On 2/28/2020 12:00 PM, donovanf@starpower.net wrote:
If you model -- or build -- a 40 meter Yagi with a nearby T-top vertical
with a 60-75 foot top, you'll discover that when the 40 meter Yagi is
turned so that its elements are parallel -- or near parallel -- to a nearby
T-top the impact on the performance of the 40 meter Yagi is severe.
There are lots of possibilities for interactions between antennas,
towers, and even feedlines. I use chokes as "egg insulators" in the coax
feeding my high (120-130 ft) dipoles to prevent their interaction with
160M verticals. Any dipole (or Yagi) can interact with an antenna
operating on the dipole's second harmonic (for example, 40 interferes
with 20). A stub designed to kill second harmonic on the lower frequency
antenna and properly placed in the line from it to maximize suppression
will also minimize that interaction.
Some of the best advice I've gotten about interaction came from old
hands N6BV and NI6T when I described the 120 ft tower I'd installed soon
after moving here. I rigged two wires sloping at 70 and 270 deg az, fed
from their base against elevated radials, with the tower acting as a
passive reflector. I spent a summer modeling interactions between that
tower, those antennas, and Tee vertical about 200 ft away. What I
learned was that the tower and sloping wires acted as a reflector for
the Tee, yielding a few dB to VK/ZL, the Tee acted as a reflector for
the sloping wires, kicking the 270 az wire to JA, and the 70 az wire to
EU. For all of this to work, the antenna not being fed needed to be
shorted, so I tweaked feedline lengths and added a stub to one antenna
and shorting the others in the shack.
73, Jim K9YC
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