Date: Wed, 12 May 2010 20:15:15 -0400
From: "Roger (K8RI)" <K8RI-on-TowerTalk@tm.net>
Subject: Re: [TowerTalk] Best mast location?
> Also not sure why they pick
> a tower height of 55' which is an odd height if comparing wavelengths of
> amateur bands,
It more than likely has to do with things not related to antennas or
even towers at all.
It's far less expensive to use standard stock lengths of material. IOW
tube, angle, and flat stock, comes in 20' lengths, but that would make
tower sections to ungainly to handle so they cut them in half. That's
why when you purchase UPS shippable tube, angle, or flat stock shipped
buy UPS or other non trucking carrier it's more expensive, but retailers
do it because it's the only way some customers can get the materials.
Yes they also sell it in 20' lengths, but better buy lots as shipping is
expensive.
## say what? US Tower corp uses 50 ksi pipe... and buys it in
21' lengths. It comes on 18/26 wheel semi trailers.. lots of em.
## they use a 4' overlap between sections, when fully extended.
Bottom section sticks up 21'. Then the 1st 4' overlap. Next section
buys you 17' [21-17] Now you are up to 21+17 = 38'. Now a 2nd 4'
overlap, then another 17' . Top of tower is now 21+17+17 = 55'.
Once they take those stock lengths and cut them in half they have to
make provisions for the tube or angle sections to fasten together. That
means overlap and in a few sections that can add substantially.
To use a different means of construction either wastes material of adds
labor. Take for instance the larger towers that use welded on, bolt
flanges. That's 6 of those, plus 18 or 24 nuts and bolts plus washers.
All of this adds up to odd sized hardware which seems expensive but
would look cheap if they made them to custom sizes. Take a look at and
the price of those really nice, heavy duty crank ups at Dayton. Now
there is both quality AND price.<:-))
## US Tower crank ups are all welded. There is NO flanges like rohn 65-G.
There is no direct mech connection from one section to the next..except via
EHS aircraft cables.
> being more than a half wavelength at 20 and less than a half
> wavelength at 40.
>
> So I, too, am reading all I can and asking all I can. On this issue,
> actually, the ARRL Antenna Handbook makes the most sense going through the
> "moment" calculations. But they come to the conclusion that there is little
> difference between the top of the tower and 5' above.
>
That's one of those: sorta, kinda, true in *some* cases, but not for a
top mounted rotator (which is not recommended ). In that case the force
at the top of the tower is the wind load of the antenna *multiplied* by
the height above the top of the tower. IOW without the mast going down
through the tower an antenna presenting a 100# wind load @ 50 MPH will
present 700# to the top of the tower when mounted 7' above the top of
the tower with a top mounted rotator. Even down in the tower that
moment/arm is applied from the rotator to the thrust bearing at the top
of the tower. That distributes the wind load between the top of the
tower and the accessory plate. A good, substantial tower will distribute
that load across the entire length of tower from the rotator to top
thrust bearing. The calculation gets a bit more complicated for a less
than substantial tower or one that is loaded to, or beyond it's ratings.
## which is why you should always use TWO thrust bearings. Then you
won't have a sideways load on the rotor at all ! Per Leeson [ dunno if he
got this part right] you take the bending moment where the mast enter's the
top bearing... and divide that number by the distance between the top bearing
and what ever is below it... like a 2nd bearing.. or the rotor. If the
distance between
top bearing and next item below it is only 2'... VS 4'...... you can kiss the
top 2'
of the tower goodbye in a big wind storm.
## What you have is a fulcrum. Imagine trying to move a boulder with a 10'
pipe,
and a fulcrum placed 5' away from the boulder. Pull down on the pipe all you
want,
and you can't get any leverage on the boulder. Now move the fulcrum just 1-2'
away
from the boulder, and now you have loads of leverage ! Same deal with mast
/top bearing/
intermediate bearing [or rotor] . The flip side to all this is.... if you
mount the 2nd bearing/
rotor too far down inside the tower, and mast material is flaky, the mast will
bend inside the
tower... which will then allow the portion of the mast..above the top of the
tower to bend even more.
## for those folks with trylons.. and tall masts.... and 2 x bearings
used..watch out. The 'middle shelf'
on a trylon is only down 30" from top of each section. So even if you mount
one bearing at top of tower,
and a 2nd bearing down 30"... and the rotor at top of next section below [8'
below top of tower].... you will
still have this huge leverage effect.. from the 2nd bearing only being down
2.5' from the top of the tower.
## I mounted the cue rack down 30' from top of tower.. but no bearing goes
there... just something to stand on.
2nd bearing is mounted 60" [5'] down from top of tower. I had to fabricate by
own cue rack from 2" x 2" x 3/16" thick
6061-T6 Angle. Rotor can either go at top of next section down... or 2.5'
below top of next section down .
That puts the rotor 8'/ 10.5' down from very top of tower. Ok, now you can't
bust the top 2.5' of tower.
## The US tower corp HDX-689 top section is a whole diff deal. Bearing at
very top. A sleeve goes
between bottom of top bearing.. and extends down a few inch's to shelf plate.
Both top bearing,
and this 2nd shelf plate are welded to tower legs. 1.315" OD legs. [.179"
thick, 51 ksi]. Loads
of Z bracing used. [7/16" rod = .4375" ]. 2nd bearing is down 4'. Rotor
is 2' below 2nd bearing.
The top section of tower is 18" wide.. and built like a tank. Both bearing
plates are 3/8" thick steel.
The 2" thrust bearing by US Tower is pretty good. It's 3" OD.. and 2.0625"
ID. The side walls of the
bearing are .469" thick steel. I prefer them vs my Rohn TB-4 bearings. I
gave up on pillow block
bearings years ago, they always eventually rust.
Later.......... Jim VE7RF
Hope that helps.
73 and good luck,
Roger (K8RI)
> Fred
>
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