Hi Chris --
You are not sufficiently aware of weather conditions on ridge tops of
this elevation in western Virginia.
Wind will exceed 60 mph in gusts on a regular basis. As pointed out by
K7LXC, many Virginia counties have higher wind loads specified in the EIA
standards.
Furthermore, these locations will have icing conditions every winter.
The ridge tops will be in the cloud layer for long periods during winter.
Sometimes this will cause rime ice to build up -- not too bad as rime ice
usually has a lot of air in it. Other times the clouds will be
super-chilled water droplets that freeze on contact with any object, forming
a solid layer of ice. Freezing rain storms are also an annual occurrence,
usually 1/4 inch radial ice each year but 1/2 inch will occur at least once
every 2-3 years.
You must design your antennas to account for ice load as well as wind
load. Ice load designs are typically more difficult and costly than designs
for pure winds. You can discuss with the local power company the
environmental conditions which they use on their high tension lines when
crossing the mountains in your neighborhood -- this will give you some more
input beyond the EIA standard.
You must use shock absorbers on your rotators to account for the gusty
winds (aka W6QHS), and very good brake mechanisms or worm gear drive. Your
rotator should use a spline mechanism to connect between the rotator and the
drive pipe. Without these mechanisms, you will be climbing your tower
several times each winter to re-align the antenna directions.
Even when designed properly to the standards, you should plan on a
disaster wind+ice storm every 25 years or so that will destroy all of your
antennas. These storms are too expensive to design for -- get good
insurance and be prepared to rebuild. If properly designed, your antennas
will come off the tower before the tower comes down.
Be sure to clear all trees away from both the tower and the guy wires, so
that no tree can fall on a guy or the tower.
And, if you have antennas mounted below other antennas, think about what
will happen when ice breaks off the higher antenna and falls on the lower
one.
FYI: I have had a QTH on a 1800 foot ridge top in Maryland for the last
19 years. The power utility reported a similar storm in the mid-70s. Their
design criteria is to meet both:
a) 80 mph steady winds plus gusts (+25%) with no ice.
b) 1-inch radial ice simultaneous with 60 mph steady winds plus gusts
(+25%).
My towers were guyed AB-105 (24" on a face). The best rotators had worm
gear drive and spline connections, with a W6QHS "BMW" shock mount to the
drive pipe. Prop pitch rotators will also work successfully (but you need a
good clamping mechanism to join the prop pitch to the drive pipe).
My disaster occurred around 1993: more than 1 inch radial ice from
freezing rain followed by 80 mph winds from the east. Thousands of trees
came down in the surrounding forest. At that time I had a KLM KT-34XA and a
40m 3-el delta loop on my towers. The KT-34XA simply disintegrated. Every
element half sheared off where it mounted to the boom, sometimes by breaking
off the boom-element insulator... and in other places by shearing through
the aluminum tubing. The long side of the boom ripped the long way through
ten inches of 1/4" thick aluminum mounting plate in order to fall off the
tower. The drive pipe (2.875" o.d. steel pipe, 1/4" wall), left with an
unbalanced load of half of a boom plus ice and wind, bent through a 30
degree angle at the top of the tower. The guys supporting the 40m
horizontal boom snapped, the boom folded and then ripped itself off the
tower. The towers were undamaged.
All my wire antennas are strung with rope, pulleys, and counterweights.
As the wire builds up an ice load, the counterweights rise to minimize the
added horizontal stress on the supports. (This works until the ice load is
too thick to go through the pulleys.) Without a counterweight system, the
wire antennas will break under ice loads or will pull the towers over. (My
wires survived the big storm, except where trees fell across the antenna or
the descending coax line.)
In PVRC there is another contester on a similar ridge in northwest
Virginia. Repairs to his yagis due to ice damage are an annual problem,
especially on 40m. He has had no tower problems, but his tower is guyed and
48" on a face (200 ft high).
These are great radio locations but you must be prepared for them. This
is NOT southern California. If your existing yagis have not been designed
for this environment, they WILL fail within the first year or two.
-- Eric K3NA
-----Original Message-----
From: owner-towertalk@contesting.com
[mailto:owner-towertalk@contesting.com]On Behalf Of kg6ar@pacbell.net
Sent: 1999 November 23 Tuesday 03:04
To: towertalk@contesting.com
Subject: [TowerTalk] TOWER SELECTION
Gentlemen I need help:
I will presently be installing a new tower installitation in Western
Virginia on a ridge that is approximately 2200 ft in elevation. I will
have a wind factor of approximately 60 mph tops in this area. The area
in prone to heavy lighting storms and am considering PolyPaser
equimpent. What I would like to know if it would be feasable to use a
tower such as a heavy duty US tower or would it be better using
something in the line of a Roh 45? The tower will be in the range of 55
to 72 ft with a 2 element 40 and a force 12 tribander system? Any help
would be appreciated. Direct replys would be appreciated because I
would hate to tie up the reflector.
73 and thanks for your help
Chris KG6AR
So Calif Contest Club
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