My 160 transmit antenna is a shunt fed vertical (overall DC/RF height is
about 116 ft) consisting of 94 ft of Rohn 45 guyed at three levels with
1/4" EHS, 15 ft of mast & a VHF vertical. Ground system is about 90
radials in a small yard...about 30 to 70 ft long. At the top is a CC
shorty forty, a KT-34A, long boom 2M & 432 Yagis.
The shunt rod attaches at about the 70ft level.
Impedance was measured and adjusted using a broadcast type Delta OIB-1
Bridge. A simple matching system using a series vacuum variable feeds
it. It has played exceptionally well.
Last year I added a second fixed, side-mounted KT-34A at about the forty
foot level. I expected some interaction. To my surprise, an impedance
sweep of the 160 feed point indicated essentially the same parameters as
before the addition of the side-mounted Yagi. After a years worth of
contesting & DXing on 160 with this arrangement, I have come to the
conclusion that the system performs the same as before.
My suspicions are, that at the top of the tower, the CC shorty forty is
the main constituent of the capacity hat and the other Yagi's are just
"along for the ride." The second KT-34A way down the tower seems to have
little or no effect on the RF characteristics of the structure.
Except for the impedance measurements, my results are anecdotal (Had I
thought of it, I would have taken field intensity measurements). It
would be interesting if someone could model the system before and after.
73...Don, K9NR
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