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Author Topic: Tower extention material?  (Read 3255 times)
W8JI
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« Reply #15 on: July 10, 2011, 02:16:34 PM »

I see why you are confused, because he said he had a "tribander". That confused me too. If he really had a tribander, that tower would be down in the first puff of wind. :-)

He cleared up tribander by saying it was really a three band VHF/UHF vertical antenna. He doesn't have a beam. :-)

He only has a very low wind load vertical and a dipole. It is probably very marginal even for that! It good for maybe 20 feet or so above the house bracket, if the bracket and anchors are good.



How about this scenario:

  • Mount your rotator just above the sleeve on the tower.
  • Use a 10' Radio Shack mast and mount the boom of the tri-bander about a foot above the rotator clamp.
  • Find a sealed, flange-mount bearing with an ID that fits over the Radio Shack mast and locks with two set screws and secure it to the top of the mast.
  • attach the center of the dipole to one of the flange holes on the bearing and make SURE the ends of the dipole slant up so as to not tangle with the tri-bander.
  • Route your coax from the dipole down the mast then along with the tri-bander coax.

The tri-bander will rotate and the dipole center will not, because it is mounted on the bearing.
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KB9MDQ
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« Reply #16 on: July 10, 2011, 03:36:01 PM »

Ya... sorry for the confusion. I was just wanting to take all the crap off and run a main pipe higher so I could mount the dipole so it would be level with the trees to the north and south so the ends of the dipole were fairly level with the middle.

The three band VHF/UHF vertical antenna would stay at the height it is already at since the coax is already cut to length.

No rotor. Don't need it since the antennas I am using are the dipole and the 2m/440/6m vertical.

I would assume if wind caused it to give it would give where the mast mounts to the tower at the top.

No that tower would never support a large ham antenna. There appears to be 8' of 1 1/2 crap mast above the tv rotor in the picture so I figured a 2" pipe of the same heights would work and be sturdier than what is currently up there.

The tower is not rusty but I already planned to check the base. I installed it back in '96. It is in the ground with sackcrete in the hole. Will just want to check the integrity of the metal at the base.


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W8JX
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« Reply #17 on: July 10, 2011, 03:54:01 PM »

Ya... sorry for the confusion. I was just wanting to take all the crap off and run a main pipe higher so I could mount the dipole so it would be level with the trees to the north and south so the ends of the dipole were fairly level with the middle.

Trees are really not going to have any effect on HF dipole for 2 reasons. First being trees have little effect on HF frequencies and two, even if they did, radiation angle of dipole will be above them.
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N4CR
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« Reply #18 on: July 10, 2011, 04:16:13 PM »

I would assume if wind caused it to give it would give where the mast mounts to the tower at the top.

Your assumption is wrong. The most leverage is where the longest distance to the fulcrum is. That would be at the brace near the roof line. That is where it will most likely break first.

What is your budget for a new roof when that tower goes through it? That's the real question. If you are cash strapped, won't it be a problem when your roof has a big gaping hole in it? You can't predict which way the wind will blow a tower over or where it will land.

This is cheap TV antenna tower. Pushing it's limits is a very bad idea. How are your guy lines anchored? How many of them do you have?

The accepted minimum standard for quality ham radio tower is Rohn 25. If you look at it online, you will see that each square formed by the cross members is braced by a diagonal brace, creating triangles. Your tower doesn't have those. That means that your welds take all of the twisting stress that the angular braces take on a properly designed tower section. And that force is spread over a much smaller distance, the length of the pinch weld.

Can we talk you out of this?
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
W8JX
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« Reply #19 on: July 10, 2011, 04:48:42 PM »

The accepted minimum standard for quality ham radio tower is Rohn 25.

There is no minimum standard because there are applications where Rohn 20 would work  and places where even  Rohn 25 will not work and Rohn 45 or 55. is warranted. That being said when is minimum standard depends entirely on application.
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KB9MDQ
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« Reply #20 on: July 10, 2011, 05:06:51 PM »


Can we talk you out of this?

Since the roof is metal I was trying to get the dipole higher to help the 80 meters. At its current height the 40 M is 1/4 wave above the roof but not to 80M

I could always mount it at the current height. I still think I would remove the crap off and simplify the arrangement at the top of the tower to one basic pipe (eradicate the rotor) but keep the current height. I haven't measured it but from the current mast is about 8' including pipe/rotor and 1 1/4 yellow mast.

BTW, the hinged part was constructed at a local fabricator back in the mid 90's.

Regardless, I still need to inspect the base to determine what the tower is like at the concrete. If it is rusty the tower will need to be removed.

The house/tower/site is owned by a nonprofit park and we were thinking of mounting a Wi-FI antenna on it to pipe public wi-fi to areas of the park.  Maybe someone in the area would donate a tower but if I start asking, probably only old TV towers will be coming out of the woodwork though.

Around the corner their is an abandoned house with a non guyed wire tower that has got to be maybe 80' high! The style is wider at the bottom and narrow at the top. I thought about asking about it but I ain't getting my fat but up it.  Grin






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W8JX
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« Reply #21 on: July 10, 2011, 05:27:10 PM »


Since the roof is metal I was trying to get the dipole higher to help the 80 meters. At its current height the 40 M is 1/4 wave above the roof but not to 80M


Generally a Dipole does not start acting like a dipole (directional wise) until it is close to at least 1/2 wave of ground. This means over 100 feet on 80.  I doubt that even a metal roof (unless it is very large) is going to have much impact on 80m. Also even a loaded and shortened 80m dipole is going to have a lot of wind load and no rotor kinda make it worse in that you cannot turn it sideways in heavy winds to lighten load. One more comment, it is not hard to make a tower self supporting to 70 ft and more with minimal wind loading. The hard part is making it being able to handle a lot of wind load at top created by bigger antennas.
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