[TowerTalk] Making an insulated base for 80 meter vertical from Rohn 25
brahmangou at aol.com
brahmangou at aol.com
Wed Dec 23 22:38:36 PST 2009
Hello All,
Thank you for all of your questions and suggestions.
The fluid characteristics of plastics was what scared me about the
commercially available tower leg insulators. Like the fellow's mobile antenna, a
tower leg is under compressive and expansive loading. That is why I wanted to
get the insulating material away from the legs and use it on the base in a
purely compressive application.
To answer some other questions, 6 sections of Rohn 25 weighs about 240
lbs. A few extra pounds for the phillystrand and stinger. The antenna will
probably weigh in at 260lbs or so when up.
The Rohn tilt base is 15" square. That comes out to 225 square inches of
compressive area onto the 1" thick HDPE (high density polyethylene) base
insulator. That makes a little over 1 psi static down force onto the
insulating pad.
There will be additional forces that remain out of the calculation, the
amount of tension on the guys, and the amount of torque applied to the base
bolts. I shouldn't have to go crazy torqueing down the base bolts as there
is minimal lift on a tower section.
The PTFE (teflon) 3" round x 1/2" thick insulating washers will be under
the force of the torque of the base bolts and any flexing of the steel
plate from wind loading the upper sections only. It will amount to whatever that
force is against the area of about 6.5 square inches. If it shows to be a
problem I can fabricate 6 new ones from fiberglass. I'd like to try it with
PTFE first though because of the superior UV and insulation properties.
I hope this answers some of the questions posted. Looking better or worse
now?
Marty AB5GU
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