You guys are way above my head :-)
I had no idea the resonant freq goes up and down as the antenna travels
on its way up.
I thought it just went up on a steep curve till about 15 feet then
gradually after that.
I can see why some head scratching as we pass thru the "decrease zone"
"what did I do now" hihi
73,
Bob
K6UJ
On 6/1/16 2:11 PM, jimlux wrote:
On 6/1/16 1:45 PM, Hans Hammarquist via TowerTalk wrote:
It seems to me that Steve's, G3TXQ and Dan's, AC6LA diagrams coincide
pretty well. You find the lowest frequency at about 1/4 wave and max
frequency at about 1/2 above ground. What I don't understand is how
the frequency start to decrease at the high increases before it
reaches 1/4 wave above ground. In my simple mind the fx should start
to increase as soon as the antenna "leaves round".
To me, and obviously I'm wrong, the capacitance to ground should
affect the frequency with the result that the fx should increase when
the ground capacitance decreases up to the point where speed of light
start to have an effect.
Anybody with a "simple" explanation?
It's basically "not simple" but what you're really looking at is the
mutual impedance of the dipole with the "image" in the soil. If you
look at those graphs of mutual Z for two dipoles with variable
spacing, you'll see a lot of similarity.
What you see as the feedpoint impedance is the combination of the
impedance of the dipole itself in combination with the mutual
impedance of the image antenna.
And the "other antenna" is a very lossy one.
With a fair amount of math:
Chapter 23 in Orfanidis's book http://www.ece.rutgers.edu/~orfanidi/ewa/
http://www.ece.rutgers.edu/~orfanidi/ewa/ch23.pdf (page 1057, figure
23.3.2)
There's a simpler plot in the ARRL Antenna book, but it's not online,
and I don't have it handy.
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