The first question to ask is how many antennas you plan
to put on the 100 ft R25 tower and where you would like
to place them.
K7NV has done some analysis of tower guying and reported
that increasing the size of the TOP guy to the next larger
size is beneficial when loading the tower to the MAX.
Going from 3 to 4 sets of guys is also beneficial, keeping in
mind that this will affect where you can place other (lower)
antennas. I would stick with 3/16 inch cable for the lower
guy levels.
Simple trigonometry will tell you how much benefit there is
in moving the guy anchors out further than 80%. I would
not expect much improvement going from 80% (51deg)
to 90% (48 deg).
Note that R25 is rated for 18, 960 LBS maximum vertical
load on the bottom section and 5310 LBS allowable tension
per leg. (Note that ratio exceeds 3:1....interesting!)
This from my 1978 catalog.
de Tom N4KG
On Fri, 24 Dec 1999 17:33:16 EST KI7WX@aol.com writes:
>
> In a message dated 12/24/99 13:27:29, you wrote:
>
> >Is the tower 25G or 45G? I don't think I'd do it for 25G - the legs >
> >don't have the leg capacity to take 67% more guy tension preload.
> This is my >'non-engineer' perspective.
>
> I'm not sure that's quite right Steve. For a 190' 25G tower my 1992
> Rohn
> catalog shows six guys per leg. If the bottom section of a 190 foot
> tower
> can support six guy lines plus the additional weight of an extra 90
> feet of
> tower weight it's hard to see why 5 guys on 100 feet would be a
> problem.
>
> What additional guywires gains us in additional capacity/safety
> would be nice
> to know. I've considered guying my (planned) 100 footer in four
> places and
> using 1/4" EHS instead of the suggested 3/16". My logic is that the >
> additional guywire will help the tower resist twisting and the
> thicker
> guywire is less elastic which whould help control bending. The
> added cost is
> a fraction of a 100' tower project so it seems a good idea. None of
> this
> will stop a hurricane wind, but's it is a bit of over engineering
> which seems
> helpful.
>
> I'd be interested on additional input to this whole line of thought.
>
> Particularly from people that could model some hypothetical towers
> and show
> why different things are good or bad. A good example (well for
> me...) would
> be to compare a 100' 25G tower built to Rohn specs vs the same
> height tower
> guyed at the top with four equally spaced guy wires of 1/4" EHS.
> I'd also be
> interested in the difference between the suggested 80% anchor to
> tower
> distance compared to 85 or 90%. Logic suggests that an additional
> guywire
> per leg (4 vs 3), heavier wire and additional tower-anchor spacing
> would all
> be good ideas.
>
> In anycase it's a real shame that the "data" we all have to work
> with is
> either the not always applicable Rohn Catalog specs or anecdotal
> evidence
> from other projects. We have easy access to great tools like NEC
> for
> modelling antennas and horrible tools for figuring out how to hold
> them up in the air!
>
> Cheers,
>
> Mark
>
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