On Oct 24, 11:53 am, "R. Fry" wrote:
"Michael Coslo" I'm still left with the increased bandwidth phenomenon. None of the above
would seem to account for this.

____________

The reactance of a conductor with a relatively large cross-section changes
slower with a change in frequency than one having a small cross-section.

Therefore its impedance bandwidth remains below a given limit over a greater
frequency span than one of a smaller cross-section.

RF

If you plot the feedpoint Z over frequency as R and X on a
rectangular scale (not a Smith chart), you get a spiral. thin
antennas have a big spiral crossing the R axis at X=0 very steeply
(implying narrow match bandwidth), while fat antennas have a smaller
diameter spiral.

As someone else has pointed out, the spiral eventually converges to
something like R=377 when frequency is very high.

The "why" for all of this does not admit of a simple explanation.
(thereby providing nice grist for EM textbook writers, and brain
bending work for EM students)
It's the trying to understand why (the actual measured data has been
around for at least a century) is what prompted the work of folks like
Schelkunoff, Hallen, King, and others. They came up with good answers
for very specialized cases (inifinitely thin wires, conical antennas,
etc.). The fact that "real" antennas tend not to look like the
idealized ones with the analytical models led to the development of
finite element methods, in particular, the Method of Moments, which
NEC and it's ilk are based on. The idea had been around for a while,
but fast computers made it possible to do for interesting non-trivial
cases.

There are similar analytic models that are "pretty close" for Yagis,
for instance.