Topband: T Top Verticals and yagis

[Jim Brown]

On 2/28/2020 12:00 PM, donovanf at 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