[AMRadio] Tower Base Insulator for 160m Vertical


D. Chester k4kyv at charter.net
Sun Apr 26 15:33:38 EDT 2009


> A friend of mine is thinking of putting up a 125' tower for 160m use on 
> AM.
> He's collecting up old TV tower sections
> from folks who are no longer using them for TV, and the plan is to build a
> vertical out of it. Here's the $65K question he threw at me
> and I thought I'd open it up to the group. What can he reliably use as 
> base
> insulators for this tower/antenna project??
>
> Anyone had any experience with building a vertical like this, and have a
> suggestion?
>
> Mike - N3EAQ

My first choice would be to find an old broadcast station base insulator and 
make an adaptor plate to fit the tower on  hand.  That's what I did for 
mine.

The next choice would be to use a large power pole or substation insulator, 
something at least 3-4 inches in diameter and at least 6" tall, solid 
ceramic.  Fabricate some kind of metal base plate to mount it onto the 
concrete pier and some kind of metal adaptor to fasten it to the bottom of 
the tower  section. That has worked for me for previous verticals.

You want something that pivots and rotates freely at a single point at the 
base, and depends only on the guy wires to hold the tower vertical. This 
allows the tower to sway in the wind and rotate slighty in case there is 
ever some torque stress under heavy turbulent wind condx, that could pop the 
welds and make a flimsy TV tower collapse in a windstorm. You DON'T want to 
rigidly attach the bottom of the tower to the concrete base pier. As long as 
there is no other antenna on the tower, TV tower sections should be good for 
that height if they are well guyed.  Find a Rohn catalogue and follow their 
instructions for assembling and attaching guy wires and anchors.  Don't take 
shortcuts just to save a few bucks.  I learnt that years ago, the hard way.

If you use metallic guys, be sure to break them up with insulators. 
According to the National Ass'n of Broadcasters engineering handbook, the 
maximum recommended length for each section of  guy wire between insulators 
is 1/7 wavelength.  If you plan to use the tower only  for 160, you need to 
put an insulator every 72 feet.  If you plan to use it also on 80, they 
should be broken up into 36' lengths, for 40 18' lengths, etc.

For something as permanent as a 125' tower, I wouldn't dick around with some 
kind of jury rigged insulator made out of sheet plastic and wood.  That 
might be OK for TV push-up masts and water pipe or gutter pipe verticals, 
but I would never use anything like that with a tower that I planned to have 
stay up for decades.

I have seen a few small tower base insulators at hamfests, although rarely. 
I have also seen large ceramic stand-off insulators, some 3" in  diameter 
and several inches tall, that ought to hold a light-duty tower like old TV 
tower, provided there is no huge antenna array mounted on top.

Don k4kyv

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