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Author Topic: Tower extention material?  (Read 3035 times)
KB9MDQ
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« on: July 09, 2011, 10:04:40 PM »

I want to extend my 40' 3-legged 1 1/4 tube TV tower. First I am going to remove the short piece of pipe that the tv rotor mounts to. Remove the tv rotor, tv antenna, and short Radio Shack cheapo tubes that are above the rotor.

I want to extend the tower maybe another 10-20' and remount my triband antenna and top it off with a 80 meter dipole for now.

Question is. Do I extend using galvanized water pipe, black gas pipe, or metal conduit?

I am guessing the pipe is is 2".

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W8JI
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« Reply #1 on: July 10, 2011, 03:22:27 AM »

I want to extend my 40' 3-legged 1 1/4 tube TV tower. First I am going to remove the short piece of pipe that the tv rotor mounts to. Remove the tv rotor, tv antenna, and short Radio Shack cheapo tubes that are above the rotor.

I want to extend the tower maybe another 10-20' and remount my triband antenna and top it off with a 80 meter dipole for now.

Question is. Do I extend using galvanized water pipe, black gas pipe, or metal conduit?

I am guessing the pipe is is 2".



The pipes you mentioned are not good. They are the wrong types. 15-20 feet is out of question with any type of pipe or mast on a TV type tower. Before getting into the pipes or mast or mast size here are a few very important questions:

What type of tower is it?

Is it guyed or bracketed, and where and how?

How big is your tribander?

What type of rotator?

Where do you want to put the tribander, how high on the mast? Did you want to put the dipole above the tribander, and if you did how do you plan on not having the dipole snag up under the tribander??


73 Tom
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W5GNB
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« Reply #2 on: July 10, 2011, 05:38:17 AM »

if I were in your situation, I would abandon the TV Tower and put up something like a Rohn 25 with proper base and guywires.  At 40ft, your already streaching the limits wth that TV tower and adding any antennas simply intensifies the danger of a disaster.

73's
Gary - W5GNB
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G3RZP
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« Reply #3 on: July 10, 2011, 05:44:22 AM »

I agree with W5GNB and W8JI. Towers are one place it does not do to skimp on things.
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W8JX
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« Reply #4 on: July 10, 2011, 06:18:34 AM »

I agree with W5GNB and W8JI. Towers are one place it does not do to skimp on things.

Ditto, you are pushing envelope here. Towers can be very unforgiving. If it was just for say a vertical ground plane for VHF/UHF which has low wind load and no torque you could extend it but not as panned.
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W8JI
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« Reply #5 on: July 10, 2011, 06:50:29 AM »

I once used TV tower (Amerite or American) at 130 feet, many people do, but it has to be guyed correctly and rated to handle the height and wind load. The top post of most TV towers is not especially strong, and will not normally accept much over 1.5 inch OD mast.


That's why it is important to know the type of tower.

Even with the best tower, ten feet of mast can have considerable bending moment. Gas pipe, steel water pipe, and conduit are not very strong.
That's why it is necessary to known the antenna types.

 
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KB9MDQ
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« Reply #6 on: July 10, 2011, 08:02:38 AM »

This is not my house. It is provided w/ my job. The tower came w/ the house. I am short on cash and love the hinged design. (I am not a climber).

Since I am the director I was nice enough to let myself modify the tower as shown:

Hinged:http://www.kconlineinc.com/potawatomi/images/Hindge.JPG
The tower is mounted at the base and at the peak of the house. Note the triangle bracket on top of the tower on the part of the tower that hinges. If you look closley you will see that a cable runs from the top of the tower down to the base of the hinged tower in a triangle to strengthen the tower. Tower has been up since '95.

Here's a look at the entire tower bent over: http://www.kconlineinc.com/potawatomi/images/tower-side.JPG

Here's the view of the mast. http://www.kconlineinc.com/potawatomi/images/top.JPG
To clarify, the tribander is only a 2mm/440/6 meter stick. rotor, et al comes off. Because it is a TV tower I will never be able to put a rotor on the tower for ham radio. I am just wanting to get a dipole higher and prefer the tower vs. the adjacent trees for convenience and shorter distance to shack in basement.

BTW, the tower is not guyed. Yes as it originally stood before modified it had a slight bend to the east. I reversed it 180 deg and after redesigning it and especially adding the angle cable it hasn't budged since '95 despite rare 80-90+ mph micro downburst winds that took some trees out in recent years. . It appears to be quite sturdy.

Dipole will be mounted above the stick antenna (both on the new pipe. No pulley since the antenna swivels.

I can also visit the local fabricator I frequent for pipe if that works better. Since it is only a dipole I was thinking basic pipe would work even if I am limited to 10' extension.
« Last Edit: July 10, 2011, 08:09:42 AM by KB9MDQ » Logged
W8JI
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« Reply #7 on: July 10, 2011, 09:37:07 AM »

By tribander you mean a small three band vertical. That's less wind load than a TV antenna.

