Hi Dick --
Rope: One of the problems with rope guys is that many kinds of rope are
quite stretchy. The purpose of guying is to keep the
aluminum tubing aligned in a column; this keeps downward forces (which can be
substantial) from collapsing the column (column
failure). To do this, any guy (rope or other material) needs to be
well-tensioned so that a sideways movement of the guy point
doesn't merely take up the slack in the guy.
The total amount of stretch in material increases as the material is made
longer; i.e., elongation under a particular load add a
certain percent to the overall length. Long pieces of rope (e.g., the 85 feet
needed to guy to the 60 ft level at a 45? angle) will
therefore stretch more when the wind loads up the antenna (or a tree limb falls
on the guys).
Consider investing in low-stretch rope for at least the longest guys, such
as those with amarid fibers. These are available at
marine supply stores or, e.g., from New England Rope company. Low-stretch
ropes are quite strong, so you will be able to use a
smaller diameter rope.
The thinner the aluminum, the more important it is to have an adequate
number of low-stretch guys.
W1KM's approach to getting enough leverage to raise the antenna is an
excellent one.
73,
-- Eric K3NA
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com]On Behalf Of Dick Green
Sent: 2003 October 6 Monday 00:08
To: Tower (Tower); YCCC (YCCC)
Subject: [TowerTalk] follow-up on 4-square conversion
Thanks to all who responded to my request for info about converting my 40m
4-square to an 80m 4-square.
I got lots of interesting ideas on extending the elements, from ultra-simple
(add a loading coil at the bottom) to fairly complex (anything to do with
irrigation tubing.) I've settled on simply extending the existing elements
with six 6-foot sections of T-6063 drawn aluminum tubing, ranging in size
from 1.125" to .375". I'll use 4-way guying in two places (say, 35' and 60')
with black Dacron rope. It will take nine guy anchors, five of them shared.
Any suggestions on the height of the guys, minimum diameter of the rope (can
I use 3/32"?), and how to attach the guys to the tubing would be welcome.
I'm considering using rivets instead of the old cross-cut and hose clamp
method to build the extensions. Of course, this means I'm not going to use
the old telescoping trick I used to erect the 40m elements. Since two of the
existing elements have been bent from flying tree branches, and all four of
them have been whipping around in the wind unguyed for six years and may be
loaded with dirt, leaves and other debris, telescoping is either impossible
or inadvisable. So, my plan is to build hinges onto the mounting posts and
raise the fully-extended elements. Some believe I can just walk them up by
myself. The elements weigh only 20 lbs, but I'll probably ask at least one
friend to help. W1KM had a novel method: place a 30' extension ladder
vertically against the post, run a rope from the top of the ladder to the
middle of the element, and tilt the ladder away from the element to the
ground!
As for the matching network, that is a more complicated question. Quadrature
feed is not optimal for an 1/8-wave spaced array, and it will take a
modified Lewallan network, or something similar, to obtain a pattern with
about 5 dB forward gain and 20+ dB F/B. There's an example of just the
network I need in the ON4UN book. The inductors look easy to build, but I'm
puzzling over where I would get the caps and how much power they have to
handle. Any help in that regard would be most appreciated.
It is also possible to use a standard quadrature feed, such as the Comtek
box. However, the pattern is not as good -- only about 4dB gain and 9-10dB
F/B. I would imagine the bandwidth would be fairly narrow as well, but I
only care about CW anyway. As winter is fast approaching, one option would
be to install a Comtek box now and work on building an optimal phasing
network over the winter. I would guess I could sell the Comtek box later for
a good percentage of what I pay for it. The good news is that both matching
methods use 1/4-wave feed lines, so I could just swap in the new network
when it's ready.
Can anyone see flaws in this plan or suggest a better one?
73, Dick WC1M
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free,
1-800-333-9041 with any questions and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|