On 4/10/2016 12:05 PM, Ken K6MR wrote:
Thanks Joe. I was thinking about wind vectors but didn’t work it through.
I used 45 degrees here because the element and boom components were
nearly the same. If one had long boom 6 or 10 meter antennas it would
probably be necessary to solve for the angle of maximum force somewhere
between broadside to the boom and 45 degrees. With short, thin
elements a long boom is a much larger contributor than the elements.
73,
... Joe, W4TV
With these numbers it should be simple to add a torque compensator to make up for the
higher torque generated by the reflector’s larger wind load. With only a 22 ft.
boom the rotator needs should be minimal. I also had to add a small counterweight to
the reflector end of the boom to compensate for coax and balun weight.
Ken K6MR
From: Kevin Stover<mailto:kevin.stover@mediacombb.net>
Sent: Sunday, April 10, 2016 07:34
To: towertalk@contesting.com<mailto:towertalk@contesting.com>
Subject: Re: [TowerTalk] W6NL 40m Moxon
Thanks for the input Ken and Joe.
I used 9ft in the ARRL mast calculator and end up with 131 mph survival
wind speed on a 3" x .25 wall 87,000 ksi mast.
The antennas are an Optibeam 11-5 at 70' and the Moxon 15ft above, and
still under max load of the 70' Rohn 55 tower. Now I've got to figure
out what to turn it with.
On 4/10/2016 6:55 AM, Joe Subich, W4TV wrote:
I suppose with some trig you could figure the total projected area
at various wind angles. One going down and one going up. Would some
angle that gives equal projected area from each figure be the max?
You don't need to be concerned with equal areas. The force (load) on
the element will be broadside to the element and will vary with the
angle of the wind to the element. The force (load) on the boom will
be broadside to the boom and vary with the angle of the wind to the
boom. Those forces add as a vector sum which is maximized when the
wind is at 45 degrees to the boom/elements.
The area of largest force is:
sin(45) * (2.88 + 2.76) + cos(45) * (3.67 + 1.24 + 1.20)
= 0.707 * (5.64) + 0.707 * (6.11)
= 8.31 sq. ft.
The boom value will need to be increased somewhat to account for a boom
to mast plate and the element values increased to account for element
mounting hardware, particularly if the original U channel is used. I'd
allow between 8.5 and 9 sq. ft in any tower/mast loading calculation.
73,
... Joe, W4TV
On 4/10/2016 12:51 AM, Ken K6MR wrote:
I’ve wondered this myself but never bothered to model it. Here are
some rough figures from Yagi Stress:
Reflector element without the tee: 2.88 sq ft.
Driven element without the tee: 2.76 sq. ft.
Reflector tee (each): 0.62 sq. ft.
Driven tee (each) 0.60 sq. ft.
Boom: 3.67 sq. ft.
Now the hard part: how do they add? I’m an EE so my logic may be
totally flawed. YS doesn’t allow for the tees in a single model so I
can’t have it do everything at once.
Since the tees are parallel with the boom, I would think you would
just add the area of the tees to the area of the boom which should be
the projected area with the wind blowing 90 degrees to the antenna.
That would give 6.11 sq. ft. for that case. Wind straight on would be
just the elements: 5.64 sq. ft.
I suppose with some trig you could figure the total projected area at
various wind angles. One going down and one going up. Would some
angle that gives equal projected area from each figure be the max?
My first one is built from scratch and mounted to the side of the
tower so I wasn’t concerned with wind area. The XM240 is at the top
of a mast so when I convert that one I’ll have to be more careful.
Ok you mechanical types: does this look reasonable??
Ken K6MR
From: Kevin Stover<mailto:kevin.stover@mediacombb.net>
Sent: Friday, April 8, 2016 19:18
To: towertalk@contesting.com<mailto:towertalk@contesting.com>
Subject: [TowerTalk] W6NL 40m Moxon
I need to figure out the wind area of the W6NL 40 Moxon antenna.
The antenna starts like as a Cushcraft XM-240 which they say is 5.5
sqft.
The Moxon is going to be more but how much? I'm guesstimating 8 sqft.
--
R. Kevin Stover
AC0H
ARRL
FISTS #11993
SKCC #215
NAQCC #3441
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R. Kevin Stover
AC0H
ARRL
FISTS #11993
SKCC #215
NAQCC #3441
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