On Fri, 13 Nov 2009 07:28:34 -0600, "amdx" wrote:

My original thread seems to have died, still wondering how the folded loop
is matched to 50 ohms. (probably isn't? cheap, but works?)
I've added a dimensional drawing and some more pics.
If more info is needed let me know.

See:
http://802.11junk.com/jeffl/antennas/mfj1800/
The annotated NEC2 file is attached to the "Main" JPG. Cut-n-paste
should get you something to play with.

This is NOT an exact representation of the MFJ1800 antenna. The
elements are round, not flat. The driven element is a squared off
approximation. Some of the dimensions are questionable. Note that
the feed impedance is normalized to 288 ohms, the characteristic
impedance of a folded dipole, not 50 ohms.

Here is a drawing and some more pics.
http://i395.photobucket.com/albums/p...intFileJPG.jpg

I need an overall length, measured from the CENTER of the driven
element (the plastic screw hole) to the CENTER of the last director.
That's because a tiny error in the spacing between elements at 1.060"
(2.69cm) grows rather rapidly when multiplied by 14 elements.

NEC works in wavelengths or meters, not inches. Think metric and
forget about using inches.

Please check the length of the first and 2nd directors. I don't
believe MFJ would make them the same length as the other directors.

Also, measure the coax balun cable dimensions. Mostly, I'm interested
in the:
1. Shield to shield length.
2. Center pin to start of folded dipole length. (i.e. exposed center
wire length).
3. OD of center conductor wire.
4. ID of shield.
5. A good guess as to the type of dielectric (foam, solid, or PTFE).
6. Any markings that might identify the coax.

I'll do a better job of modeling the folded dipole driven element
later. Modeling all the asymmetrical elements is going to require
more time than I want to spend. Maybe if I get inspired to RTFM. It
seems easy, but looks tedious.

What I've deduced from the model so far is:
1. The characteristic impedance is over 300 ohm without the coax
balun/matching/whatever section.
2. A 1/4 wave coaxial matching section isn't going to work as the
highest commonly available impedance coax of 93 ohms will only match
50 ohms to about 173 ohms. That thing doesn't look anything like a
4:1 balun (unless there's something we missed under all that shrink
tube).
3. With a suitable 4:1 balun, the antenna is slightly shy of the
advertised gain spec of 15dBi:
http://www.mfjenterprises.com/Product.php?productid=MFJ-1800
with a gain of 14.3dBi. Close enough.



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