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Author Topic: Towers, Verticals, and Masts, oh my!!!  (Read 2069 times)
KG4WXP
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Posts: 165




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« on: August 18, 2012, 08:36:50 AM »

Here's a question..... as I've never messed with towers before.

Say that there is a small 15 foot tall tower.... nicely installed....

How much mast would be ideal (and safe) if I was wanting to put up a 2m/70cm vertical, via the tower? (As far as how much mast could be above the tower before it would be unsafe)

The vertical is about 8'4" tall.


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W9GB
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Posts: 2616




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« Reply #1 on: August 18, 2012, 02:03:05 PM »

5 foot mast is common for "off-air" TV and FM radio antennas with roof tripods or small towers.

IF you could go to a 10 foot mast .. Unknown.  
Too many unknowns about your situation, to make any creditable recommendation.

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K3VAT
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Posts: 708




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« Reply #2 on: August 18, 2012, 02:57:39 PM »

Quote from:  link=topic=84814.msg615751#msg615751 date=1345304210
Here's a question..... as I've never messed with towers before.

Say that there is a small 15 foot tall tower.... nicely installed....

How much mast would be ideal (and safe) if I was wanting to put up a 2m/70cm vertical, via the tower? (As far as how much mast could be above the tower before it would be unsafe)

The vertical is about 8'4" tall.

There was a thread on the calculation of forces on antennas/masts/towers and a number of parameters that you could vary within the calculator.  Might want to search for that as it had lots of good info.

GL, 73, Rich, K3VAT
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WB6BYU
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Posts: 13171




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« Reply #3 on: August 18, 2012, 04:47:04 PM »

Several things you need to specify:

1) the type of tower, whether it was designed to be guyed or not, and the size
of the base.  The stresses are quite different on a guyed tower vs. unguyed

2) the size, material, and wall thickness of the mast, and whether it is spliced.

3) The expected wind speed you want it to survive.

4) How you plan to install the mast and antenna.  I say this because, while a
properly guyed TV push-up mast can support a triband yagi (at least temporarily),
both times I've seen crews attempt it on Field Day the mast buckled.  That was
because they tried to extend the mast first and tip the whole thing up into place
with the antenna on top, rather than tipping it up first and then extending it once
it was vertical.  The act of installing a mast + antenna can often put more stress
on the mast than it will encounter once it is up.


Our local ARES group has a similar antenna up on a 15' mast, with no problems
so far.  But the mast is a piece of high-strength steel tubing that was salvaged
from a collapsed greenhouse rather than a flimsy piece of rolled #14 steel.  In that
case the weakest link is the base of the tower it is mounted on.
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N4CR
Member

Posts: 1665




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« Reply #4 on: August 18, 2012, 08:39:31 PM »

If wind survivability is no issue then you can put a 10 foot mast under your vertical. Down forces aren't really stressful for an installation like yours. However, side forces (like high wind) can apply considerable leverage and to understand how much of that you can withstand would require someone with appropriate knowledge to inspect what you have.

Is it a free standing tower? Is it bracketed to a house? Is it guyed? Is it old and rusty? Is it a stamped leg tower or tubular? Horizontal braced or triangular? So much unsupplied information that it's impossible to give you a correct answer.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
K3VAT
Member

Posts: 708




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« Reply #5 on: August 19, 2012, 02:07:41 PM »

Several things you need to specify: ...

Dale, thanks.  Since you started the 'list' - can you endorse my recommendation to use, for instructional purposes, the WD9P mast calculator?  http://www.math.niu.edu/KARC/mast/  I've mentioned this a number of times in this forum and no one has ever commented.  How are folks learning the mechanics of their tower systems and how wind speed, mast material (strength), rotor placement, antenna placement, etc all affect the system?  IMHO this is the most instructional tool to understand forces on one's tower system.  73, Rich, K3VAT
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WB6BYU
Member

Posts: 13171




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« Reply #6 on: August 19, 2012, 04:12:10 PM »

That looks like a useful page page.  It does give a good start, though it doesn't handle
aluminum hollowbar, and some hams may not know what to enter in a few fields.

I think it is important to be clear that it uses "mast" to mean the portion of the installation
between the top of the tower and the antennas, while the same term can be used to mean
the whole support structure when the "tower" is round.

For example, I designed some pipe masts using design equations published years ago in
ham radio magazine.  Even though the requirement was that they withstand 100 pounds
sideways pull once erected, it turned out that the stress of tilting them up into place
put more force on the mast than the top load once installed - and that was without any
antenna or rotator.
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K5KNE
Member

Posts: 65




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« Reply #7 on: August 22, 2012, 11:30:13 PM »

Just find a short piece of pipe that will work - say 5 sticking out of the top of the tower.

With 15' tower and a 5' of pipe the antenna should work fine for local contacts.  5' or so more would not make a material difference in the coverage.

You would have to get the antenna up to about 30' to make a little difference.

Do mobiles in your area work the repeaters you want to work?  If so, why even bother with a little tower? Just use a slip up tv antenna mast hooked to the house.

Walter  K5KNE
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