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Author Topic: need help on math with a2 b2 c2 issue  (Read 3483 times)
AG5T
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« on: January 20, 2013, 03:47:18 PM »

If a tower base is 40 feet away from the power lines and the tower is 34 feet plus an 18 feet vertical, if the tower were to fall, would it fall into the power lines? If 52 feet is too high, what's the safest height for the tower and antenna? The power lines are probably 15 feet or higher above ground.... I was trying to use a2 b2 c2 but there are too many variables. Any help?
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K2CBI
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« Reply #1 on: January 20, 2013, 04:14:41 PM »

Tower and antenna can't exceed 42.72 feet.  So if the tower is 34 feet high, antenna can't be more than 8.72 feet.

73, Mike
K2CBI
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AA4PB
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« Reply #2 on: January 20, 2013, 04:27:08 PM »

K2CBI is correct. Draw a right triangle. The right angle is formed by the power line height (b), the distance from the base of the tower to the Earth directly under the power line is (a), and side (c) is the distance from the base of the tower to the top of the power line.

a = 40, b = 15.  so c = sq root of a squared + b squared so c = 42.72 feet.
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AJ3O
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Posts: 124




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« Reply #3 on: January 21, 2013, 03:03:38 PM »

Marty,

Although Mike is technically right, the tip of the antenna would still just barely shave or miss the lines on the way down depending on which way the wind was blowing.

Looking at the property on Google , I see the dilemma. The power lines run through the back yards of your plan.

Would you consider or would you be allowed to put it at the very front right corner of the house looking at it from the street? If not, even moving it alongside the house to line up with the peak of your roof in the side yard would give you roughly 60 feet from the power lines to the base of the tower. This would be just enough to safely miss the lines by approximately 5 feet.

I was using the tower (34 feet) + mast to  base of antenna (4 feet) + antenna (18 feet) to equal a total height of 56 feet as a rough guess on your setup.

Any how, which ever way you decide to do this, PLEASE BE SAFE ABOUT IT! There are already far too many stories on here and in the news about how some Ham had a line or tower fall into power lines and the fatalities that ensued....

Just my thoughts.

73,

Joe / AJ3O
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KB1GMX
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Posts: 709




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« Reply #4 on: January 21, 2013, 03:36:40 PM »

If the power wires are indeed 15ft then the 42.75ft number is correct if you want them to just touch.

You don't want to get even close to them.

For all practical purposes 40FT it the never exceed answer.  As the power lines are on an easement you
can't safely fall there.  This is not a math derived number as likely the lines are on the edge of property
and it's frowned upon if the tower and antenna fall on any property not yours.

That, and exceeding 40ft is shaving safety.  Shorten the tower by the amount of the antenna and mast
exceeding 40ft to stay under that height (use a 30ft tower).  Its a matter of being practical and safe.


Allison

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N6EY
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« Reply #5 on: January 23, 2013, 05:22:23 PM »

No one has taken boom/element length of Yagis into consideration either....
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________________________________
73,
Jason N6EY
________________________________
WA2TPU
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Posts: 201




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« Reply #6 on: January 23, 2013, 05:59:48 PM »

to AG5T.......Ever think about putting up a Delta loop....maybe with traps in it?? Food for thought. I'd rather be SAFE than FRIED.
Best regards and many 72/73.
Don sr. --WA2TPU --
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AJ3O
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Posts: 124




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« Reply #7 on: January 24, 2013, 02:57:12 PM »

No one has taken boom/element length of Yagis into consideration either....

He stated that he wanted to place a vertical on top of the tower. But you are correct. If he wanted a Yagi beam on top of the tower, the distance needed WOULD be greater. Unless he mounted a mini-beam tri-band antenna on a mast connected to an eave mount or a 5 foot - 6 foot roof mount tower. That may be another option.

73,

Joe / AJ3O
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