Hello TTs
Before I place my order for a pair of Force 12 antennas I thought I would
put my design before the TT participants for a reality check and comments.
First, let me thank those of you who have made comments and suggestion off
the reflector.
My plan is to put a Force 12 C31XR and a EF-240s on top of a 58 foot
foldover 25G. The hinged which is at the 28 ft point will be PINNED and
LOCKED. I will NOT be winching the antenna using the tower winch but rather
I will use a crane with a bucket that can reach to 80 ft from just outside
my parameter fence. The foldover will come in handy for dressing the tower
before I erect the antennas.
Currently the tower has 2 sets of four 3/16" EHS guys attached the 28' and
58' points. I am using large diameter screw anchors that have been embedded
in 1.5' x 1.5' x 6' concrete at a distance of 37' from the base of the tower
(my fence line on a residential lot). The portion of the screw anchor above
ground has been tarred to prevent corrosion. The base of the tower sits on a
concrete block 3' x 3' x 3.5'. In the past 30 years we have rarely had
extreme winds gusts of 90 mph and an extreme hourly speed of 60 mph. I am
using 70 mph as my design speed.
The Rohn table indicates that 60' 25G tower with two sets of three guys
attached at the 28' and 55' points and a guying radius of 48' can a handle
a top round load of 16.8 sq ft at 70 mph - no ice (we rarely get icing
here - it is too cold :-). The total area of the Force 12 antennas is 13.6
sq ft. My calculations indicate that if I place the C31XR at the top of the
tower and the EF-240s 10 ft above that I will add about 1 sq ft of mast area
and I will need a mast with a yield strength of about 40,000 psi. Good mast
materials are hard to find in this area but I might be able to locate some
42,000 psi stainless steel. Barring that I will have to reduce the
separation between the two antennas to 9 ft and use 6061 schedule 40
aluminum with a yield strength of 35,000 psi.
My rotator is a T2X that will be mounted as far down the tower as possible -
likely within 10 ft of the base of the tower. I have a thrust bearing at the
top of the tower and I am going to place another bearing about 10 ft below
the top of the tower. I have not quite decided how to handle the coupling
between the 18 - 20' top mast section and the lower section which will run
down the center of the tower to the rotator. Also, I have not decided what
type of material I need for lower section of the mast.
That is about if folks. Your comments and suggestions are welcome. I would
rather error on the side of safety and if need be I will forego the 40m
yagi.
cu .... sylvan
----------------
Sylvan Katz
Saskatoon, Sask
VE5ZX & G0TZX
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