On Fri, 20 Nov 2009 08:52:25 -0800, Jeff Liebermann
wrote:

On Thu, 19 Nov 2009 13:20:01 -0800, Richard Clark
wrote:

My model shows a more benign mismatch to a 72 Ohm load.
Richard Clark, KB7QHC

I built several models of the antenna folded dipole assembly. The
simple rectangular rod folded dipole yielded about 300 ohms. A
slightly better simulation of the rounded ends, but still using a
round rod, was about 260 ohms. Converting it to a flat wire ended up
about 280 ohms. I never got anything even close to 72 ohms. It's my
http://802.11junk.com/jeffl/antennas/mfj1800/

EZNEC shows:
Impedance = 73.13 + J 18.78 ohms
which, I suppose, could have the reactance driven out if I shift
frequency.

Show my your NEC2 deck and tell me what I did wrong, and maybe I'll
believe that it's 72 ohms. Incidentally, the possibility that I
screwed up somewhere in the model is quite real:
http://802.11junk.com/jeffl/antennas/mfj1800/mfj1800.txt
(I'll convert this mess back to a macro form so it's easier to read
maybe this weekend).

I don't have an option of NEC2 deck. One thing you might check, and
is something I reported about, is does your model have the loop
symmetrical to the plane of the directors/reflector? I followed all
of Mike's dimensions and I note that your lobe characteristics don't
show his lack of driven element symmetry - mine do. Again, I have
modeled only the three elements (Ref/Dr/Dir) as the additional
directors are unlikely to perturb drive point Z as much as to push it
from 73 Ohms up to your high 200s (triple?).

Another point, as I have described, I used 1/4 inch diameter wire in
place of larger flat sheet metal elements (which I note you try to
replecate, but only once). True, 1/4 inch is not as big as any flat
dimension, but as Roy reports on equivalence, flat is not the same as
diameter, but flat performance is closer to a smaller diameter round
wire. Hence the 1/4 inch.

73's
Richard Clark, KB7QHC