[TowerTalk] Re. Aluminum mast calculations

Thu, 8 Oct 1998 18:17:40 EDT

In a message dated 98-10-07 14:26:57 EDT, donald.s.tucker@exgate.tek.com
writes:

> Question 2.  I have a 2 inch 6061-T6 1/4 wall 24 foot piece of tubing
that
> I acquired to use as a mast in the above tower.  Want to mount my TH7DXX,
my
> KLM 40M monopole, and a 12 ele 2M beam. My intention was to place a plate
> with a bearing where the original rotator goes, and put a new rotator
plate
> further down the tower, to take any stress off the rotator.  My concern is
> how much mast I could (should) leave out with the antennas noted and if I
> could sneak in a  small 12/17M beam. >>

Let's examine the question and put some numbers on it.

Here are numbers I got from Hygain and Force12 catalogs (sorry, no KLM
catalog) which should be about right for your situation (Hygain has lowered
their effective areas (available on their web site, see archives for
discussion of this point) so these are conservative for the Hygain
antennas):

Antenna                             area (sq ft)    load (#) (@ 80mph using
25#/sq ft)
TH7                                         9.4                  235
Disc1 (40m 1 el)                    2.7                  68
214FM (2m 14el)                   1.7                  43
WARC 2/2 (12/17m yagi)     3.1                  78
mast                                        .1/ft                 2.5/ft

Calculated mast bending moment resisting strength: 1628 ft# (see Leeson's
"Physical Design of Yagi Antennas" for the technique).

One possible solution:

Antenna      Hgt above tower(ft)    bending moment (ft#)
TH7               .5                                   118
WARC 2/2     5                                   390
Disc1          10                                    270
214FM        12                                    516
mast             12                                  180

Total                                                  1474

This load should be safely supported by your mast.  Other solutions exist
and depend upon what you consider to be adequate stacking distances.

On a personal note, I have used aluminum masts in the past and currently
with no problems.  I used a sleeved mast to support a stack of 402BA,  204BA
and DB1217.  Those were the days when I lived in the country.  Now on a
quarter acre city lot I have a semi homebrew 2el tribander (trapped DE,
monoband reflectors) on an aluminum mast on a Heights crankup tower.  I have
to be able to do all my high work myself and like the light wgt of al masts
and towers.  If you do a little design work you should have no problems with
aluminum masts.

Hope this helps

73, Ed, K5ED (in El Paso)
k5ed@dzn.com

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