On Dec 28, 3:32*pm, Roy Lewallen wrote:
The "Method of Moments" (MOM) makes no assumptions about
current distribution on a radiator; *it computes the current distribution.
The radiated field is then calculated based on the current distribution..

Frank

Exactly correct. EZNEC (including demo) users can get a good
illustration of this by looking at the pattern from the example file
Cardioid.ez which consists of two identical elements with perfect
spacing and perfectly ratioed base currents. Notice that the front/back
ratio is about 35 dB (dropping to about 31 dB with more segments for
more accurate results), while it should be essentially infinite. The
small back lobe is caused by modification of the current distribution on
the two elements caused by mutual coupling -- although the base currents
are perfectly equal in magnitude and 90 degrees out of phase, the fields
from the two elements aren't, due to their having slightly different
current distributions. When I first saw this back lobe with perfectly
ratioed base currents, I searched through the program code looking for
what I was certain was a bug. I finally realized that the result was
correct and that the lobe was caused by the altered current distribution
which the program had correctly calculated. For more about this, seehttp://eznec.com/Amateur/Articles/Current_Dist.pdf.

The assumption of sinusoidal current distribution is strictly true only
with straight, isolated conductors which are infinitely thin, although
it's a reasonably good assumption in many other cases. Mathematical
analyses of antennas done before computers were pretty much limited to
cases where sinusoidal distribution was assumed, because a more accurate
determination of current distribution was virtually impossible to
calculate. The ability to determine the actual current distribution is
one of the very important advantages of computer analysis.

Roy Lewallen, W7EL

If current distribution is calculated correctly as stated then the
answer with respect
to the route taken of the current of a fractional wave antenna should
be available and beyond doubt
as the program is derived from Maxwells laws. The distributed current
should be DC based if current flow is thru the center
of the radiator. As far as the resistance encountered on reverse flow
on the outside of a radiator the figure provided by computor programs
should be rather interesting as I have never encountered in print
suggested figures. In accordance with some engineers the radiation
resistance but be stated as the radiation impedance to avoid sniping
Art