Steve,
Thanks for your excellent reply! That's just the kind of info I've been
looking for.
What is TOWER TECH's address? I'd like to get a copy of that article as
soon as I can.
> First of all, Grafton County is only a 70 MPH wind zone but it is
within
> 100 miles of a hurricane oceanline and it sounds like you have a spot
with
> higher wind exposure and potential wind speeds so you might want to
consider
> 80 MPH as your system planning wind speed.
Yes, but a mitigating factor here is that the tower will spend virtually
all of its time in the down position (22' high with all four sections fully
nested). In the down position, it will be shielded by thick tall trees less
than 50' away on three sides (the trees on the fourth side are 500 feet
away). I could plant some trees for further shielding on the fourth side,
which gets windy during the famous New England Nor'easters. That would
provide more visual screening as well (in about 10 years...) It's not a
bare exposed hill here, and the elevation is only about 700 feet. The worst
exposure will occur when the tower is raised over about 40 feet. I've lived
in this town for 25 years and can't remember ever experiencing 70 MPH winds
(no, that will happen the day after the tower goes up...) I'm sure we've
had gusts up to 50 MPH, but that would be somewhat rare. More typically, a
really big wind storm might have gusts to 40 MPH. However, I wouldn't tempt
fate. My intention is to check wind speeds *before* raising the tower for
use.
Still, your comments do make me wonder if the 55'/TH5 combo would be more
prudent. I could probably be a lot less concerned about checking the wind
conditions, unless there was a big storm in the area. It's really hard to
assess the differences in gain between the two options. My guess is that
the TH5Mk2 at 55' will give me at least 80% of the performance of the
KT-34XA at 71'. What does that mean? Well, I doubt that I would miss any
truely rare DX because of an inadequate antenna (at this point, the 20 QSOs
I need for Honor Roll are all very rare.) After all, I snagged Heard Island
early in the expedition on the multiband vertical (and 1500 watts) by using
a bit of operating skill. For contesting, however, I'm sure there would be
a noticable difference in scores. But I'm not exactly the most devoted
contester. Normally, I just do the ARRL SS, CQWW, and CQWPX, and prefer CW
anyway (although sometimes I dabble in the SSB contests.) I also have a
wife and a one-year-old daughter who like to see me, so it's often hard to
schedule a full weekend of operating for the contests. The available
operating hours are probably a much bigger limitation on my scores than the
antenna system.
One other factor to consider is the boom length. There's a 12 foot
difference between the KLM and TH5. Although the windload rating of the KLM
is less than the tower rating, I do wonder if that longer boom means that
twisting is more likely. In this case, is it possible that the top section
could actually turn inside the second section from the top (remember that
the rotator is at the bottom)?
Here's a thought -- what about the 71' tower with the TH5? The extra height
is attractive because I'm losing 20 feet of elevation at the antenna site
and I have some difficult hills in the way to the east of my house (towards
the top of the hill. That extra height could make a bigger difference than
the antenna. If I were to go that way, does the extra 15 feet or so of
height present any more wind damage risk (both towers have the same
windload rating)?
> Are you going to get a permit? A crank-up can make the process more
> difficult.
Why are crankups more difficult? Fortunately, we have pretty liberal rules
here in my town. The local zoning ordinances require special permits for
structures over 35 feet, but antenna towers are specifically exempted!
Evidently, someone with a satellite dish and a big hill in the way managed
to get the law changed about 10 years ago. I easily got a permit for a
Hy-Gain crankup I had planned to put up on another piece of property (but
never did.) A friend put up a 100 foot Rohn 45 with no squaks at all
(except for some curious looks.) Given where my antenna will be (totally
invisible to all neighbors, even when raised), I doubt there will be a
problem. Unless I get advised otherwise, I'll go for the permit because I'd
hate to have to take the thing down later.
> You'll probably need a crane to install it BTW. You can get
"positive
> pulldown" with many of the US Towers which means that they'll always
retract.
Yes, both models I'm looking at have positive pulldown. The crane could be
a *real* problem. The installation is down the hill in a spot that would be
somewhat difficult for a big vehicle to reach. As it is, cement mixing will
probably have to be done in small batches with a portable mixer. I'm
planning on getting the tower raising fixture which allows the tower to be
tilted over for maintenance. I think this will eliminate the need to put
the tower in vertically to begin with, but I'm not sure (no doubt, US
Towers will tell me.) If it can be installed horizontally, it still has to
be raised a few feet off the ground to mate with the hinge on the base (the
55' tower weights about 650 lbs; the 71' tower weight about 850 lbs.) This
is something I'll be discussing with my installer (I forgot to mention that
I'm getting help from a ham buddy who is a professional tower rigger --
he's put in a handful of the U.S. Tower crakups, and has looked over the
site -- his opinion, BTW, is that the bigger beam is OK, but I think he's
basing that solely on the windload rating -- I want to be more sure than
that.)
73, Dick, WC1M
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|