[aprssig] Helix Groundplane?
noskosteve at yahoo.com
Fri Dec 14 03:24:04 EST 2007
--- Russ Chadwick wrote:
> That Steve Noskowicz wrote:
> >..."method of images" ...
> The "method of images", ... a phase term ...
Ah yes. Thanks for re-awakening that dark, distant past...
> ...ground plane and a vertically directed line of current...
> ...maximum gain in the horizontal.
Make it a 1/4 wave real element right "at" the plane and...
This can be viewed (no pun intended) as if the real element and the image
make up a standard dipole - oriented vertically, but "piercing" the plane. The
polarities (phases) work out as such. In other words, the phase of the Image
is opposite that of the real element, just like in a dipole.
This makes the comparison of a ground-plane antenna and a dipole easy to
Keeping the same phase reversal in mind...:
> ...horizontal...plane and a horizontally directed line of current
> above the plane...
> ...null in the horizontal gain broadside to the line of current.
> ...maximum gain in the vertical.
To visualize this, I consider a real, 1/2 wave dipole spaced 1/4 wave above
the plane. The image is another dipole, now 1/2 wave "below" the real one, but
opposite in phase.
First, being 180 degrees out of phase, there will be a null which is
broadside to this "Dual dipole array" (an Adcock - makes a simple DF broadside
Then, in the (vertical) plane containing the two dipoles, this gives
in-phase summation , or a two element, bi-directional "beam" also called an
end-fire array... which is radiating, mostly, perpendicular to the plane (up).
All fits my own "mental model" so far, but for the last part:
> A vertically directed helix ...horizontally directed current
> ...so I would expect ...
> ...increase vertical gain and decrease horizontal gain
While I follow Russ's reasoning 100%, I still have a problem resolving the
right-hand / left-hand issue. I have no mental model of the entire helix to
begin with, so I till can't resolve the effect of the handedness & image issue.
NOW.... For your listening pleasure. Drifting OT just a bit:
How come mirrors reverse left and right, but not up and down? (Move your
right hand and the image moves its left. Wiggle your toes and the image
doesn't wiggle its ears) How do 'they' know the difference between left-right
Lay down and the mirror "knows" you did...'cus it now reversed up/down.
73, Steve, K9DCI
73, Steve, K9DCI
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