[Fwd: Re: [TowerTalk] Rohn FK 25 & KT34XA]
Stan or Patricia Griffiths
w7ni@teleport.com
Mon, 12 Jun 2000 23:48:35 -0700
Jim Nipper wrote:
> Subject:
>
> Message-ID: <39418C5A.4DA2@southeast.net>
> Date: Fri, 09 Jun 2000 20:31:22 -0400
> From: Jim Nipper <jnipper@southeast.net>
> Reply-To: jnipper@southeast.net
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> To: Bill Aycock <baycock@HIWAAY.NET>
> CC: Tower Talk <towertalk@contesting.com>
> Subject: Re: [TowerTalk] Rohn FK 25 & KT34XA
> References: <3.0.5.32.20000608200504.00a48b20@HIWAAY.NET>
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> It is correct the the Rohn book says that their (prior) 68 foot foldover
> should be guyed at 25 feet from the base of the tower, using four guys
> at each level. This guying distance is about 35% of the tower height.
>
> I wonder if one could go up to 88 feet, on a regular Rohn 25 tower, and
> guy at 35 feet from the base, using four guys?? This would be about 40 %
> of the tower height.
>
> The antenna would be about 12 square feet.
>
> I also have a related question. Do many of you fellows still use those
> antenna climbing masts made by ILX (I think that is the name). This
> ladder mast is abt 12 feet long I think, and of course the tower is
> normally guyed at the very top of the top section. It has been said
> here that the tower should not be guyed at any place other than the top
> of the top section. But what is the difference between having a tower
> with a ladder mast sticking 12 feet above the top of the tower, compared
> with guying a tower at the BOTTOM of the top tower section ?? Would not
> that top section be the same as the ladder mast ??
>
> I do know that the first answer is to ask an engineer. I agree. But I
> want the second answer, hi !!!
>
> 73 and tks to all !!!
>
> de Jim K4PYT
>
Hi Jim,
There are a LOT of ways to guy a Rohn tower safely. Rohn's catalog shows a few
good examples. All of Rohn's examples have been checked by Rohn engineers as
being safe. If I had any doubts, I would definitely consult an engineer, and I
know this is not what you wanted to hear. There is a great deal of risk in doing
anything else . . .
By the way, I just got a new batch of "Consumer Catalogs" from Rohn and there is
a new section in it I have never seen before called "Mounts". In the Mounts
section on page MS-62 (yes, they number the pages now), is a very interesting
series of guying diagrams for "#25 Guyed TVRO Towers". You really need to get a
copy of this catalog and look for yourself but the bottom line is this: These
towers use the normal 3 guy wires per level BUT the guy anchors are only 50% of
the tower height from the base of the tower. A note with these diagrams says
they are for 70 MPH winds but the square footage is HUGE . . . "maximum effective
projected area of 48.9 square feet" and apparently, this load rating is for all
of the towers shown on that page which vary from 30 to 80 feet. The 80 footer
has two set of guys, one at a height of 40 feet and the other at 80 feet, anchors
40 feet from the base. Rohn calls these "TVRO" towers (TVRO means "televsion,
receive only") but how can it matter what you intend to use the antenna for as
long as you don't exceed the design load? It says right there that it will hold
a 6 foot diameter dish which, if you simply take the formula for the area of a
circle with a 3 foot radius (pi R squared), gives you 28.86 square feet and that
does not include any of the mounting hardware for the dish. I don't know quite
what to make of all of this . . .
There are a few other pieces of interesting information supplied with these
diagrams:
They are not really new drawings. They are dated 1985 and redrawn in 1989. Even
so, I have NEVER seen them in any Rohn Catalog I have ever looked at before, and
I have looked at a lot of different Rohn Catalogs over the past 30 years or so.
There are similar guying diagrams for "#45 Guyed TVRO Towers" with a rating of 87
square feet Maximum Effective Projected Area at 70 MPH and for "#55 Guyed TVRO
Towers" with a rating of 94 square feet Maximum Effective Projected Area at 70
MPH. These towers are also guyed three directions and with anchor points at 50%
of tower height on towers up to 80 feet high.
I would be very cautious before loading my tower anywhere NEAR this much with HF
beams. Remember, Rohn said these were diagrams of "TVRO Towers" and they expect
to see dishes on top of them and not long boom Yagis.
If you stick with the "do what the manufacturer says" philosophy (which is not a
bad philosophy if you don't have a qualified engineer to tell you otherwise), you
won't put anything on these towers but TV receive antennas. But, on second
thought, where, in ANY Rohn Catalog have you ever read it was OK to put a ham HF
beam on ANY Rohn tower? The closest thing I have ever seen Rohn say about this
is where Rohn clearly warns you not to exceed a boom length of 10 feet on a BX
tower. The implication, of course, is that shorter boom antennas should be OK
for BX towers. Nothing is said by Rohn about ANY boom length for 25, 45, 55, or
65 towers that I have ever seen. They don't say anything about boom length on
these latest drawings either . . . So what conclusions can we draw from all of
this? My conclusion is that I don't have enough data to draw a conclusion. The
twisting stresses on guyed towers are a very complex engineering issue that I
don't pretend to begin to understand well enough to give any advice on.
There has been a lot said here about rotator ratings and how just specifying a
rotator in terms of how many square feet it can handle is not good enough and I
certainly agree with that. The "K" factor that K7LXC talks about is a step in
the right direction but, as Steve himself says, it is not the complete or final
answer. Well . . . , I think we need to take a similar look at towers and how
they are rated. Square footage does not tell the whole story. It does nothing
to address the issue of rotational stresses due to stopping and starting
rotators, sidemounted antennas, and winds on unbalanced loads.
I am not really offering any answers here, because I am not a qualified
engineer. I am only sharing some new information I saw yesterday in a new Rohn
book and sharing the questions that it raised in my own mind . . . Get your own
copy, read, and share your views. I will be very interested to hear them.
Stan w7ni@teleport.com
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