side of the house and the shop on the west side about a million and six times for that one size wrench, socket, or drill bit that I didn't have, or that thing I didn't even think I might need.  

Those of you in the southeast US watching the tropics know that there's a broad area from just north of Puerto Rico and Hispaniola (the island containing the Dominican Republic/Haiti), stretching north up over and east of the Bahamas up to around Cape Canaveral.  Even farther north as you go farther offshore. The NHC says there's a good chance that will develop into either a tropical storm or a non-tropical cyclone, all of which means there's a chance that it goes from a rainy/messy week to a storm that forces me to crank over the tower again.  More on that later.

But first, here's the leg that was broken and mangled repaired with the tube up inside the leg, held in place by two 1/4-20 stainless bolts at 90 degrees to each other and offset center-to-center by one inch.  

Anybody here a mechanical engineer?  The strength in bending is proportional to the moment of inertia of the tube.  The existing 1.50" OD tube has a polar MOI of 0.068 in^4.  The tube sticking out of the bottom of that tube has an MOI of 0.097 in^4.  Smaller OD but thicker walls.

The angle aluminum is new for general purposes; the old piece had undergone some corrosion and it seemed prudent to replace it.  The angle is held by another 1/4-20 stainless bolt and locking hardware.  In the slab itself is a 3/8" bolt with a stainless nut on it which had also suffered some corrosion but running a threading die over that cleaned up the threads and made it so the nut threads all the way down to surface of the concrete slab.  

This is that area with the tower cranked up and bolted in place to the house bracket. The two holes that get bolts through them are drilled with the "loose clearance" size recommended on standard threading charts, about 1/64 of an inch larger in diameter than the bolt size.  By loosening the 1/4-20 bolt on the tower leg, it easily allowed me to lower the tower into place. 

The house bracket and the 2x4 that clamps the tower to that bracket was fussy and made me walk back and forth for different hardware the most.  

I think this needs to be replaced and I've been telling myself for years to make an alternative out of aluminum bar stock in the shop.  I end up replacing the wood 2x4 every couple of years, and it's annoying.  In the big picture, as has been talked about here often, I don't want to use a house bracket and I'm seriously studying how to emulate mounting pole concept that Aluma (the tower manufacturer) uses.

Finally; an overview.

By the time I finished this up yesterday, it was too late to spend any time looking at the antennas, so I did that today.  It turns out the antenna I repaired, the bigger one on the bottom in this view, was at least as good as before the elements snapped off, but the smaller antenna visible above it was markedly worse.  Ironically, I never swept that antenna when the tower was cranked over because it looked fine.  For those who understand this, the swept VSWR went from 1.1 or 1.2:1 up to over 5:1.  It's rather strange.  

The short version of the story, though, might well be that with the messy weather coming up that might turn uglier, maybe the answer is to crank the tower back over so I can access the back end of the antenna.  When I put that antenna up last January , I worked on it by turning the antennas to point south which puts the connector closest to the ground.  In this picture, it's pointed north.

I don't see anything about cranking it back over again for another week that would be negative. Thanks again to the people who suggested fixes and approaches that helped me converge on a way to get this done.



18 comments:

  1. Looks good. That's a smaller tower than I thought you had. I've seen antennas act like that, and it was usually a loose/corroded connection, or some damaged coax.
    I moved some antenna stuff around today for the basement shop. Got my FM antenna up higher and in the clear, and the GPS antenna for my Master Standard is now up nice and high. If it's "nice" tomorrow, I'll hammer drill the hole through the foundation to run the coaxes through.

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    1. Yeah, that's what I'm thinking. Something happened at the N-Connector and matching network. The cable near the ground looks fine, so naturally problems have to be at the end that's hard to get to. Murphy's Law is a fundamental law of nature.

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    2. +1 on a pinched coax or connecter that has come loose or is damaged.

