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Re: Topband: T Top Verticals and yagis

To: topband@contesting.com
Subject: Re: Topband: T Top Verticals and yagis
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Fri, 28 Feb 2020 15:52:58 -0800
List-post: <mailto:topband@contesting.com>
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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