The tower you have is weaker than many other types of TV tower. It looks like it has the end-welded braces, not the wrap around braces, so that is the weak point (assuming it is 16 gauge).

I would not go 20 feet above it. I would keep it at ten feet and just use heavy duty TV type masting or heavy duty chain link fence top rail. The entire thing is marginal, and I have no idea how sound the hinge is, but maybe you will get lucky. I sure wouldn't put more than a dipole and that three band VHF UHF vertical on it.

If it were mine and I was forced to use it, I would not install a heavy mast or anything longer than ten feet. I would prefer the mast to fold over first rather than the tower. :-)

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W8JX
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« Reply #8 on: July 10, 2011, 10:46:26 AM »

Looks like a old Rohn 20 tower. I help a guy put up 125 feet of it once with a big heavy stationmaster vertical at top for a repeater. We guyed it every 30 feet and it worked out fine and went through several storms too. He could safely go higher  "IF" is was guyed.
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AA4HA
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« Reply #9 on: July 10, 2011, 11:34:32 AM »

Normal mast extensions are available up to 10' long. I have special ordered a 20' mast section at one time, it was expensive and very heavy. The wall thickness was around Schedule 40.

The amount of stress it placed upon the tower at the point of mounting was quite significant. That was on a Rohn 65 (much bigger tower than what you are using). and the antenna was only a 16 dBi, 900 MHz grid-parabolic. I would not have tried doing the same thing with a tower like a Rohn 25, much less a 20 (or no-name brand).

That mast section is going to act like a giant lever on the tower section and even the slightest amount of wind is going to multiply the forces on the tower.

TV type towers are designed for a limited amount of loading and even actions like house bracketing (that it should be already) or guying is only going to go so far with a light duty tower. One of the effects you will not be able to fight with guying are the torsional (rotational) forces on the structure. I have seen towers corkscrew themselves into the ground when the vertical forces are no longer "vertical" on the legs and the cross-braces give away.

Somewhere around here I have a tower loading calculator that Andrew used to make available for a download. You still may find it but I do not think it even had a table entry for such a light tower.

Tisha Hayes
AA4HA
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
K6AER
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« Reply #10 on: July 10, 2011, 11:52:33 AM »

Tom and John have told you about every possible that what you are planning is at best a disaster waiting to happen. I know you are on a budget but if you plan on climbing this modified TV tower you are going to be writing checks you can't cash.

New TV towers are at best marginal and the tower dynamics of a used tower can be much reduced from rust, metal fatigue and other factors. Many new hams forget you will want to add more antennas and you will fall into the trap of it has not fallen down so far and one more antenna  should not be a problem. 

Wind pressure on the tower rating goes up by the square of the wind speed. Add to that the gust factor and a tower that stays up at 50 MPH will topple at 70 MPH. I have been climbing towers and erecting them for 40 years and when gravity is involved you only need to make one mistake.

I highly suggest you find a local ham with large towers to advise you on your first tower project.
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AD4U
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« Reply #11 on: July 10, 2011, 12:55:56 PM »

NONE of the material you listed is rated for structural use.  They are designed to hold water, gas, or wires.

Dick AD4U (PE)
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KG4RUL
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« Reply #12 on: July 10, 2011, 01:03:22 PM »

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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KI4SDY
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« Reply #13 on: July 10, 2011, 01:12:15 PM »

It is the unknown age of the TV tower and the mounting brackets that scare me. What about the base? Is it in concrete or just stuck in the ground? If it has no concrete base, I would dig it up for inspection first and if good, install one. If it is in concrete, how much of the metal is really left? Check the concrete for any rust stains and base material spalling where the tower tubes go in. That is a sure sign of in the ground structural problems!  A real prescription for disaster! Shocked

In consideration of all the above cautions, I would remove the old rotator, find a good 8 foot mast and install guying to what you have now. I would not put an H/F beam on that tower! You can add some sloper dipoles that double as extra guys, but I wouldn't go above the top of the the tower, itself.  Undecided

A better idea is to use those tall trees in the background for your 80 meter dipole. You need as much height as you can get anyway.  Wink

This probably should have been listed under the "Tower Talk" string.  Smiley
« Last Edit: July 10, 2011, 02:57:17 PM by KI4SDY » Logged
W8JI
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« Reply #14 on: July 10, 2011, 02:07:23 PM »

Looks like a old Rohn 20 tower. I help a guy put up 125 feet of it once with a big heavy stationmaster vertical at top for a repeater. We guyed it every 30 feet and it worked out fine and went through several storms too. He could safely go higher  "IF" is was guyed.

That tower is very clearly NOT Rohn 20.

Rohn 20 is a zig zag brace that looks identical to Rohn 25 G except it has thinner brace rod, thinner wall tube, and seven horizontal zags per ten foot instead of 8 like 25G.

What he has is a cheaper crimped tube brace TV tower. Dozens of companies made that stuff.

Again, Rohn 20 looks identical to 25G except for zig zag rod size and the number of zags per section.

73 Tom

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