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[aprssig] Helix Groundplane?

Russ Chadwick russ at wxqa.com
Thu Dec 13 13:35:38 UTC 2007


Steve Noskowicz wrote:
>  
>    Then...then  Approaching this from another school of thought, if the plane
> really is a "ground plane", it MUST have a reflection.  An image, as we learned
> in fields class, using the "method of images" (a mirror is a perfect analogy),
> WILL have an effect on the antenna pattern and feed point Z.   Stop me, stop
> me, my brain hurts.
>
>   
The "method of images", mentioned above by Steve, also has a phase term 
that causes things to work like this (for the correct length, spacing 
and frequency):

Given a perfect horizontal ground plane and a vertically directed line 
of current, the fields of the current and its image will interfere 
constructively in the far field to produce a maximum gain in the horizontal.

Given a perfect horizontal ground plane and a horizontally directed line 
of current above the plane, the fields of the current and its image will 
interfere destructively in the far field to produce a null in the 
horizontal gain broadside to the line of current.  They will interfere 
constructively to produce a maximum gain in the vertical.

A vertically directed helix of current (depending on the pitch) will 
have predominately horizontally directed current and so I would expect 
the ground plane to increase vertical gain and decrease horizontal gain 
over that of the radiating element alone.

Russ   KB0TVJ







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