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Re: [TowerTalk] Yagi antenna fundamentals

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Yagi antenna fundamentals
From: k1ttt@arrl.net
Date: Mon, 9 May 2005 14:21:24 -0400 (EDT)
List-post: <mailto:towertalk@contesting.com>
the 'appearance' of what they do is the reason for the names.

the how is the harder part.  because the elements are different lengths it
affects both the phase and the magnitude of the current on the parasitic
elements.  the longer or shorter they are the further from resonance at
the driven frequency they are, this affects the magnitude and phase of the
induced current so that the elements produce fields that add in one
direction and cancel in the other.  the best way to learn about this is to
model a 2 element yagi and watch the current in the parisitic element as
you sweep the frequency over a wide range so that the parasitic changes
from a 'director' to a 'reflector'... watch both the magnitude and phase
of the current in the element relative to the driven current and figure
out how it adds to the produce the pattern.

of course the other factor that affects the magnitude and phase of the
current is the distance from each of the other elements.  this is the even
harder part to simulate because each element's currents depend on all the
other element's currents and the distance away each element is.

> I am trying to understand why the so-called reflector has to be longer
> than the driven element and the so-called director has to be shorter than
> the driven element for the standard configuration Yagi. I have been
> reading antenna books and also trying to rationalize what is going on.
> Secondly, I am trying to rationalize why the longer element is called a
> reflector and the shorter element a director.  I posted this to Towertalk
> perhaps two years ago and only got simplistic (?) answers.  Hopefully some
> of the great gurus here can provide an explanation.
>
> I think that I understand the fundamental principle of the Yagi i.e. the
> induced currents in the parasitic elements cause a re-radiation in all
> directions.  For compass bearing points around the array, the currents in
> all of the elements are vectorially added.  In some directions the vectors
> will add, but in other directions the vectors will subtract.
>
> What bothers me is that the currents at the end of the elements are low so
> I would think that the ends would not have much effect on the pattern?
>
> I think that I have the answer for my second question.  For a resonant
> driven element, a longer parasitic element will have vectors on the side
> facing the driven element that will add to the vectors from the driven
> element.  Hence the "reflector" appears to reflect the wave.  For a
> shorter element the vectors on the side away from the driven element will
> add to the vectors from the driven element.  Hence the "director" appears
> to direct the wave.
>
> k7puc
>
> _______________________________________________
>
> See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless
> Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with
> any questions and ask for Sherman, W2FLA.
>
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>


_______________________________________________

See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless Weather 
Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions 
and ask for Sherman, W2FLA.

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