How to make short self supporting decent performance 80 and 160
verticals is an *interesting* design problem ;) . My goal is to make
one for 160m so I can get away from a dense tree area of RF absorbers
and T supports.
For 80m, a great vertical I had at a prior QTH was 40ft of 3" irrigation
pipe (guyed) on top of a metal barn roof. That design had a capacity
hat of 8 x 6ft 0.375" dia 6061 tubes and a perimeter wire. I think that
would scale easily to 40ft of R25 and perhaps a small stub mast to
support the hat and tune to lower in the band. 40ft of R25 is ok self
supporting. It you only have 28ft then add some 2" mast which will have
lower wind load than R25.
One mistake I made was to use solid Al wire as the perimeter for the
hat. It fatigued and broke from wind vibration. A high flex copper
antenna wire some clips that reduce the strain at the hat tips
connections would be better. The pipe was base hinged so came down
easily for fixing. An R25 version should have a base hinge, but you
will need a falling derrick rig to raise and lower the assembly based on
short R25 towers we used on Field Days.
Grant KZ1W
On 12/30/2021 02:00, N1BUG wrote:
Hi Towertalkers,
I spent much time this summer rebuilding after storm damage last winter.
As part of that I upgraded from inverted V's at 90 and 105 feet for 30m
and 40m to an OptiBeam OB1-4030 at 108 feet. I expected improvement but
Wow! I wasn't prepared for what I observed! The old antennas on another
tower were left up for a few weeks to make comparisons. Unbelievable. I
now consider "inverted V" roughly synonymous to "dummy load".
So... 80m is now my worst performing band, still using an inverted V
there. I can work stuff with a shunt fed tower on 160 that I cannot work
on 80. I'm thinking a quarter wave vertical for 80 would probably be an
improvement and surely no worse. Plus it would eliminate a set of ropes
that are somewhat in the way of other projects. But I don't want to have
another guyed structure for reasons I won't get into for now.
I looked at the DX Engineering self supporting 80m verticals. Nice, but
they won't fit the budget any time soon.
So here is a question: Would 30 feet (real height 28 feet) of Rohn 25 be
OK freestanding with a 35 foot whip on top? I have a 30 foot whip made
from aluminum tubing, tapering from 1.5" down to 1/2". It could be
extended to 35 feet with a section of 1.75" OD 0.120 wall tubing I have
laying around. I'm confident this whip would be fine, but not sure the
Rohn 25 would be OK self supporting with this on top. A 1992 Rohn
catalog says 6.4 square feet of antenna with no ice, dropping to 2.5
square feet with 1/2" ice. That is for 70 MPH wind and assumes one 5/8"
feedline on each tower face. There would be no feedlines and being
sheltered by trees this thing would never see 70 MPH winds. 50, maybe.
Any experience or wisdom on this?
My main concern is not that this thing would come crashing down, but
that every Rohn 25 tower I have ever taken down that did not have guys
on the topmost section had seriously elongated bolt holes. I'm not sure
what the deal is there since Rohn seems to be saying they should be OK
with small antenna loads.
Another option might be 28' of tower, air wound loading coil, and
somewhat shorter whip.
Base insulators could be a problem (budget-wise) but I see no reason not
to ground and shunt feed a 1/4 wave vertical. I wonder if it might even
cover CW and SSB by adjusting the shunt feed capacitor without needing
to move the tap point on the vertical? Comments?
Happy New Year to all and thanks in advance for any comments or
suggestions regarding my vertical thoughts.
73,
Paul N1BUG
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