Once again, I really learned a bunch from the
replies to my query. The most "convenient" layout
would have put one face parallel to the trade
winds. Now I know thats the weak direction, I'll
reorient.
Thanks & Aloha,
Jonathan KH6X
& Helen KH6T
Herbivore Ranch
kalepa@maui.net
Jonathan,
There can be as many guying configurations as you
can think of. The most used and cheapest is
three guy anchors. The strongest resultant force
vector is in line with one of the anchors. The
weakest point (86%) is 30 degrees from any guy
anchor. One of the advantages of 4 anchors is
that the weakest point occurs in line with one of
the anchors and the strongest (140%) is exactly
between two of the four anchors. Figure the
vector sum of the forces to get the worst case
(weakest) direction, then you must always design
for the weakest direction, because the storm that
gets your antenna will come from that direction.
Up to 200 foot, I would much prefer to have the
first 4 feet of tower embedded in concrete. Lets
moisture drain out the bottom of the legs and
provides a degree of self support when you stack
the first sections to the first set of guy wires.
However: from an analysis point of view, there
is little difference. Either way you want your
tower exactly vertical.
Wendell Wyly W5FL w5fl@flash.net
--------
Hi Jonathan,
What is the height of the tower you plan over on
Maui?
I had a heck of a time getting the zoning
variance and
use permit for 55 feet here on Kauai. Hurricane
Iniki
helped me; after getting a lot of traffic off
the island
after the storm, the planning commision granted
my
permits at their first meeting following return
to nearer
normal life over here. But I am supposed to keep
it
below 30 feet during most daylight hours; my
compromise
with the neighbors. It is a tubular crank-up,
so has no
particular favored wind direction as far as the
trades go.
I have tried different parking positions for the
tribander,
but mostly just leave it aimed toward the
mainland, more
or less boom directly into the trades. It's a
Mosely, so is
heavy and plenty strong, bit element tubes, so
they don't
move or bend in the usual trades. It was new in
December,
after I got the permits following Iniki. I had a
quad up before,
of course Iniki turned that into a pretzel!
73, Jim, KH7M
On the Garden Island of Kaui
PS: KH6T was my sencond choice on the 610V;
KH7M was
third, and that is the one I got.
>From the geometry of 3 equally spaced guy wires,
it is evident that the load carried by the guy
wires
IS EXACTLY EQUAL with the wind coming in-line
with
a single guy or bi-secting a pair of guys.
The WORST CASE is if the wind is in line with a
FACE
of the tower, 30 degrees from a single guy wire,
placing
ALL of the load on that single guy (direction)
with an
additional 15% load due to the geometry.
FOUR WAY guying prevents the 15% additional load
from geometric considerations, but puts an
additional
load on the tower due to pre-tensioning. (Not to
mention
the additional cost and effort!)
de Tom - N4KG
Hi Jon. I have been seeing your updates on the
reflectors. Nice to have you
on-line. Have you been doing OK? How is Helen
doing? We miss seeing you two.
I don't know the "official" answer, but I think I
would put the one leg of
the tower to face the wind and make sure that
anchor is well set.
>Does it make a difference whether the tower base
>is embedded or on a pier pin ?
I don't think this makes much difference. But if
you do have a lot of winds,
most of the time, the pier pin is probably
better. But that's a lot more
trouble to get the first 30/40 feet installed.
Susan and I still want to come see you sometime.
Especially during a CW
Contest. HI. BTW, Susan is now K5DU.
Take care.
73, Richard
K5NA@BGA.COM
http://www.realtime.net/~k5na
Pat, I believe you have misconstrued each point I
made.
I did NOT say the tension in each guy was equal.
Let me re-phrase my comment:
If the wind comes from an angle that exactly
bisects two guy wires,
(perpendicular to a tower FACE), the tension in
those TWO guy wires
is EXACTLY EQUAL to the tension that would result
in a SINGLE guy
wire if the wind were coming in the direction of
that single guy wire.
The worst case is when the wind comes IN THE
PLANE of a tower
FACE (NOT perpendicular to the face), which is 30
degrees removed
from a guy wire. This places one guy
perpendicular to the wind
and therefore there is no load on that guy, and
the other guy is
away from the wind, also carrying no load from
the wind. The
tension in the single guy (30 degrees from the
wind) must be
15% higher than the wind force due to the 30
degree offset.
I hope this "word" picture clarifies the
situation. I do not have
graphics capability on my E-mail.
de Tom - N4KG
On Fri, 14 Feb 1997 16:14:31 -0500 (EST) Pat
Masterson <bat@grumman.com>
writes:
>
>I dont understand your thinking on this:
>
>On Fri, 14 Feb 1997, T. A. Russell wrote:
>
>> >From the geometry of 3 equally spaced guy
wires,
>> it is evident that the load carried by the guy
wires
>> IS EXACTLY EQUAL with the wind coming in-line
with
>> a single guy or bi-secting a pair of guys.
>
> The guys on the back of the tower (lee side,
down wind) will tend to
>slacken a bit as the tower leans toward them.
The cable inline to the
>wind will obviously get tighter. The load on the
cables (tension) cant
>
>ever be equal with some wind blowing.
>
>>
>> The WORST CASE is if the wind is in line with
a FACE
>> of the tower, 30 degrees from a single guy
wire, placing
>
> The face of the tower is 60 degrees from the
guy cable.
>
>> ALL of the load on that single guy (direction)
with an
>> additional 15% load due to the geometry.
>
> If the wind is on a face, then the load is
spread evenly to the 2
>guys
>attached to each edge of that face. They get
tighter, and the back guy
>
>gets looser. Where does that 15% get calculated
from?
>>
>> FOUR WAY guying prevents the 15% additional
load
>> from geometric considerations, but puts an
additional
>> load on the tower due to pre-tensioning.
>
> Yep. More guy wires will add to tower
compression.
>>
>> de Tom - N4KG
>>
>>
For big towers, a pier pin equalizes the
compressive forces on one
point. During construction, the necessity for
temporary guys is a big hassle.
For anything less than a BIG tower (yours
isn't), sticking the legs in
the concrete is fine. As long as it's plumb, the
leg forces will be within
spec.
I am not talking for the manufacturer, just
my professional opinion.
73, Steve K7LXC
TOWER TECH -- professional tower supplies and
services for amateurs
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