On Tue, 17 Nov 2009 18:35:15 -0800, Jeff Liebermann
wrote:

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.

My model uses 1/4" elements, and for just the loop, Z = 150 Ohms @
2250 MHz

For the Reflector, driven loop, and director, the Z = 72 Ohms @ 2450
MHz

The loop ends are 8 segments(22.5 degrees). EZNEC complains of
len/dia ratio being too small, but there's nothing to be done about
that. 20 wires total, 62 segments total (for just the first three
elements).

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

I went with the dimensions provided - inches no problem.

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.

This is imparting too much engineering for their product.

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.

THIS is where error is going to intrude, big time. I think there's
too much attention to this detail for what it is supposed to do - but
I have already written on that subject.

To answer the original question, the folded loop Z drops due to the
proximity of the other parasitic elements (a normal consequence).
However, as to calling it a folded loop seems to be straying from
conventional usage as those loops are rather sweeping (large). This
may be deliberate if my data conforms to the usage found.

As a side note on just three elements, there is a degree of
up/down/side look due to the loss of co-planar symmetry of the loop
with the other elements. Adding more directors would probably hide
this.

73's
Richard Clark, KB7QHC