On 7/4/2011 11:49 PM, Jim Lux wrote:
> 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.
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.
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|