Steve,
When you find the article, please let us know the issue date. I'd like to
see if the way I did it bears any resemblance to the right way.
I developed my own method for calculating anchor placement and guy lengths
for the 110' Rohn 55 tower I put up on my steep hill, which has a difference
in elevation of 30 feet from the uphill guy anchor to the downhill guy
anchor. In addition to the need for different length guys and anchor
placement distances, the task was complicated by the fact that the tower
would sport a stack of three 4-el SteppIRs on TIC rings, with a combined
turning radius of 24 feet. The top SteppIR would be above the top guy, so it
wasn't a factor. Drawing a diagram of the tower, turning radii and guys on
graph paper showed that there would be plenty of clearance for the bottom
SteppIR. But middle SteppIR would come very close to the top guys, so it was
important to ensure that the angle of the top guy provided enough clearance
to accommodate the turning radius of the middle SteppIR.
I started with premise that the guy attachment points would be at the
Rohn-specified heights of 95', 63' and 33'. Ah, you say, these are the
attachment points for a 100' tower, not a 110' tower! True. But my design
called for an extra 10 feet of tower to be used in lieu of a 10' mast. In
other words, the top antenna, a full-size 2-el 40m beam, would be installed
a few inches above the top plate of the top section, supported by a short
aluminum mast (2" dia, 1/4" wall.) I felt this was comparable to a 100'
tower with a 10' mast, so the guy points could be the same. I believe the
bending moment on the tower is the same, though the extra section presents
more windload than a mast. (Note: For added safety, I used 4D anchor blocks
specified by Rohn for a 110' tower at 110 MPH wind speed.) Another
consideration was that I wanted to mount the SteppIRs just above the guy
points, and HFTA liked 96'/64'/34' for the stack.
With the guy attachment points chosen, I drew a diagram of the tower on
graph paper, with the three SteppIR turning radii (+1 foot for each TIC
ring) shown at their intended heights of 96', 64' and 34'. I then drew in
the guys, allowing for five feet of clearance for the middle SteppIR. This
resulted in an anchor distance of 92 feet over flat ground, just a little
more than the Rohn spec of 88 feet. Given the height of the tower and the
distance to the anchor points, I was able to use the Pythagorean theorem to
compute the angles of the guys at their tower attachement points.
The idea was to ensure that the angle of the guys to the tower, when placed
on my hill, would be identical to the angle of the guys over flat ground.
For various engineering and cosmetic reasons, one guy had to be uphill of
the tower, one guy had to be downhill of the tower, and the third guy had to
be at about the same level as the tower. I used a transit level to determine
the elevation differences in each of the three directions out to a distance
well beyond where I estimated the anchors would wind up. This required
multiple measurements for the uphill and downhill guys because the transit
level's measuring rod is only about 8 feet high, which is less than the
difference in elevation from the tower to each anchor.
Once the terrain profile was known, I used the target guy-to-tower angle to
compute the length of each guy and the distance of its anchor point from the
base of the tower.
Finally, I drew the tower with the computed guy lengths for each direction
on graph paper, and visually determined the lengths of Phyllistran and EHS
leaders needed to ensure that the Phyllistran was at least 10' above the
ground.
The computations are in an Excel spreadsheet, and I've scanned the graph
paper diagrams into JPEG files (about 1 MB each.) I'll be happy to email
these to anyone who needs them.
DISCLAIMER: I'm neither an engineer nor a mathematician. Follow my procedure
at your own risk.
73, Dick WC1M
> -----Original Message-----
> From: k7lxc@aol.com [mailto:k7lxc@aol.com]
> Sent: Wednesday, January 13, 2010 1:13 PM
> To: towertalk@contesting.com
> Subject: [TowerTalk] Guy article in CQ
>
> Howdy, TowerTalkians --
>
> Several years ago there was an article in CQ about dealing with
> varying terrain and different lengths of guys that used a bunch of trig
to
> get
> everything calc'ed out. I'd like to get a copy of the article. If you
could
> possibly pass one along to me I'd really appreciate it. Tnx!
>
> Cheers,
> Steve K7LXC
> Champion Radio Products
> Cell: 206-890-4188
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