IT'S ALIVE!!! IT'S ALIVE!!!
An old Frankenstein movie line....
In 95+ degree heat (and humid), three locals helped me push it into the sky.
7:15AM breakfast until 6:30 PM Miller time. I lost 6 pounds in one day.
(at this rate, about a half a dozen more antennas and I'll be looking like
my college days).
Needless to say, I'm pumped! Why? Because the hours of modeling and
agonizing at the vertical mill and machine lathe paid off. Thanks for all
the suggestions and encouragement from the Towertalkians. It now stands at
67.5' above the Minnesota corn fields.
Ok ok. You guys told me it wouldn't work. It didn't. The original plan
that is. I was going to push small gauge tubing to 68'. I foolishly
brushed aside a few hundred years of collective towertalkian experience and
brashly professed "With enough guy supports and you can make a garden hose
stand to 68'." Go ahead. Say it. "I told you so, I told you so..." At
about 45 feet, it felt like I was handling a garden hose. With that out of
the way, here is what stands in my field...
A few weeks ago, I harvested some nifty stuff out of a dumpster. 13 planks
of 2" x 2" square fiberglass tubing -- 9' long, 1/4" wall. It's cool stuff.
I've never seen anything like it before. It's yellow in color and strong.
The cool part is that it is fiberglass--the bummer is that it's fiberglass
(non-conductive). It was used by Innotec to fabricate a temporary clean
room (at great expense no doubt).
Somebody on towertalk called me stubborn. Yep that's me. We grabbed a 500'
spool of #10 wire I have had here for about 4 years looking for a project.
It screamed to be pressed into service. I coupled 5 lengths of the tubing
together with 3 ft pieces of 1.5" square tubing, which had to be pressed in
with a rubber mallot--a very tight fit. Screw holes were drilled and tapped
near the junctions. The 45' length was clad on 4 sides with the #10 wire
using 1" x 1/2" black HDPE blocks (drilled on 2 axis) to support the wire.
#8 hardware attaches them to the fiberglass.
The top uses the chain link fence top rail--the swaged stuff. A 21' hank
and a 6' hank make up the top (pressed about 4.5' into the square) I made
shims out of some 1.5" diameter fiberglass tubes. I used the lathe to turn
down the inside diameter about .030" to make a tight fit into the square
tube. The whole shootin' match was hauled out to the field. I don't think
it weighs 50 lb.
I even found a PVC plumbing cap that fits the fence top rail material snug.
A tap with the rubber mallot and it's on for good. I have wondered how the
PVC will behave as a cap on a full sized 1/4 w/l vertical. For all I know,
it may vaporize in a few weeks of use.
The supports for the 40 meter resonant sleeves are made out of electric
fence supports (fiberglass again). Using the HDPE, these 4' long rods
support a pair of wires on each side of the main mast to 33'. Each side has
2 - #14 antenna wire held 8" apart. The spacing is 18" off the main mast.
Gary Breed K9AY suggested I leave lots of room to adjust the spacing to get
the feedpoint correct (thanks for the tip Gary). The pair of wires were
intended to simulate a larger diameter tube--more bandwidth was needed than
a single wire could produce.
The nifty part about this vertical is the ground. My XYLs job jar has had
"remove the rape pen" for some time. Without getting into details, it's a
small building now used as storage and a woodshed that was built to breed
pigs (25 years ago). The pen out back was made from 1/4" steel rod. 16'
rods were laid side by side and welded together with 1/4" steel rod spaced
every 6". The 6" x 6" grid makes a panel 54" by 16'. I had a bunch of them
(still do). I laid a 54" x 54" square of the stuff down around the 4 x 4
post that supports the antenna. I then welded 4 panels on each of the four
sides of the square. The ground forms a screen that is 54" wide in the
shape of an "X" around the tower. An 8' copper clad ground rod was driven
in and welded right to the grid. I was anxious to see how this plays
without extra radials over excellent ground.
Using a well casing as a boom, I fabricated two large hinges--one for the
antenna and one for the lifting boom. The guy anchors are set 25' from the
base (4 of them). All guys supports were measured and fitted the best we
could. The east set goes to the lifting boom instead of the anchors. I can
actually lift the thing with my bare hands! I use the old pickup to do the
job for me since I'm basically lazy. Once the lifting boom is at 45
degrees, I have to lower it the rest of the way by hand--better control.
One she's standing up, I transfer the guys from the boom to the anchor.
Done!
At the feedpoint, I wanted a good ground connection between the vertical
mast and the earth--lightning protection. I have looked at air gaps (one of
those is installed too) but didn't like the idea of forcing a couple
thousand volts on my feedline before she arcs. Instead, I use two 1:1
baluns. The first provides the coupling to the antenna. Unbalanced antenna
to balanced. The second provides balanced to unbalanced coax. Care was
taken to ensure that the antenna had a DC path to the ground rod.
Polarization is important at DC.
I need to shorten the antenna to what the modeling predicted -- 66' There
were enough unknowns in the model to strip my confidence in cutting the
length the first time. Same with the 40 meter sleeve. I modeled it to 33'
but installed it at 34' intending to trim. The spacing seems about right.
Over perfect ground, at resonance, the 80 stick should be 38 +/-j0. It came
in about 100 kHz low but 44 ohms at resonance. 3.500 = 45 +j5. When I
shorten it, it should be fine. On 40, the same feedpoint impedance was the
target. I set it to 18" and hoped for the best. At 7.0 it's 44 +j20.
Resonance is 6.9 or so. I need to drop it to shorten it a bit.
The big unknown was the diameter that the 4 - #10 wires spaced ~ 2" x 2"
apart. Mininec cannot model a 2" parallel wire at 80 meters. A single pipe
of 2.25" was chosen for the model. Changing it to 1.25" in the model
matched the results exactly. Anybody have any idea how to estimate the
effective diameter of a wire cage?
There you have it. I'll post the results of some testing once I have it
tweeked and we get some activity on 80 starting up. Thanks for the
bandwidth and all the help.
Ford-N0FP
ford@cmgate.com
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