[TowerTalk] How To Avoid Telescoping Mast Colapse

Wayne Willenberg wewill747 at gmail.com
Wed Nov 16 09:04:04 PST 2011


I am in the planning stages of a new antenna project.  It is going to be a
simple, center fed, 40-Meter dipole (using a 1:1 DX Engineering Maxi-Core
balun) connected to RG213.  (My county has a maximum wind speed of 100mph.)

The supports I intend to use at each end are Rohn H50 telescoping masts,
raised to their maximum, about 44.  I am following Steve’s (K7LXC) first
rule – follow the manufacturer’s directions.  So, the stays I am using are
25 guage-galvanized wire sold by Rohn and the radius of the anchors for the
stays will be per Rohn’s specs.

Steve’s second rule, “don’t do something the manufacturer doesn’t tell
you,” creates several issues.  First, the instructions for the Rohn masts
don’t indicate the length or type of the anchors that should be used.  So,
I plan to use the recommendations from the ARRL Antenna handbook.  I plan
to use 1” OD galvanized pipe driven 4 feet into the ground.  QUESTION 1:
Are these anchors sufficient?

The second embellishment I plan to use is ¼ inch galvanized thimbles at the
anchors and at the collars of each section of the mast.  It seems to me it
is better to have the guy wires wrapped around a thimble rather than going
through the rough edges of the collars and anchors.  QUESTION 2:  Is this a
good idea?

At each point where a guy wire goes through a thimble, I plan to use a
series of 3 -- 3/8 galvanized wire clamps (i.e., the type that has a U-bolt
that goes around the two cables.  Each end of the U-bolt is threaded.  The
U-bolt extends through a flat part, and nuts are tightened on the threaded
ends of the U-bolt and thereby clamp the wires together.) QUESTION 3: Is
this form of attachment correct?

The ends of the dipole opposite the antenna wire from each feedpoint will
be connected to H1200 Phillystran.  The Phillystran will go through a small
marine-grade stainless steel pulley at the top of the mast and then
continue to the bottom of the mast.  The center of the dipole is heavy
because of the DXEngineering balun (rated at 2,000 W) and the RG213
connected to the balun.  My reason for using Phillystran for the dipole
halyards is the strength vs weight ratio.  I know I can’t make the dipole
horizontal to the ground, but I hope to avoid an acute angle on each side
of the feedpoint by exerting significant horizontal force on the halyards.

I plan to tension 3 of the sets of guy wires just enough to take the slack
out and to make the masts plumb.  However, the fourth set of guy wires
opposite the each end of the dipole will have turnbuckles for adjusting the
pull in a direction opposite the dipole wire.  I intend to have the guys
exert significant force to try to take some of the droop out of the dipole.
 I know some of this force will be directed downward through the mast and
try to buckle the mast.  QUESTION 4: Is this additional force on the fourth
set of guy wires a problem?

My final general question:  What is the primary reason for causing
telescoping masts to fail?  Am I setting up a failure situation by planning
to have more force in the guys opposite the dipole wires?

Thanks for your help.

Wayne KK6BT

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