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[TowerTalk] Modeling

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Subject: [TowerTalk] Modeling
From: (Bob Nielsen)
Date: Tue, 5 Feb 2002 10:12:14 -0800
On Mon, Feb 04, 2002 at 09:08:30PM -0800, Al Williams wrote:

> > There is a reason that 12M Yagi's interact with
> > tribanders and 10 and / or 15 meter beams...
> What is the mechanism that causes an element longer than the driven
> element to re-radiate in the same direction as an element shorter
> than the driven element but on the opposite side?
> Please, answer not too heavy in math, physics, and geometry but also
> not simplistic.

Fairly simplified version:

Each element radiates in both directions, but the resulting pattern is
the vector sum of the radiation of all of the elements.  This is
dependent on the location of the elements and the phase of the
radiation from each element.  

A parasitic element radiates because the radiation from the driven
element induces a current in the parasitic element.  This is known as
mutual coupling.  If the parasitic element is longer than resonant
length the phase of the re-radiated pattern from that element is such
that the vector sum of the patterns of it and the driven element will
add in the direction away from the parasitic element (hence the term
reflector) and subtract in the direction toward the parasitic element.

Likewise, if the parasitic element is shorter than resonance the
patterns will add in the direction of the parasitic element (director).

Bob, N7XY

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