[TowerTalk] Antenna Mast - guidelines for ratio in tower / out of tower

[john at kk9a.com]

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 at 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