On 7/4/11 7:05 PM, w7xu@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.
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