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    3. Not sure exactly what the cause was, but it appears to be that the Coax Seal ("monkey shit" as it's known) that caused it. After taking the connection apart, cleaning everything apart, I put a 50 ohm termination on the cable and it was fine. After cleaning up the rest, of the outside of the connectors (the insides were shiny and new looking) it went back to looking like the first impedance sweep that I saved in January.

      But the tower is staying down until this mess out near Puerto Rico decides where it's going.

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    4. Oh, GAWD....I HATE that stuff! I use adhesive-lined heat-shrink tubing, but that can be very inconvenient at times. Scotch 33+, or self-fusing silicone tape are what I use now, and if it **really** has to be weatherproof, I'll use a layer of 33+, a layer of butyl rubber, and a top layer of 33+. One of my friends goes so far as to apply a layer of "Scotchcote" over all that, but that's a bit extreme, even to me!

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    5. Belay what I said there. A few very weird things have gone on and I don’t really understand what it is. I wrapped some tape around the cable to antenna connector joint and it went back to high VSWR. Took the tape off and it stayed high. The only constant is that the results are fine if I put a termination on the cable. There’s too much to go into here. More to follow.

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    6. Is that a 6M beam above the LPDA? Now it sounds like something wonky in the matching network between the connector and the driven element....

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  2. you shudda been an orthopedist! the repair looks like it'll hold up through a Cat 7. Halevai!!!
    livng on a Gulf island off the west coast of Florida, I watch (very closely) those counter-clockwise spirals (coming all the way from Senegal) that pass over the Windwards and Leewards at the eastern end of what I call "The Channel" that ends in a "nipple" between Cancun and that little finger hanging off the western end of Cuba. Once through that squeezepoint , depending on the winds, they'll hit any Gulf shore from Tampico to Marco Island.
    Yes. I watch 'em very closely.
    Then there are the ones (like Ian of recent fame) that appear to arise on the western side along that stretch from Oaxaca, Mexico to Buenaventura, Colombia, cross the isthmus, and then can wiggle back and forth in that little bay between Merida and Tampico for days appearing to die at times before they, too. head into the Gulf to build up a fine head of steam.

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    1. This one has been made an Invest this morning - area to investigate. Considering how desperate they've been to start calling things "Pre-Tropical Cyclone" this year, I'm surprised they didn't call it that. It might be extra-tropical but still a crappy day. Too soon to have model runs on it.

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  3. The repair looks good, very professional looking!

    I will say,TBH, the slab bolt seems under sized - my fathers rule of thumb for “good steel” bolts was 1/8” for every 1,000# of working load - and there is a heck of a moment arm there for wind loads. I imagine there is a asce or same standard.

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  4. SLS/Artemis is scheduled for a launch the 14th. Wonder if they are going to have another roll back. They cant catch a break.

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    1. If they did another rollback they'd have to scrap the whole thing.

      Wait... Who am I kidding? Maybe they should scrap the whole thing but they could also intimidate various engineers until a few agreed to pencil whip that it's fine. Then fire those engineers when it falls apart on ascent.

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  5. Forgot to add the tower repair looks great. I also vote for corrosion/bad connection on the upper antenna. Let us know how that turns out please.
    Regards & 73

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  6. Any bets that Starship launches before SLS?? Starship may be pretty close to a full stack static fire and if that happens pretty quick.... and SLS rolls back, then.... of course one would also have to roll in the odds that the FAA might not approve a Starship flight before and SLS launch. Talk about pencil whippng :)

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  7. stress= (the bending moment times the distance from the neutral axis)/the moment of intertia

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    1. Compression or tension, John. Of course, there's shear in there as well.
      Welcome, everybody, to Strength of Materials 101 course practicum!

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    2. Yup! But that's on the other axis.

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  8. Distant observer here. My view is closer to the sand than most. If your above ground equipment is acting unusual, perhaps your ground plane is being corrupted by the many dissimilar metal connections. Just a guess. Thanks for your posts and engineering views. Duck when that next one comes through!

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