Another possibility.
Put the rotary 40m about 1' above the tribander and at right angles to it.
The extra few feet of height lost won't amount to much. Also you would
still have the advantage of pointing it at the signal of interest.
That will help with the load, but won't impact the torque.
73 de Brian/K3KO
On 8/21/2013 17:56, Gene Fuller wrote:
These numbers looks like calc's for bending moment at the top of the tower.
What about torque ?
Would it be possible to put the rotor plate lower in the tower and pick
up some extra restraining moment with "overkill" in the mast ?
However, this still wouldn't help the torque problem. If he's stuck
with this tower and the given antenna family, how about a leg anchor
plate and torque bars at the top of the tower ? This would also help
the bending moment problem. Pretty soon a more heavy duty tower, or less
ambitious antenna system, starts looking good !
With some extra $$$ and a little engineering there will be a good answer.
Gene / W2LU
----- Original Message ----- From: <TexasRF@aol.com>
To: <RShirbroun@newportlabs.com>; <towertalk@contesting.com>
Sent: Wednesday, August 21, 2013 12:41 PM
Subject: Re: [TowerTalk] Question on mast height above tower
Randy, a quick look at your proposed system shows that at 90mph, the
total
force applied to the top section is 666 foot pounds. 195 fp for the 40m
ant, 315 fp for the 6m beam and 150 fp for the Triband beam.
If that force was from a single large antenna it would be the
equivalent of
22 sq ft mounted one foot above the tower top.
This is with NO safety factor and does not include loading for the 2"
mast
or feedlines.
This is more than the tower rating and you will have to decide if the
risk
is acceptable. The bottom line is that the system would fail with a wind
speed somewhat less than 90mph, maybe 70mph or so.
The mast proposed is an overkill. My computer program shows that a 2" OD
mast made with 1026 DOM, .125" wall will fail at 130 mph. This
material is
available at Texas Tower of course!
Hope this helps.
73,
Gerald K5GW
CEO Texas Towers
In a message dated 8/21/2013 8:25:38 A.M. Pacific Daylight Time,
RShirbroun@newportlabs.com writes:
I didn't get any comments or suggestions to my previous post, so I'll
try
again.
If this is a dumb question, feel free to tell me!
I have just erected a new HDBX 40 in place of my 30 year old HBX 48,
sacrificing 8' of height to gain some strength and load capacity (and
peace of
mind!). The stock rotator plate allows 2' of mast below the top
plate and
I
will be using the heavy duty Yaesu thrust bearing (along with the Yaesu
heavy
duty 2800 rotator plus the absorber plate). I have added steel angle
braces
to reinforce the rotator plate.
I'm looking at using a 15' chrome/steel 2" mast (in place of my
previous 9'
mast), so 13' of the mast would extend above the tower. The mast would
support
a TX38 tribander just above the top plate, a 6 m beam half-way up, and a
40m
rotatable dipole near the top, 12' above the top tower plate and the
thrust
bearing. The mast, of course, is very heavy, weighing around 75lbs.
The
tribander weighs 40lbs and with 5 ft2 surface area; the 6 m. beam weighs
10lbs
with 1.5 ft2; the dipole weighs 10lbs and is 0.5 ft2.
Is this too much mast for this tower? (BTW - I'm aware the boom length
exceeds the 10' maximum for this tower, but the HBX 48 handled a similar
tribander for 30 years, with occasional severe ice-loading, without any
problems.)
Thanks and 73,
Randy, ND0C
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