Topband: Full size Rohn 25 vertical vs. shunt fed tower

[Milt Jensen, ARS N5IA]

Ken,

	I first started using a gamma matched 80' tower with a Hornet TB-1000
on top back in 1973.  It worked well and I had a lot of fun with it for a
number of years.

	In 1991 got serious about 160 Meters and erected a separate Rohn 25 tower
of 120' height.  I gamma matched it, and it worked a bit better than the
original mentioned above.  I installed eight elevated, insulated radials at
about 10' height and the performance improved a bit more.

	I have tall (60' or so) mulberry and pecan shade trees here and even though
they are deciduous, I concluded that a good portion of my RF was being 
eaten up by the trees.  In 1995 I set out to improve the 160 transmit antenna
to a model which would the "best" I could construct on my small (125' x 190')
lot.

	After much studying, I concluded that the "best" antenna I could erect
would be a full sized 1/4 wavelength Ground Plane which would put the feed
point at or above the top of the shade trees.  To make a long story short, I
split the existing 120' tower apart at a convenient section (67' above
ground) and inserted a homebrew insulated section.  I constructed the
insulated section from surplus electric power switch gear insulators.

	I then added sections to the tower to make it 125' above the insulated
section.  This makes the total structure 192', keeping it under the 200' FAA
limitation for non painted 7 lighted or strobed towers.  The three anchor
points are 5" steel pipes concreted 6' into the ground and extending 6'
above ground.  They are filled with rebar and concrete also.

	The guy wires are broken up with insulators at 57' intervals.  The four
elevated radials are insulated from the tower structure and each radial is
composed of four, # 12 copperweld wires in a 1 FT. square cage
configuration.  The radials go out at a slight downward angle to the tops of
55' high wooden poles.  Feed point impedance = 37 Ohms.

	In all aspects this antenna is a "classic" Ground Plane.  The results?
Absolutely superb performance.  I seriously doubt if a single element
antenna could be more effective that this design.  Perhaps another percent
or two improvement could be squeezed out by having more radials, but the
incremental increase is not worth the time and expense of doing so.

	So, with all that said, get as much as you can up as high as you can and
you will be very pleased.

	73 de Milt, N5IA




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