----- Original Message -----
From: "Steve London" <n2icarrl@gmail.com>
To: "Topband" <topband@contesting.com>
Sent: Friday, January 11, 2013 4:12 PM
Subject: Topband: 160 shunt fed tower update
When we last left this adventure, I was shunt feeding my 110' tower on
160, which supports a multitude of yagis and wires. Unfortunately, I was
coupling so much energy into the 80 meter wire array that the 80 meter
baluns were heating up. There were a number of interesting and useful
suggestions on this reflector.
As an experiment, I disconnected all of the feedlines going from the 80
meter switching/tuning box at the top of the tower to the individual
elements of the 80 meter antennas. The 80 meter wires were still in place,
just not connected to the tuning/switching box. That solved the balun
heating problem, and greatly changed the shunt feed point and spacing
needed for 50 + jX at the tower base feedpoint. I left it that way for
several weeks, while I evaluated how well the shunt fed tower actually
worked. Based on comments from topband readers and performance in the Stew
Perry, I was satisfied it was working effectively.
The next step was to build an insulated bracket to support the 80 meter
switching/tuning box to keep all of the 80 meter coax lines electrically
insulated from the tower. No torroids. No grounding of the 80 meter
feedline to the tower. I was hoping this would keep the 80 meter array RF
isolated from the shunt fed tower. No such luck. The shunt feed needed
significant adjustment. There is significant coupling, despite having no
DC connection. The good condx to EU most evenings this past week have been
very frustrating as I struggled to work "easy" stations that were no
problem to work in the past with the old sloping dipole. I have to
conclude that the coupling is having a detrimental effect.
Yes, I could experiment with torroids, etc., but my conclusion after
reading comments here is that nothing is guaranteed to fix the 80 meter
coupling problem, short of removing all the 80 meter wires.
Back to the drawing board. Maybe a dedicated 160 tower is in the works.
Hope to work some of you in the NAQP. Listen for the weak signal from New
Mexico !
73,
Steve, N2IC
I dont know what you are using for 80M Steve and why all the feeds, etc are
located at the top of the tower.
In the 80's I had a 100' Rohn 25 with a stack of yagis and shunt fed on 160
which worked well once I went to 4' x 50' galvanized 2"x4" rabbit fencing
running from the base as spokes and with lots of interconnecting wires
between the sections.
Then I worked on improving my 80M signal with an inverted vee, 3 switched
slopers and a delta loop. The vee feed was taped to the tower, the sloper
feeds came to ground where the relay box was, and the delta loop was about
6' from the tower and the feed went to ground level. I believe the loop was
in place with the slopers for awhile but not certain; I think I tried using
it as a reflector. About 60' and 125' away were 80M phased verticals.
Nothing affected 160M tuning or performance that I remember and if so the
tuning was very minimal. That Omega match shunt feed worked great in all
directions. The delta loop was the final choice for 80 and its horizontal
signal complimented the verticals when needed. I moved before I could try a
reversible 2 el loop.
Carl
KM1H
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Topband Reflector
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