Topband: T Top Verticals and yagis

Fri Feb 28 17:08:09 EST 2020

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 at arrl.net> 
To: topband at 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 at contesting.com> On Behalf Of donovanf at starpower.net 
Sent: Friday, February 28, 2020 3:00 PM 
To: topband at 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 at arrl.net> 
To: topband at 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 at contesting.com> On Behalf Of Guy Olinger K2AV 
Sent: Friday, February 28, 2020 12:16 PM 
To: W7RH <midnight18 at cox.net> 
Cc: topband at 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 at 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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