"Roy Lewallen" wrote in message
. ..

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, see
http://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

Thanks for the info. A very interesting link. I have experimented
with NEC models of phased arrays, and found the same problem
with a small back lobe. I followed the procedure in the ARRL
Antenna book, which involved calculating the elements in a 2 X 2
complex Z matrix, for a 2 element phased dipole array. The
results appeared to be very good, but I never actually built it.

Frank, VE6CB