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[TowerTalk] Modeling

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Subject: [TowerTalk] Modeling
From: (Tom Rauch)
Date: Tue, 5 Feb 2002 19:56:02 -0500
> 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?

1.) The current induced in the second element by mutual coupling  
is reversed from the direction of current in the driven element. 

2.) If you make it longer than resonance, phase will lag more (it will 
act like it has a series inductor).

3.) If you make it shorter than resonance, phase will move towards 
a positive direction.

What happens as we move towards the reflector is the wave is 
delayed in space by the reflector element spacing. Now if the 
reflector is set VERY close to resonance, it reradiates 180 degrees 
out-of-phase (remember current moves the opposite direction as 
the exciting wave)  with the energy coming at it from the driven 
element. That is why radiation cancels behind the self-resonant or 
nearly self-resonant reflector element.

The director is tuned reasonably far off resonance and leads phase. 
Since it inverts phase 180 from reradiating, plus it leads phase from 
being short, the total radiation is the equivalent as if it were fed with 
lagging phase!

Antennas fire in the direction of lagging phase, so it fires towards 
the director.    

73, Tom W8JI 

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