The interaction of a grounded steel tower, I have found, can be
minimized somewhat by having the end of the 80 meter antenna more than
25' away from the structure and supported with non conducting material
like braided nylon or Phylistran and having the angle of declivity at
least 45 degrees or even less. There may also be some useful
directivity in the direction of the slope caused by the tower acting as
an a periodic reflector. Another way of of feed the 80 meter section is
at the bottom to ground by making the wire 90 feet of a 3/8 wave
vertical sloper and fed to a ground system consisting of some radials
and a series capacitor to remove the inductive reactance. The feed will
be close to 60 ohms for a coax feed at that point. Doing this would
remove feed line interaction with both 160 and 80. Additionally it might
be possible to bring the 80 mete bottom back to the 160 feed point and
be able to have a common coax for both the 160 meter match (if it also
is just a series capacitor to a shunt). I am somewhat reluctant to
advise something I have never tired this way...but the 90' bottom fed
wire sloped off a 90 foot tower with the 25 foot insulated extension
works well in the past for me with it's own coax feed.
73,
Herb Schoenbohm, KV4FZ .
On 8/26/2013 12:49 PM, N1BUG wrote:
I've done exactly what Herb suggested: hung a sloper for 80M off the
side of my 160M shunt-fed tower. Neither antenna knows the other is
there
and both work pretty well - meaning I'm satisfied with what I get out of
them.
Interesting. I have tried verticals, slopers, inverted V's and other
80 meter antennas on or near my 160 meter tower with little success.
The impedance of any such antenna seems to be severely altered by the
nearby tower. I suspect it depends on the electrical length of the
tower. Mine is close to an electrical half wave on 80 - 100 feet of
Rohn 25 with a 7 element 6 meter yagi, approximately 30 foot boom
sitting at 103 feet.
At one point in time the 160 meter shunt feed could be made to provide
an excellent match on 80 simply by changing the series capacitance.
However, after I cut down several nearby trees that no longer works.
73,
Paul N1BUG
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Topband Reflector
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Topband Reflector
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