I had a six element 20M Yagi on a 65 foot boom that was 200 feet away and
65 feet lower than my 40M Yagi.
If the 40m Yagi was pointed to Europe it was pointed at 20M tower. I could
turn the 20m Yagi so the boom was pointed at the 40m Tower and of course no
problem. I could watch the SWR on the 40M Yagi go higher and higher
peaking when boom of 20M Yagi was broadside to the 40m antenna.
Fortunately the 20M Yagi was trussed with 1/4" EHS and the simplest
solution was to fasten a thick G10 plate to the truss support and attach
the turnbuckles to that. Problem solved.
Now having more fun with much simpler stuff :-)
73...Stan, ZF9CW (K5GO)
On Fri, Feb 28, 2020, 5:08 PM <donovanf@starpower.net> wrote:
> 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
> >
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