Scott,
I have no background or technical knowledge on this subject; however, I have
the same mast that you have. It is mounted in a heavy duty 71 ft motorized
tower with about 7 ft inside the tower (prop pitch rotator) and 14 ft
sticking out. It has worked well for me.
For almost 25 years I had a 4 el KLM 40m yagi, 42 ft boom about 6 in out of
the tower, and a 5 el KLM yagi, 42ft boom at about 8 ft above the 40m yagi.
The remainder supported a boom strut. Those masts are made to flex. When I
received the tower and mast (used) I placed the ends of the mast on saw
horses, to paint it. There was about 6 in of sag just while setting on the
horses.
I can tell you that during 70 MPH winds the mast at the very top would flex
at least one foot, maybe more, but it always held. We may have had higher
winds, but I have no way of measuring them. The 20m yagi finally had to be
repaired so now I have a HyGain 204BA in its place.
Hope this helps,
Ray,
N6VR
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of john@kk9a.com
Sent: Monday, March 23, 2009 2:02 PM
To: TOWERTALK@contesting.com
Cc: W5WZ@arrl.net
Subject: [TowerTalk] Antenna Mast - guidelines for ratio in tower / out
oftower
I do not have a mechanical engineering background, however since no one else
has answered your post I will give it a try. I believe that 1/3 in the
tower and 2/3 above the tower works fine for most mast installations.
John KK9A
To: <towertalk@contesting.com>
Subject: [TowerTalk] Antenna Mast - guidelines for ratio in tower / out of
tower
From: "Scott Dickson, W5WZ"
Date: Mon, 23 Mar 2009 06:10:00 -0500
I've seen discussion on the importance of selecting the correct material for
a mast. I don't recall ever seeing a discussion on the ratio of mast in the
tower versus above the tower.
I have a 2" x 0.25" by 21 ft galvanized 1026 DOM mast. It will hold a Force
12 C-51XR, (weighs 250 lbs, wind load 18 sq ft, mast torque < 900 in-lbs)
installed 2 feet above the thrust bearing at the top of the Rohn 55G tower.
The rotor is M2 Orion 2800.
Currently, I do not have any other antenna that will be on the mast.
Perhaps I'll install a 6 meter beam above the C51 in the future.
It seems to me that the length of mast in the tower is as important as the
length of mast above the tower.
What rules / guidelines / best practices are used to determine where to
install the rotor?
--Scott, W5WZ
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