[TowerTalk] Mast ratio in and out of the tower

Jon Casamajor k6el at comcast.net
Mon Mar 23 20:06:51 PDT 2009


I’ve had mine up for over 20 years. The mast is Chrome Molly, 2”x 20’. I
have 4’ of it in the tower below a large thrust bearing. The tower is a
Tristao HDX572 equivalent to the US Tower of the same model number. I’ve had
20²’ on this tower for the whole time pretty much. I had a KT34-XA and the
402CD 40m Yagi, plus some other stuff with no issues. We do get some nasty
winds here, although not like the tornado country but occasionally 75 MPH.
I’ve had no problems with either the Tail twister that rotates the house
here or the mast. My current stack is a bit larger and you can see it at QRZ
on the K6EL page. I am confident that this stack will be here long after I
am. I’ve added 10’ to the top of my mast and I think I’m still within a good
safety margin at least for my area assuming my stack is “parked” properly
when the winds kick up. We don’t get many violent downburst type
thunderstorm stuff
occasionally, but pretty rare. Four or five feet inside
the tower, with a good thrust bearing would be fine in all but the most
violent conditions.




k6el at comcast.net


To: <towertalk at contesting.com>

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

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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