Hi Sylvan,
I think your planned installation will exceed the tower rating.
You have the wrong antenna areas for evaluating the tower.
The Rohn allowable areas are projected areas (see Dwg C870484 R4 note 2).
The antenna areas given in the Force 12 catalogue are effective areas based
on EIA-RS222-C which reduces the projected area by multiplying it by 2/3.
If you look in the "Antenna Specification" area of the Force 12 manual, you
will find under "Wind Load", that this is explained.
Thanks to Force 12 for telling us what their numbers mean!
So,
The C-31XR at 10.7 sqft effective area is 10.7 x 1.5 = 16.05 sqft projected
area.
The EF-240S is 2.9 sq ft effective x 1.5 = 4.35 sq ft projected.
Running a 2" od mast down to the 10' level adds 8.33 sqft of addirional
projected area.
You can make that tower carry more load by moving the top guys up to the
tower top, putting the lower ones at midpoint, and using 1/4" cable on the
top set.
Remember, when you put an antenna up high on the mast the total allowable
area will be reduced because it takes less load up there to create the same
tower load.
This might be one of those times where spending a bit on engineering will
pay off.
--
73, Kurt, K7NV
http://yagistress.freeyellow.com
Sylvan Katz wrote:
>
> 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
>
> --
> FAQ on WWW: http://www.contesting.com/FAQ/towertalk
> Submissions: towertalk@contesting.com
> Administrative requests: towertalk-REQUEST@contesting.com
> Problems: owner-towertalk@contesting.com
--
73, Kurt, K7NV
http://www.freeyellow.com/members3/yagistress/
--
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|