[aprssig] Helix Groundplane?
russ at wxqa.com
Thu Dec 13 07:35:38 CST 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
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.
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