Hi Paul,
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.
Been there. Done that...
73
Frank
W3LPL
----- Original Message -----
From: "Paul Christensen" <w9ac@arrl.net>
To: topband@contesting.com
Sent: Friday, February 28, 2020 12:45:19 PM
Subject: Re: Topband: T Top Verticals and yagis
I would hate to give up the efficiency of an over-resonated T, even if field
strength isn’t significantly better than an L design. I think two design issues
help to avoid pattern distortion to nearby HF Yagis.
First, as Frank suggested, keep the length of the horizontal T wire
non-resonant on any given band, including non-harmonically related bands like
30m. It's not necessary to select a T top wire length that results in resonance
at the base feed, at least with a single omni antenna.
Secondly, there's a difference in interaction between a T that's installed
broadside to a Yagi versus one that's installed perpendicular off the end of a
Yagi. For example, If a T is installed between two towers that are spaced 150
ft apart and a 40m Yagi is atop one of the towers, the T top wire is never
broadside to the Yagi. It's looking straight down a piece of wire.
It's no different than a Yagi director that's rotated 90 degrees to the driven
and reflector elements. In fact, an existing Yagi design file could be modeled
that way only with a rotational change to the director's Cartesian coordinates
and a change in distance from the DE. No doubt modeling will still show some
interaction but what remains should be cured with an optimum, non-resonant
director length.
Paul, W9AC
-----Original Message-----
From: Topband <topband-bounces+w9ac=arrl.net@contesting.com> On Behalf Of Guy
Olinger K2AV
Sent: Friday, February 28, 2020 12:16 PM
To: W7RH <midnight18@cox.net>
Cc: topband@contesting.com
Subject: Re: Topband: T Top Verticals and yagis
That’s an awful lot of effort just to keep a T top. There are a number of
effective alternatives to the traditional T top without the interaction
drawbacks. Inverted L is only the dirt simple one without the skip-zone-making
high angle hole in the pattern.
73, Guy K2AV
On Fri, Feb 28, 2020 at 9:46 AM W7RH <midnight18@cox.net> wrote:
> The discussion has involved horizontally polarized Yagis. Perhaps use
> a vertical 8 circle array on 40m! LOL And keep your T-Top!
>
> Bob, W7RH
>
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> W7RH DM35qj
>
> "It has become appallingly obvious that our technology has exceeded
> our humanity." - Albert Einstein
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