Roy Lewallen wrote:
I'm getting pretty convinced that the problem is the use of lumped loads
for the inductors. With this short an antenna, I'd expect the inductor
currents to be quite different at the ends(*), making the lumped load
models inadequate. This can lead to pretty severe errors.

(*) due to inductor radiation and unsymmetrical coupling of the inductor
to the rest of the antenna and to ground.

Over on qrz.com, W8JI reported that he measured a 60 degree phase
shift through a 100 uH coil at 1 MHz. He also asserted that the
flux density is highest in the middle of a coil. Since the current
is proportional to flux density, that means the current in the
middle of the coil is higher than at the ends. These things are
perfectly consistent with what EZNEC reports when the distributed
network helical coil inductor is used instead of the lumped circuit
load inductor.

Essentially the only time the currents at each end of the coil are
equal is when it is installed near a standing-wave current maximum
point where the slope of the current is already close to
zero whether it be in a wire or in a coil. The phase of the standing-
wave current is relatively constant whether it be in a wire or
in a coil. (The standing-wave current doesn't rotate like a normal
phasor.) The phase shift caused by the coil happens in the forward
and reflected currents, not in the standing wave current which is the
sum of the forward current and reflected current. Not much
changes when part of a wavelength of wire is replaced by a large
loading coil. The current waveform, though warped somewhat by the
high fields inside the coil, still very roughly follows the classic
cosine shape of a wire. After all, no matter what, the current at
the tip of an antenna is zero whether it be a wire or a coil. If
a coil is placed at a standing-wave current node, the phase at each
end of the coil will be opposite, i.e. current is either flowing in
both ends at the same time or out both ends at the same time. Such
is the nature of distributed networks.
--
73, Cecil http://www.qsl.net/w5dxp