I did some cleanup, error correcting, and tinkering with the model of
the MFJ-1800 2.4GHz Yagi-Uda antenna. Mostly, I tried to make the
model as close to the original antenna as possible and recovering the
macros to make it easier to tweak:
http://802.11junk.com/jeffl/antennas/mfj1800/
The NEC2 file is under the "main" mfj-1800.jpg page. Cut-n-paste
works.

Data sheet, of sorts:
http://www.mfjenterprises.com/Product.php?productid=MFJ-1800

One remaining issue is that the stamped 2mm sheet aluminum does not
have a circular cross section suitable for easy modeling. By trial
and lots of errors, I finally settled on a diameter equal to the 2mm
thickness of the sheet metal. This is probably not correct, but does
result in the best looking gain pattern and VSWR curve.

My thanks to Richard Clark KB7QHC for help with the cross section
problem, and answering my questions.

Note that the antenna shows about a 200 ohms impedance at mid band.
That settles the debate as to whether the antenna, without the current
balun, is 50 or 200 ohms. It's 200 ohms and the 1:1 current balun is
quite inappropriate for matching or whatever. The exposed center
conductor forms a series inductance or radiator, and certainly doesn't
help. With a proper 4:1 matching xformer, the antenna seems quite
usable, but as is, it's junk. If I had one to play with (and my
HP8620c sweeper elects to cooperate), I could sweep the antenna to be
sure.

The coax is RGS-303 PTFE 50 ohms:
http://wireandcable.thermaxcdt.com/item/high-performance-coaxial-cables/mil-c-17-coaxial-and-twinaxial-cables/m17-111-rg303-id-70-?&seo=110

Additional photos:
http://s395.photobucket.com/albums/pp37/Qmavam/

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558