On Fri, 20 Nov 2009 10:46:40 -0800, Richard Clark
wrote:
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.
No problem. Just post or email me the .EZ file. 4NEC2 will import it
or I'll just switch to EZNEC. (I promise not to complain about your
using inches).
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?
Unfortunately, yes. I got lazy and planted the folded dipole centered
about the axis of the other elements. I don't think (guess) it would
make much difference, but I'll move it to the correct position this
weekend.
Got any theory as to why the vertical and horizontal patterns are so
different? That only appeared when I switched to the flat wire folded
dipole. They were symmetrical with the wire rod driven element.
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?).
Yes, something is different. In past tinkering, I've found that 3
elements is sufficient to characterize the feed impedance, where the
additional elements just improve the gain and pattern.
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.
Well, I used a rod with the same circumference as the flat
(asymetrical) elements. Methinks the element diameter would have an
effect on the bandwidth of the antenna, but not on its characteristic
impedance.
73's
Richard Clark, KB7QHC
thanks much...
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558