<<I am planning a 160m vertical with elevated radials, using Rohn 25G tower
sections. I plan to have the tower insulated at 30' above ground.
Rohn is very proud of their 25TG insulated section and I have heartburn paying
over $2K for the section, so I am looking for alternatives to insulate this
tower. There will be 130' of tower above the insulated part.
Someone recommended using fiberglass rod machined down to fit inside the 3 legs
and I am currently investigating this option.
Any other ideas / comments / recommendations would be greatly appreciated.
73's
NJ0F>>
The 25TG is a tapered base section designed to go with a separate base
insulator. It's basically a regular 10' section with the bottom 2' or so of
the legs bent inwards, and about a 5/8" thick round steel plate welded to the
ends. From the bend in the legs to the bottom, there are three solid sheet
metal plates welded to the legs, replacing the normal zig-zag.
I bought my tower brand new in 1980, purchased from Hill Radio in the same town
where the Rohn manufacturing plant was. At that time I paid $38 each per
regular 10' section, and as I recall, the 25TG section was about $110. Just a
little over double a regular section. Hill Radio is still in business, and out
of curiosity I checked their website recently, and they were asking $900 for
the 25TG. But the kicker was, even back in 1980 the matching base insulator
cost $510!
I purchased the 25TG section, and found a used base insulator that had been
taken from a broadcast tower. The BC tower was larger than Rohn 25G, and the
three holes in the tower base plate did not match the mounting holes in the
base insulator. I went to a scrap yard and scrounged a piece of steel plate
about the same thickness as the 25TG base plate, took it to a machine shop and
had them cut it into a round disc, and drill 3 holes matching the tower base
plate and 3 more matching the insulator mounting holes. I had threads tapped
into the six holes, and bolted the adaptor plate to the tower base and then
bolted the insulator in place from the opposite side, using the other three
holes. My 127' tower has now been in service for over 30 years, and I am just
now making preparations to re-guy it.
This year at Dayton a friend of mine picked up a Lapp ceramic base insulator
that looks exactly like the one that goes with the 25TG, for $40. These may not
be so hard to find these days, now that there are little Mom and Pop AM
broadcast stations that have gone dark because they could no longer make it in
the economy. Just a matter of finding the insulators before they go to the
landfill, since most Hammy Hambone types who might salvage a tower just want
to bury the bottom of the tower directly in the concrete base and have no
interest in the base insulator.
You should use a single base insulator that allows the tower to pivot and sway
with the wind, with the insulator and its base mounting plate acting as a ball
and socket joint. When the tower is buried in concrete in a typical ham
installation, the base is fixed rigidly in place and can't move, so the
movement resulting from the wind bends and twists the tower structure causing
undue stress that can eventually cause failure at the welds, and metal fatigue
on the rest of the tower structure, usually not a problem with a typical 60'
ham tower, but a cause for concern when you are going up 160'. For the reasons
mentioned above, I would be leery of using a separate insulator in each leg of
a rigidly planted tower, whether homemade or the individual leg insulators sold
by Rohn (also very expensive). When I was planning my tower over 30 years ago,
the Rohn engineers told me they did NOT recommend using those insulators for an
AM-type tower.
I would consider 30' of 25G or maybe 45G with the base buried in concrete,
heavily guyed at the top, and a heavy plate attached at the top. Then mount a
broadcast type base insulator on the top plate, attached to a 25TG section or
suitable substitute, and continue the guyed tower on up to the full height in
the normal manner, so that the base insulator pivots at the 30' level. I have
seen photos of AM BC towers with an elevated base insulator like that located
in flood-prone areas.
There has been a fellow at Dayton the past 2 or 3 years who custom builds heavy
duty tower hardware compatible with Rohn 25 and 45, and even finishes off his
work with hot-dipped galvanising. He brings examples of his work and I believe
he could easily fabricate a top plate section to mount atop the bottom 30' of
tower, and a base adaptor to custom fit a broadcast type base insulator to the
bottom end of a regular 10' section of 25G. I even suggested that he should
make base sections that would convert a regular tower section to the equivalent
of a 25TG. He said he had never had such a request, but that he could easily do
it if one provided him with the specifications.
The guy gave me his business card a couple of years ago, but unfortunately the
black hole in my shack has long ago sucked it up. Maybe someone reading this
will remember seeing this guy at Dayton and be able to tell you who it was.
Don k4kyv
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