I went with an extra 10-foot section. There are lots of advantages to that,
including not needing a heavy-duty mast if you set the top antenna just
above the top plate. In fact, since you don't have a rotor, a short piece of
pipe will probably do.
The tower is Rohn 55, 110' total, including the extra section. It's guyed at
33', 63' and 95', with TIC rings one foot above each guy bracket. The guy
points have more to do with placement of the stack of antennas on the TIC
ring than available distance to the guy anchors.
So there's 15 feet of tower above the top guy point. I have a 12' aluminum
mast with 1/4" wall, seven feet below the top plate and five feet above the
top plate. The mast supports a Cal-AV 2D-40A (16 sq ft windload and 165 lb.)
The boom-to-mast plate sits an inch or so above the thrust bearing in the
top plate, so there's virtually no bending moment on the mast from that
antenna. I recently added a 6-el 10m monobander at the top of the mast, five
feet above the top plate. It's only 4.1 sq ft of windload at about 25 lbs. I
don't believe the monobander presents any significant bending moment to the
mast. The mast is turned by an M2 Orion.
I've climbed the unguyed portion of the tower many times and it's pretty
darned solid. I have no worries about it withstanding the 20 sq feet of
windload above the guy bracket. Rohn 45 isn't as rugged as Rohn 55, but you
should be able to find out if it'll handle the spec.
73, Dick WC1M
-----Original Message-----
From: John W [mailto:xnewyorka@hotmail.com]
Sent: Monday, January 09, 2012 11:23 PM
To: TowerTalk
Subject: [TowerTalk] How to get another 10' in height? Add a mast or add
another section?
If I have a Rohn 45G tower of a given height, let's say 90', guyed per the
Rohn book, and rotating from the base, hosting several side-mounted yagis
for 10m (3), 15m (2), and 20m (1), and I want to add another antenna up at
100' (a small 40m 2L) that also rotates with the tower, which one is the
safer/better installation: adding a mast that protrudes 10' above the top
of the tower (and for some distance down into the tower), or adding another
10' section and sidemounting the new antenna at the top of it?
This is a theoretical question, because I haven't built the tower yet, but I
plan to.
The first reply many might have is probably: Why don't you just build it to
100' per the book, instead of 90'?
The answer to that is that I don't have enough real estate to put guy
anchors 80' from the base, I only have 72' available.
Also, I have not even seen hardware that allows adding a FIXED mast at the
top of a Rohn tower - sort of like a thrust bearing that can't turn. How is
this typically done?
Thanks & 73,
John
W2ID
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