[TowerTalk] Base insulators for verticals

[Jim Lux]

On 7/4/11 7:05 PM, w7xu at iw.net wrote:
> I'm looking for some suggestions on how to insulate the base of  Rohn 25 tower so
> it can be series fed.  This is for a 4-square on 80 meters.  I already have the tower
> on hand, so I'm not really looking for alternatives to using tower as the radiating
> elements.  I've looked through the archives of TowerTalk and Top Band, but I'm
> wondering if anyone has some other ideas.
>
> The options I've found are:
>
> 1)  Base insulators carried by DX Engineering, with one insulator in each leg of the
> tower.   They go for about $450 pertower.  I've seen the arguments that having
> an insulator in each leg is less than ideal, due to torque, etc.,  but each tower will
> only be 60 some feet tall.  This is the lowest cost commercial alternative I've found.
>
> 2)  Commercial AM tower insulator.  I did some investigating and was quoted about
> $1100 per insulator, with of course just one insulator per tower.  I've been looking
> locally for several years and can't come up with 4 used ones, so the used route
> hasn't proven to be a viable option.
>
> 3) Power line insulators.   I checked with my power company and they won't sell
> these.
>

Los Angeles DWP ha a salvage yard where they sell these sorts of things 
(post insulators) for the scrap metal value. You might need to call 
around to find how your power company disposes of stuff like this.  In 
general, they're leery of selling to individuals because of the hazmat 
problems with oil filled whatevers, but often there's a way to get 
hooked into the system.


> 4) Rohn insulated tower section --  These cost about $2000 each now, plus shipping.
> Ouch.
>
> 5)  Home made insulator?  I'm willing to go this route if it doesn't involve finding some
> one-of-a-kind items that I probably won't be able to locate locally.   Also, for those that
> have made insulators with Teflon sheet, etc., I also wonder how the shunt capacitance
> introduced into the system affects performance.
>


You want to use some form of glass/epoxy composite (i.e. fiberglass). 
Mcmaster-Carr (http://www.mcmaster.com) has a huge variety of structural 
members in various shapes (tubes round and square, angle, rods round and 
square, plates) in a variety of suitable materials.

Or, your local metals dealer may have fiberglass structural components. 
One brand is Extren. It costs about the same as aluminum in the same 
size, and is about the same strength (although it's more flexible than 
aluminum)

But, really, what you want is something like a 1/2" thick high density 
polyethylene sheet under the bottom section, right?  As in a big cutting 
board?   You'll probably want to get some appropriate fiberglass bolts 
to keep the base from sliding off, etc.