Define interaction. Do you mean that 15' is enough so that the
interaction has little impact on the 15m VSWR. Or do you mean that 15
feet is enough to reduce the interaction levels for gain, f/b and
pattern skewing to less that 1 dB. My guess is that 15' the point were
vswr is no longer impacted. I doubt 15' is enough to prevent skewing
and other performance robbing issues.
For contesting, 80% of the time this will be a non-issue.
I suspect most of your quality EU run on 15m will be generated from the
60/30 stack with the top 1 left for mults. Maybe it will be a small
issue for running JA's when you need the top or the top 2 antennas in a
Now when your not on 15m don't leave the top antenna tuned for 15m. park
it on a less offensive band.
I really think there will be a significant interaction on 10m but it
will not show up much in vswr.
Since we must make sacrifices with multi-band antenna on a single tower,
live with the small issue. I'm sure you will be able to fine tweak the
15m so the swr is ok. Don't assume rotating the 40m 90 degrees will
help either. Put up the antennas as planned and test to see if you can
live with the interactions. Every setup is different.
If you went by the effective aperture of each antenna, you would not put
the 40m on the same tower or within 50 feet.
Principal Antenna Design Engineer
DFSS Six Sigma BlackBelt
1011 Pawtucket Blvd
Lowell, MA 01853
From: firstname.lastname@example.org [mailto:email@example.com] On Behalf Of
Dick Green WC1M
Sent: Monday, April 10, 2006 2:05 PM
To: Tower; YCCC
Subject: [YCCC] guy placement questions
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
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
between the 40-2CD and the 4-el SteppIR on 15m (some) and 17m
At least 15' of separation is needed to minimize the interaction. If the
SteppIR is at 90', then the 40-2CD should be at least five feet up the
Question 1 - Guy elevations: Rohn specs call for guying at 33', 63' and
The stack models fine at 96'/64'/34', so this would be a reasonable
In that case, the 40-2CD would be 10' up the mast, at 110'. In both
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
mast above the top guy versus five feet of tower and 10 feet of mast
the top guy?
Question 2 - Mast length: In the 90'/60'/30' case, I would put eight
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
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
Question 3 - Guy radius: The SteppIRs need at least 25' of turning
(24' plus one foot for the TIC ring.) The middle SteppIR will probably
clear the top guy if the guy radius is at the Rohn nominal 80'. If I put
guys 85' out (slightly over the spec of 80% plus or minus 5%), then I'll
about 3 feet of clearance. If I put the guys 90' out, then I'll get
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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