[TowerTalk] Base insulators for verticals

[K8RI on TT]

I should have added that when ever machining, grinding, or filing 
fiberglass wear a good quality, well fitting respirator.  Change from 
work clothes and shower before changing into clean clothes and going 
into your living area.

73

Roger (K8RI)

On 7/5/2011 2:46 AM, K8RI on TT wrote:
> On 7/4/2011 11:49 PM, Jim Lux wrote:
>> 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.
> A few things to remember about fiberglass: The stuff is really abrasive,
> subject to UV deterioration, and often affected by weathering.
> Unprotected fiberglass is very sensitive to water caused deterioration
> which will reduce its strength and decrease its voltage breakdown.
> It can be made using either epoxy or Vinyl ester resin among other
> forms.  I prefer the Vinyl ester resin as it has a much lower viscosity
> (much thinner) than epoxy and that lets you use a fiberglass cloth to
> make a much stronger piece/lay-up for the same size and similar weight.
> Vinyl Ester also has a higher critical temp, BUT a much shorter working
> and pot life.  It's also exothermic like most Epoxies.  The resin has to
> be purchased in gallon batches, promoted with what is called Conap and
> the catalyst is "MethylEthylKeytone Peroxide", or its just called MEKP
> which means Hazmat shipping and you do not want to get the promoter any
> where near the catalyst.   MEKP is also the basis for some other very
> nasty stuff. Keep it cool and well away from the promoter and there
> should be no problems. I've been using the stuff for over 10 years.
>
> As to Epoxy, you can go down to the local marina and purchase "West
> System epoxy" in any thing from small packages to gallon lots with a
> pump that dispenses properly measured amounts.  It's less expensive,
> much longer shelf life, does not have to be promoted, but that stuff
> does get *HOT* when it cures.  Hot enough to burn your skin. I know!
> It's also a much higher viscosity so a less dense cloth is used.
>
> Cut the cloth on a 45 degree bias and wind it on a metal form sprayed
> with a silicone release agent.
>
> Something that's been missed is fiberglass can be machined (with
> caution) and good carbide tools. It's also about as cheap to purchase
> the rods as to make your own and a whale of a lot less messy.
>
> Instead of resorting to a sheet of plastic or fiberglass, purchase a
> solid rod of fiberglass about a 1/4 inch lager OD than the OD of the
> tower legs.  Turn down to the proper size to fit the appropriate tower
> leg BUT leave about a half inch long section in the center that is full
> diameter.  The ends of the tower sections will fit tightly against this
> section and the larger diameter will also add to the voltage breakdown
> between the bottom of the tower and ground.  Then drill out the smaller
> diameter  sections for a snug fit on the bolts.  You can drill the holes
> a bit over size and coat the surface of the rod and inside of the holes
> with either vinyl Ester resin or Epoxy. Then trim that to be a snug fit.
> That way you do not have an abrasive surface rubbing against the bolts
> of tower legs.
>
> The bolts should not break before wearing or elongating the holes in the
> tower legs. Ungraded, galvanized bolts should be far more than strong
> enough let alone going to a grade 5 or 8. (or SS)
>
> NOTE, Unprotected fiberglass should fit *tightly* into the tower legs.
> If they are not tight they can move. If they can move they are just like
> moving sand paper up and down in those legs and against the bolts. If
> you see a gray, or rusty powder forming around them, it's time to pull
> them out, clean them, and then give them a good coating.
> Once finished and assembled either paint the junction with a good UV
> resistant and non conductive paint or just give them a good coat of
> liquid electrical tape.
>
> If desired (and you have the tools) you can drill out the center of the
> insulator so moisture can drain through it.
> Incidentally, if you look up the shape of the normal tower base
> insulators you can make that larger center section as long as you wish
> and contour it to give a longer surface distance between the ends of the
> tower legs.
>
> 73
>
> Roger (K8RI)
>
>>> 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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>>
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