Only 1/4" exposed? Not much of a gap for an antenna feedpoint unless you
operate barefoot or qrp all the time. granted, the dc sparkover voltage of
a 1/4" gap is on the order of 5kv or so in dry air... and 1500w at 50ohms is
only 275v or so if I punched that through the calculator right. but you
are talking the conduction along a rather large surface area that will be
wet, contaminated with metallic salts that run down the vertical, and may be
porous if you don't keep it well sealed. Take a good look at how power line
insulators are designed, the actual gap between the hot and ground metal
pieces may only be 1/2-1" for 20-40kv of line voltage straight through the
insulating material... but trace the surface distance between them and you
get anywhere from 4" to maybe 24" for that same voltage. The extra surface
area along the glass, porcelain, hard plastic, or silicone rubber surface is
to make it able to handle that contaminated rain water.. and in most
insulator designs there is a method used to keep at least part of the
surface dry. This approach is also seen in commercially available
insulators for verticals, there is always at least several inches of
insulating material between the conductors.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> -----Original Message-----
> From: n8de@thepoint.net [mailto:n8de@thepoint.net]
> Sent: Friday, October 09, 2009 21:36
> To: jimlux
> Cc: towertalk@contesting.com
> Subject: Re: [TowerTalk] Insulators
>
> Have already cut the rod with a hacksaw with a new blade .. no
> degradation of performance from first cut to last.
>
> I tested the first 6+" length by stressing it with the lower tower
> section and the tower base. Very strong stuff.
>
> I agree that some kind of lacquer needs to be put on the insulator to
> protect it from the 'weather demons' even though less than 1/4" is
> exposed to the elements.
>
> I'll let you know next Summer how it's holding up.
>
> Don
> N8DE
>
> Quoting jimlux <jimlux@earthlink.net>:
>
> > n8de@thepoint.net wrote:
> >> FYI:
> >>
> >> I discovered a new source for insulator material that I will be using
> >> on my 80m tower verticals.
> >>
> >> At the local 'feed store', I found 1+1/4" fiberglass ROD with a few
> >> small pre-drilled holes ... it's designed to be used as an electric
> >> fence post. The rod is VERY STRONG and should hold up quite well.
> >>
> >> The cost for a 6' rod is about $6 .... quite inexpensive. Cutting it
> >> into 6+" lengths is quite easy.
> >
> > I used to use a lot of structural fiberglass in various forms. Whatever
> > you cut or drill with will get dull very quickly when working with glass
> > composites.. The glass is quite hard (harder than the steel in your
> > cutting blades).
> >
> > If you have a diamond grit blade that works real well (like a tile saw).
> > A carbide toothed blade works fairly well, too. An abrasive cutter
> > also works, but is messy, melts the plastic matrix, and makes dust to
> > boot (which as was pointed out, is nasty stuff).
> >
> > Don't contemplate taking a 6 foot rod and hacking it into 6"
> > insulators.. the rod might be cheap, but the sharpening costs or new
> > blades won't be.
> >
> > I haven't tried it, but cutting it with a shear, axe, or tree branch
> > loppers might actually be a better strategy, since it would tend to
> > "cleave" the glass fibers, and it's easier to sharpen the blade when it
> > gets dull.
> >
> > _______________________________________________
> >
> >
> >
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> > TowerTalk@contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
> >
>
>
>
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