I've tried the modeling, the results are confirming the theory if read
correctly.
First of all I've to say that since I agree the shorted coaxial line
approach, I did avoid to spend too much time to exactly model Ken's tower
and antenna.
The model arranged has been a 30mt square tower made with 2 sections of 15
meters and the feed cable has made runing inside the lower of the sections.
The feed connection is at the joint of the 2 sections (15m from ground) and
modeled made using four arms connected the tower corners.
Each section of the tower corner is connected to the adiacent at the base,
at mid heigt, and at the top.
To further simplify the model a perfect ground was modeled and the yagi on
tower's top replaced by two opposite horizontal legs.
The first noticeable result is that nearly nothing radiates (no current) in
the upper portion of the tower (up from the feed), the second is that the
antenna impedance is that of a shorted line, nearly a pure reactance, .8
+j130.
Inmedance that fully confirms what expected.
Now, what comes out and is in line with Tom's words is that practically
speaking some radiation take place (eff. about 5%) and from a tower that's
close to a full quarter wave the radiated energy it's not so small for
making qsos.
I'm afraid this "efficiency" derived, in the model, because of inherent
imperfect screening and unwanted modes other than TEM, or "spillings" but I
couldn't evaluate like that how much it was one and the other.
To have an idea about it I tried a quick check using different materials for
different parts of the tower or placing resistive loads (imperfect joint) on
the corners. In both case the currents on the outside changed significantly
likewise the radiation efficiency.
As a curiosity, It could be intersting knowing which capacitor Ken used to
match such inductive load to a 50 Ohm and about the bandwith of such system.
73,
Mauri I4JMY
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