>Hey Folks,
>
>I am in seriously contemplating putting up a tower here at my QTH in
>Florida, and was wondering were I could get information on how to make wind
>survival versus tower size tradeoffs, guy selection, etc. I am considering a
>60 to 70 foot guyed tower with a something like a Force-12 C-3 Tribander
>stacked under a Cushcraft 402CD. Being in Hurricane country, I would like to
>be able to do wind survival comparisons between Rohn 25, 45 and 55 to see if
>the heavier tower sections are worth the added expense. I just got a copy of
>Dave Leeson's book "Physical Design of Yagi Antenna's", from what I have
>seen so far, an excellent book, but it seems focus mostly on antenna and
>mast design - no too much about towers. No sense putting up a antenna and
>mast rated for 120 mph if the tower is going to buckle at 95 - HI HI.
>
>Please respond direct to me at mtope@iu.net. I'll summarize and post the
>responses on the reflector.
>
>Many thanks!
>
>73 de Mike, AD4VH
Hi Mike,
I can certainly sympathize with your desire to know how to correctly design
your tower installation so it won't fail. We had quite a thorough
discussion here a couple of months ago where it was pretty much concluded
that all of the engineering design expertise needed to do this job right
could be put into a software program that could be used by the average ham
much the same way as antenna design software is used today. The problem is
that anyone who writes such a program is assuming a massive amount of
liability in the event that one of the tower systems designed using the
program fails. When a tower comes down, real damage can occur to people and
property. If an antenna blows apart, the pieces are generally light enough
that nothing gets hurt (except the antenna!)
It turns out that there actually IS a place where you can get the
information you seek. The Rohn tower company offers various services to
their customers regarding evaluating designs using their towers. Here are a
couple of examples of the services they provide and what they typically
charge for them:
Product review of Standard Catalog Towers (for other than catalog antenna
loads): $1000 per tower.
I believe this is the service you are asking for. What you need to do is
get ahold of a copy of the "Rohn Ham Tower Catalog", determine the square
footage of the antennas you plan to use, and see how your installation
compares to those published in the Rohn catalog. Once difference I noticed
is that Rohn never talks about stacking any antennas on a mast above the
tower. Their loads are always shown to be at one place -- right at the top
of the tower. $1000 may seem like a lot to pay for this information, but it
just reflects the cost of Rohn's liability insurance which they MUST have if
they are going to pass out this kind of information.
Here is another service you might need if your city requires it before
issuing a building permit:
Foundation Drawings and Calculations (Soil analysis by others) (Anchor
Blocks, Base Piers, Pier & Pad, Caisson, Mat): $1,500 per type.
I am not aware of any ham who has ever taken advantage of this service from
Rohn. It just costs too much. Besides, I suspect you won't really be able
to get a good picture of the "tradeoffs" without having Rohn submit a bunch
of different designs for $1000 EACH . . . Oh yes, the above prices are
DEALER PRICES and you will have to pay considerably more unless you are a
Rohn dealer . . .
Probably the real way to get this done is to cultivate a personal friendship
with a qualified structural engineer and offer to swap him an equally
valuable service if he will design your tower. You could give him a free
heart transplant, for example, if you were a cardiovascular surgeon . . . or
perhaps a new Ferrari if you are a car dealer . . .
Stan W7NI@teleport.com
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