Background: I've got a permit for 100' of Rohn 55 that will support a
three-stack of 4-element SteppIR yagis and a 40-2CD. The 40-2CD will be on
the mast and all three SteppIRs will be on TIC rings. HFTA shows that
90'/60'/30' spacing is best for the stack from my location. Not too long
ago, a TowerTalk poster said models he did for SteppIR showed interaction
between the 40-2CD and the 4-el SteppIR on 15m (some) and 17m (significant.)
At least 15' of separation is needed to minimize the interaction. If the top
SteppIR is at 90', then the 40-2CD should be at least five feet up the mast
(105').
Question 1 - Guy elevations: Rohn specs call for guying at 33', 63' and 95'.
The stack models fine at 96'/64'/34', so this would be a reasonable choice.
In that case, the 40-2CD would be 10' up the mast, at 110'. In both cases,
the 40-2CD will be 15 feet above the top guy. Are there any structural
advantages/disadvantages between having 10 feet of tower and five feet of
mast above the top guy versus five feet of tower and 10 feet of mast above
the top guy?
Question 2 - Mast length: In the 90'/60'/30' case, I would put eight feet of
mast above the top section (40-2CD five feet up, boom struts three feet
higher.) In the 96'/64'/34' case, I would put 13 feet of mast above the top
section (40-2CD ten feet up, boom struts three feet higher.) At the two
proposed guy placements, how much of the mast should be inside the top
section (i.e., how far down should the rotor be?) Is it best to have the
rotor close to the top guy point? If so, I would need 20' of mast in both
cases. True?
Question 3 - Guy radius: The SteppIRs need at least 25' of turning radius
(24' plus one foot for the TIC ring.) The middle SteppIR will probably not
clear the top guy if the guy radius is at the Rohn nominal 80'. If I put the
guys 85' out (slightly over the spec of 80% plus or minus 5%), then I'll get
about 3 feet of clearance. If I put the guys 90' out, then I'll get about 5
feet of clearance. How do I compute the expected sag in the guy wires?
(Note: The plan is to use phillystran guys.) Does 3 feet leave enough
margin? Does 5 feet leave enough margin?
Thanks in advance for any learned answers to these questions!
73, Dick WC1M
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