In a message dated 97-06-10 13:55:54 EDT, JC_Smith@designlink.com (JC Smith)
> We used the actual mast height when asked for that number. For location of
> vertical on the mast we added half the vertical's height to the mast
> (it's essentially cylinderical), so we in effect told the program we were
> attaching sometiing to a part of the mast that doesn't exist. It didn't
> but did we get the correct answer?
Here are two ways to look at it; then you can compare the results. I
don't think the vertical would be adding anything significant to to the
bending moment forces on your mast.
First, you can use the square footage of the vertical as the top
antenna. I used an R7 @ 2.25 sq.ft. at a height of 20 feet on the mast; it
gave me a bending moment of 16,176 in.lbs. at 70 MPH.
Second, you can add the vertical to the height of your 'mast'. MARC
already calculates the square footage of the mast so it would just consider
it as part of the mast load. Since there's no other 'antenna' up there, it
should give you a good indication of the additional load and bending moment.
Again using a 20 foot mast, I added the 22.5 feet of R7 for a total of 42.5
feet of 'mast'. That gave me 24,228 in.lbs. of bending moment. That's 50%
more than our first example and without any other loads on the mast.
The moral of the story? Anything's possible but I would be careful with
the whole installation so that it's reliable.
73, Steve K7LXC
TOWER TECH -- professional tower supplies for amateurs
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