[TowerTalk] Re: Sidemounts

Mon, 10 Apr 2000 14:50:59 -0700

Hi Tommy,

N4ZZ is kind of right . . .  I have some sidemounted antennas on 25G.  I also
market a sidemount kit that will fit 25G or 45G.  It is explained and shown with
photos on my now inoperative web page (sorry about that).  The web site is
supposed to be at <www.reprise.com> but it has not been working for 3 weeks
now.  The Web Master and site owner (Bill Den Beste) is working hard on bringing
it back up.

Now, about this question of how much you can safely put on a 25G tower.  This is
a rather complex problem in mechanical engineering and there simply are no
simple answers.  Steve, K7LXC, takes a rather "safe" and simple position by
stating you should "always do what the manufacturer says", but the truth is that
the manufacturer does not say it all.  In this case, Rohn is the manufacturer
and they give several examples of guyed towers in thier catalog but these
examples are by no means the ONLY safe configurations for these towers.  If you
can put a tower up EXACTLY like the examples shown in the Rohn Catalog, you
will, no doubt, have a safe installation.  Few of us can actually put up (or
would even want) an installation EXACTLY like the examples shown in the catalog.

So, most of us want to deviate some from the Rohn examples, like sidemounting
antennas down the tower and putting them on swinging gate sidemounts.  Another
real popular idea is to use more guys, closer spaced, and heavier guy wire than
is shown in the Rohn books, and then load the tower with more square feet of
antennas.  All of these things can be done safely.  The trick is in knowing how
to do the engineering caluclations so you will KNOW that what you are doing is,
indeed, safe.  Rohn offers a special service to do such calculations for you but
the cost is out of reach for most of us with average incomes.  In a Rohn Catalog
that I have dated January 1, 1997, Rohn offers the following "Product Services":

Product Review of Standard Catalog Towers (for other than catalog antenna
loads)  $1000 per tower.

Product Review of Non-Catalog Towers with Simaltaneous Product Review for
Optional Loads.  $5000 (minimum) per tower Plus $300 per hour.

There are several other "Services" they offer such as special foundation
drawings and calculations for more money than you even want to know about.

So, you can get the answers you need from Rohn.  All you need to do is give them
money . . . probably more money than you planned on spending on the entire
project.

There are people other than Rohn that can give you the answers, too.  They are
called Professional Engineers and KR7X is a good one that I know personally.
KR7X (Hank) often gives free advice right here on towertalk, but since this is
how he makes his living, I would hesitate to ask for much from him for free.
After all, he has a rather large investment in his engineering degree plus some
liability insurance premiums to cover every month since he stands behind his
work.

This brings up another way to get it done:  Take off fours years and get your
own degree in engineering . . . !  This may sound like a smart-alec answer, but
it is not meant to be.  It really TAKES a full engineering education to do this
right if you are going deviate at all from the Rohn Catalog examples.  There is
a REASON that Rohn seems to charge a lot for doing calculations . . .

Or, you can do what I have done . . . spend 45 years looking at what others have
done, note what failed and why it failed, and avoid doing that on your own
towers.  That gets you PART WAY there.  You also need to have a very good
intuitive feel for things mechanical and avoid making your own original stupid
mistakes even if you have never seen anything like your ideas in practice
before.  You just have to be able to recognize a bad idea on the drawing board
and be smart enough to not implement it in the first place . . .  This is what
the formal education will help you do, plus you will learn how to prove good
ideas (or disprove bad ideas) with valid engineering CALCULATIONS (something I
can't really do).  Some ideas that seem to pass "common sense" are really
disasters waiting to happen . . . which is why there is NO SUBSTITUTE for either
having an engineering degree yourself or hiring someone who does have one to do
the calulations for you.  Anything else is risky and the degree of risk depends
on just how wild and crazy your ideas are . . .

I know this did not help you much but, frankly, I am not qualified to help you
in this area.  You really need a Professional Engineer.

Stan  w7ni@teleport.com

PS     The largest antenna I have sidemounted is a 4 element 15 meter beam on
about a 20 foot boom.  There is a 4 element 10 meter beam about 5 feet above it
on the same sidemount.  I don't know what you mean by a "large" antenna.

Tommy Tow wrote:

> Hi Stan, a friend of mine, N4ZZ said he thought he remembered that you had
> some experience with towers and sidemounts.  We thought we would contact you
> and see if our info was correct. I am wanting to sidemount a large yagi on
> Rohn 25 and have serious doubts about the structural integrity of the
> proposed installation. We were looking for someone with some hands on
> experience with these issues.  Was his memory correct??  Thanks, Tommy WD4K

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