Thanks to everyone for the comments. The plan has been changed slightly
to allow for a two part mast.
The Create CR30 is rated for 27 sq. feet @ 90 mph. Max vertical load
1322 lbs. I estimate about 10 sq. feet of antenna weighing about 45
lbs. Combined masts about 80 lbs.
The 4130 chrome molly mast will be cut at about the 12 foot mark,
leaving me with a spare 12 footer. The mast will extend 8 foot out of
the top of tower. A 'mast to mast' clamp will attach a 7 foot 2" OD 3/8
wall aluminum mast for the top part. A local ham who climbs towers for
a living recommend this method to make it easier to service the tower.
The 'mast to mast' clamp allows the 7 foot aluminum mast to be lowered
while standing about 2 foot up on the 4130 mast.
The antennas are from bottom to top: 50MHz, 144MHz, 222MHz, 432 MHz, 903
& 1296 side mounted on same cross arm. A 146, 224, 446 vertical is on
top. The 222MHz yagi will be about where the mast to mast clamp goes.
I have not decided weather to put 222 above or below the clamp. I have
attempted to maintain 1/2 wavelength spacing between antennas.
I have run the antenna specs through the MARC program, and it states I
need 65,000 PSI minimum, hence the purchase of the 4130. The force on
the top 7' aluminum piece must be less, so I think aluminum will do. I
guess I could run the top 7' through MARC and see what is says. The
upper antennas are also smaller due to higher freq. Approximate boom
length for most yagis is about 17' to 19'. I have not selected the
exact yagis yet, but as a guide look at the C3I specs for the K1FO
designed yagis.
The Create tower has been up about 5 years with about 2 foot less mast
than the above plan, with about 1/3 to 1/2 the antenna wind load stated
above. The existing mast is 2" fence post (ducking now). The tower is
guyed per Create's recommendations. Its sits on 2 dormers about 5 foot
wide. Below the dormer I have 2x6 below the tower legs. The 2x6 are
nailed into 3 rafters. I am about to have my house re-roofed, it is a
tear off job. Roof will be bare, I am going to have 3/4" plywood added
to the existing 3/4" tongue & groove sheathing. The attic is currently
unfinished, so I can build in additional support below if needed.
This mail took a while to write, thanks for reading.
Comments?
7 3
Bob - N2SCJ
-----Original Message-----
From: K7LXC [SMTP:K7LXC@aol.com]
Sent: Thursday, March 26, 1998 10:30 PM
To: Robert.Turner@ibx.com; towertalk@contesting.com
Subject: Re: [TowerTalk] Ratio of mast inside tower vs
outside tower?
In a message dated 98-03-26 18:58:23 EST, Robert.Turner@ibx.com
writes:
> I needed 19 feet of 2'od 4130 mast for my design. However, a
24' length
> was delivered. To cut or not to cut, that is the question.
>
> Are their any guidelines as to what the minimum percentage of
the mast
> that should be in the tower? I'm concerned that if I use the
whole 24',
> with 4.5 feet inside and 19.5 feet outside, that I will over
stress the
> tower and rotor. The tower is the 9' Create Roof tower.
Rotor is Yaesu
> G-800SDX.
>
Congratulations on getting an investment-quality mast.
While you'll be
able to use it for your entire ham career, it is somewhat
overkill on your
roof tower. Don't let me, though, discourage you from
over-engineering your
installation.
I think the overturning moment on the tower base will be
the limiting
factor. While Burlington County is an 80 MPH windspeed zone,
it's within 100
miles of hurricane oceanline. I'd recommend over-engineering
everything else
as well.
If you get a BIG wind through there, I think the tower will
break off at
the base or some other place. It'll hit the ground somewhere and
be a pile of
bent aluminum. Your chromoly mast OTOH will survive unscathed.
While the additional 5 feet of mast is less than one
additional square
foot of wind loading, my biggest concern is the bending
moment/lever arm
acting on the tower and base.
Don't cut it - you can always use it in a real tower in
the future. A
5-foot chunk of chromoly doesn't get you much.
> If I stick to my original plan I will have 4.5 feet of mast
inside the
> tower with 14.5 outside the tower.
Like others, I'm a little skeptical. You didn't mention
what you're going
to put on that mast. Please elaborate before we go too ape on
this thing.
73 and GL, Steve K7LXC
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