David,
There is an easier way to raise the antenna. Raise the antenna straight up
the tower so that the elements are vertical and the boom is perpendicular to
the boom of the fixed antenna. Bring it right up to the fixed boom and then
rotate the antenna vertically so that the boom and elements just roll around
the fixed boom. Once over, continute raising as usual. You have to plan
this kind of raising so that the antenna is not upside down once the
rotation is completed. I used this method to stack a 35 foot boom 15 meter
beam over my 54 foot boom 20 meter beam. (You have to tie the boom trusses
to the boom during this manuever.) Good luck. 73 Saul K2XA
----- Original Message -----
From: "Howard Klein" <howk2@hotmail.com>
To: <k1ttt@arrl.net>
Cc: <towertalk@contesting.com>
Sent: Sunday, April 04, 2004 12:07 PM
Subject: RE: [TowerTalk] Tension on tramline
> David,
> I am plannng the raising of an 90# antenna on a 32 foot boom to 82 feet
on
> a self supporting tower. The hitch is I have a similar antenna at 40 ft.
> necessitating a tramline to pass the lower antenna. What magnitude of
force
> would I expect on the mast? Could the tramline be untensioned (my word)
once
> past the obstacle to release lateral force on the mast? I will give the
math
> a try if I can set up the necessary parameters.
>
> Howard...K2HK
>
>
> From: David Robbins K1TTT <k1ttt@arrl.net>
>
> I have an old article about this from the yccc newsletter for oct 1994.
> unfortunately this one hasn't been converted and put on our web site
> yet. I will try to get this scanned and put on my web site as I can't
> find the original for it here either.
>
> The formulas for the static case are not too bad. Imagine the angle 'a'
> is the angle from the tower to the tram wire, where 0 is straight down
> and 90 is horizontal from the tower. And angle 'b' is the angle from
> the ground to the wire, where 0 would be laying on the ground and 90
> would be straight up. W = weight of antenna hanging from the wire.
>
> ]\
> ]a\
> ] \
> ] \
> ] \
> ] \
> ] \
> ] W-_
> ] -_
> ] -_
> ] -_
> ] b -_
> =========================================
> (don't you just hate ascii graphics!)
>
> P=tension on ground end=W*sin(a)/cos(a+b)
> L=tension on tower end=(W+P*sin(b))/cos(a)
>
> Note that when the antenna is stationary the L tension is shared between
> the lifting rope and the tram wire, assuming the lifting rope is
> attached near the same point the tram wire is attached.
>
> The important things are to note that as a+b approaches 90 degrees the
> tension becomes infinite... so the more sag in the wire the better.
> trying to design it generically is tough since so much depends on the
> tower arrangement and how much room is available on the ground. I found
> it easiest to make a scale drawing of the side view, figure out how much
> clearance is needed between the antenna being lifted and the tower at
> various points(usually worst case is near the top of course), this will
> give you a minimum value for the 'a' angle and the closest point you can
> anchor the tram to the ground. then determine how far out I had to put
> the ground anchor to give a reasonable 'b' angle.
>
> other notes:
> The 'tiller' method of tilting up the antenna elements as it nears the
> tower is a great way to reduce the needed clearance over the top guy
> wires.
>
> A back stay on the tram wire is almost always a good idea, the side
> forces on the top of the tower for most reasonable angles get huge
> quickly. An extra clamp above the rotor to make sure the extra down
> force doesn't end up crushing the rotor doesn't hurt either.
>
> A quick example of the forces from the calculations I did for lifting a
> 170# telrex up a 150' tower with the ground anchor about 150' from the
> tower base at about mid point of the lift:
> Angles a=40, b=40
> P= 629#
> L= 750# (this is shared between wire and lifting rope)
> Horizontal force on mast = 482#
> Vertical force on mast = 1409# (tram+backstay+down force on lifting line
> pulley)
>
> More from old discussions on this topic are at:
> http://www.k1ttt.net/technote/tram.html
>
> _________________________________________________________________
> Check out MSN PC Safety & Security to help ensure your PC is protected and
> safe. http://specials.msn.com/msn/security.asp
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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