[TowerTalk] The Moment of Truth

"Dick Green". dick.green@valley.net
Tue, 24 Jun 1997 02:21:23 -0400

The Moment of Truth has arrived...

If you're a regular, you saw my earlier posts about the tower project I am
planning. To summarize in 25 words or less: It must be 70' tall, it must be
a freestanding crankup (tubular strongly preferred for appearance), it must
be motorized with remote control, and it must not fall over! (OK, that took
28 words...)

Needless to say, I want to get the biggest tribander up that I can. Since
the original posts, I've decided to put in a 40M 4-square, so that means I
can relax and forget about the need to stack a 2-el 40M beam with a
tribander. Lately, I've been asking about the KT-34XA and TH7DX, so you
know I'm talking about a big tribander (both are in the 9 - 9.5 sq. ft wind
load range.)

The only companies seriously in the running are U.S. Towers and Tri-ex. To
my great disappointment, I have learned a thing or two on this reflector
about windload specs and tower manufacturers. To summarize, although U.S.
Towers advertises that their tubulars will handle 10 sq. ft. of antenna
wind load at 50 MPH, their engineering specs clearly show it dropping to
1.6 sq. ft. at 70 MPH! Sounds to me like the tower is virtually guaranteed
to collapse at windspeeds over 50 MPH with any commercial tribander on the

OK, so get one of their triangulars or one of the Tri-ex models. Once
again, U.S. Towers specs their triangulars for 50 MPH. I don't have the
engineering package in hand, but I shudder to think what it shows. On the
other hand, the only reports I got of towers collapsing, and there were
several, were from Tri-ex LM-470 owners. Yeah, they might spec the towers
for 70 MPH, but why did I hear about so many collapsing when I got *no*
reports of such from U.S. Towers tubular owners (who, like their Tri-ex
counterparts, have a strong tendency to overload their towers, making my
plans look tame by comparison)? Besides, base construction for the
triangulars is more difficult (more concrete to hand mix and pour in a
difficult-to-access spot), I don't like triangular crankups, and my wife
won't either (you should have heard what she said when I told her about the
250' trench we're going to dig for the AC, RF, and control line conduits!)

So! My conclusion is: If you're going to own a crankup, it's got to come
down when the wind starts a blowin'. No problem! I can whip up a wind speed
alarm and maybe even an automatic lowering circuit (seems like all the
Tri-ex owners are doing that.) It might even be possible to keep operating
in gusty conditions if I lower the tower to 50' (more section overlap
should result in greater structural strength, right? -- Does anyone know
how much more?)

As an aside, I'm pretty impressed with the specs on the Tri-ex 70' tubular
crankup (Sky Needle)-- bigger diameter sections with walls twice as thick
-- but fully loaded it costs about $12,000! That's $5,000 more than the
U.S. Towers fully decked out (and weighs twice as much -- 2,000 lb.) For
that kind of money, I could practically buy a new U.S. Towers tubular if
the first one falls over!

So! The Moment of Truth has arrived! Decision time! I must commit to
spending the big bucks very soon. The antenna of choice for me is the
KT-34XA. The tower of choice is the U.S. Towers 72' tubular (even with it's
puny windload rating, I'm impressed with the user reports -- I can't say
that about Tri-ex.) The question is, am I crazy? Should I do this? Am I a
fool to rely on the tower being lowered 80% of the time and always lowered
when the winds kick up? Or should I get sane and just put up a KT-34A (50%
less windload -- 6 sq. ft -- and about half the weight)? Given the severe
derating at 70 MPH, does it really matter which beam I use? If the tower
will fall over in a 50+ MPH wind with either beam, shouldn't I just enjoy
myself while it's up and get the big one?

(See my next post, "KT-34A")

Thanks for putting up with all my questions --- I really am a worrier! Then
again, yesterday we had a sudden freak storm that bent some of the tall
trees on my property almost in half (an apple tree with a 4.5 inch trunk
got sheared off right at the ground level.) $7,000+ is a lot of money to
fall over in a scrap heap when something like that happens (more like
$9,000 if the beam and rotator get creamed...)

73, Dick, WC1M

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