On Sep 30, 1:24*am, Owen wrote:
So, back to the electrical performance, do you have measurement data
or can you refer me to articles that contain sound objective measurement
data that would suggest that my NEC4 model is not valid.

This topic was investigated experimentally quite some time ago by a
broadcast consulting firm in the US, which generated measured data.
Here is a clip from their paper describing the system tested, and the
results (note that the convention used for "efficiency" here is that
of the FCC practice based on the groundwave field intensity at 1 km
with respect to the power applied to the antenna system):

\\ In November of 1988, our firm supervised the construction of a
temporary antenna system in
Newburgh, New York under FCC Special Field Test Authority using call
sign KPI-204. The antenna
system consisted of a lightweight, 15 inch face tower, 120 feet in
height, with a base insulator at the 15
foot elevation and six elevated radials, a quarter wave in length,
spaced evenly around the tower and
elevated 15 feet above the ground. The radials were fully insulated
from ground and supported at the
ends by wooden tripods. Approximately ten feet above ground, a T
network for matching the antenna
was mounted on a piece of marine plywood to isolate the components
from contact with the lower
section of the tower which was grounded. Power was fed to the system
through a 200 foot length of
coaxial cable with the cable shield connected to the shunt element of
the T network and to the elevated
radials. A balun or RF choke on the feedline was not employed and the
feedline was isolated from the
lower section of the tower. The system operated on 1580 kHz at a power
of 750 watts.

The efficiency of the antenna was determined by radial field intensity
measurements along 12 radials
extending out to a distance of up to 85 kilometers. The measured RMS
efficiency was 287 mV/m for 1
kW, at one kilometer, which is the same measured value as would be
expected for a 0.17 wave tower
above 120 buried radials.

The Newburgh tests gave empirical proof that the elevated system
worked although, in an abundance
of caution, we used six radials instead of four. For the limited time
that the system was operational, the
system was stable as determined by monitoring the field intensity at
selected locations each day. The
measured base impedance was in general agreement with a tower of this
height above a standard,
buried, ground system. Results of the KPI-204 tests were submitted to
the FCC in January of 1989.//

The complete paper is available at this URL:

http://www.commtechrf.com/documents/nab1995.pdf

RF