[TowerTalk] conduit for mast

[K7LXC@aol.com]

In a message dated 5/31/01 3:34:31 PM Pacific Daylight Time, mike@kb0ozn.com 
writes:

> Ok Ok, I get the idea that conduit is a bad idea.
>  
    For a mast? Not always. 

>  Let me ask this, in general is it ok to use a ham IV with a 10 foot mast
>  given that all those antennas will be a windload of about 8 sq feet I'm
>  guessing.  This would be the order of antennas, 6m 3el beam, a 2m 15el
>  beam, a 70cm 24el beam, a 33cm 20el beam, a 23cm 25el looper, and a 13cm
>  40el looper with maybe a vertical on the top for 2m/70cm.  This is from
>  the bottom up.

    Hennepin County is an 80 MPH windspeed zone so you need to be a little 
more careful about your material selection.

    The problem with using conduit or pipe is that they are designed to carry 
liquids and are not rated for strength like tubing is. That said, big masts 
are called for when you've got big loads (typically an HF stack) and big 
winds. For your proposed installation of "little" antennas (no offense) the 
bending moment on the mast is probably going to be pretty small (a relative 
term) and may be within the limits of some conduit. 

    I ran some approximate numbers thru my MARC (Mast, Antenna and Rotator 
Calculator) Program but it was sort of meaningless without knowing the 
specifics. Give me the square footage of each antenna and its height above 
the top of the tower; then I can make a more educated recommendation. 

    If you're putting one antenna at the top of the tower, then a pipe or 
conduit may be just the ticket since there is almost no bending moment if 
you're within a foot or so of the top of the tower. For example, mast bending 
moment at top of tower = x psi; m.b.m. at two feet = 2x psi; m.b.m. at ten 
feet up the mast = 10x psi. That's why you want to have something suitable 
with a loaded mast. With one big antenna at the top of the tower, just about 
anything will handle the bending moment. 

Cheers,    Steve    K7LXC
Tower Tech 
MARC is available from Champion Radio Products

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