If a 40m yagi points towards an 80m or 160m dipole in its neighbourhood the
yagi pattern will be ruined as well.
73
Peter
-----Original Message-----
From: Topband [mailto:topband-bounces+dj7ww=t-online.de@contesting.com] On
Behalf Of donovanf@starpower.net
Sent: Freitag, 28. Februar 2020 23:08
To: topband@contesting.com
Subject: Re: Topband: T Top Verticals and yagis
Hi Paul,
A little modelling will answer the question more completely and
accurately than I can answer it in writing.
I was referring to the case where a 40 meter Yagi is on one tower and
a T-vertical with a top length of 60-75 feet is suspended between the
tower and a fairly nearby second tower. Lets say -- for the purpose of
discussion -- that the two towers are in a north-south line.
If you point the 40M Yagi East or West (perhaps +/- 45 degrees), the
top of the T-vertical will be either below the Yagi or off to its side.
If its less than 70 feet below the Yagi, the T-top will significantly
degrade
the performance of the 40M Yagi. Its is off to the side of the Yagi with
a separation of at least 35 feet or so, degradation of the Yagi will be
minimal. Detailed evaluation is best done by modelling, its very easy
to do.
73
Frank
W3LPL
----- Original Message -----
From: "Paul Christensen" <w9ac@arrl.net>
To: topband@contesting.com
Sent: Friday, February 28, 2020 8:36:56 PM
Subject: Re: Topband: T Top Verticals and yagis
Frank,
Perhaps you can clarify what you mean by "parallel." Are you referring to
the T top in parallel and *in front* of the 40m Yagi? Or, do you mean in
parallel with the Yagi but off to the side by 90 degrees as in the case of a
T strung between two towers, with the 40m Yagi atop one of the towers? In
both cases, the T is in parallel with the Yagi.
A picture would speak a thousand words here, but asking the question in
words is the best I can do!
Paul, W9AC
-----Original Message-----
From: Topband <topband-bounces+w9ac=arrl.net@contesting.com> On Behalf Of
donovanf@starpower.net
Sent: Friday, February 28, 2020 3:00 PM
To: topband@contesting.com
Subject: Re: Topband: T Top Verticals and yagis
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
>
> --
> W7RH DM35qj
>
> "It has become appallingly obvious that our technology has exceeded
> our humanity." - Albert Einstein
>
> _________________
> Searchable Archives: http://www.contesting.com/_topband - Topband
> Reflector
>
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