On Tue, 17 Nov 2009 20:58:00 -0800, Richard Clark
wrote:
My model uses 1/4" elements, and for just the loop, Z = 150 Ohms @
2250 MHz
The original has an asymmetrical cross section of .330cm x 0.200cm
(from the supplied dimensions). I decided that a suitable equivalent
would be a round rod, with the same circumferance. of 1.06cm. That's
a 0.338cm dia rod. However, I don't know if this is a realistic
approximation. Your 0.635cm diameter rods might be a bit wide.
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).
My folded dipole ended up with 4 segements (45 deg/seg) at each end
and about 42 segments total. 4NEC2 also initially complained about
the ratio of the segment length to rod diameter, but I reduced the
number of segments and reduced the wire diameter to correspond to the
copper tape thickness. 4NEC2 was then happy.
http://www.11junk.com/jeffl/antennas/mfj1800/slides/Geometry.html
NEC works in wavelengths or meters, not inches. Think metric and
forget about using inches.
I went with the dimensions provided - inches no problem.
I kinda like to do things in wavelengths because invariably I end up
recycling (plagerizing) my own designs to operate on different
frequencies. With wavelengths, that can be done by just tweaking one
or two numbers (GS scale structure card). However, this abomination
of an antenna design is unlikely to be useful at other frequencies, so
I used meters and not worry about scaling. I find that I make many
more errors when using inches than with metric.
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.
I'll be generous and not pass judgement on this contrivance until I
understand this antenna with its rather odd balun, or whatever it is.
It's possible to build an equal space director Yagi-Uda antenna but
not with equal length directors. If I feel ambitious, I might feed it
to an optimizer and see what optimizing the first two director lengths
can offer.
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.
Me too. However, I was rather guarded in my criticism since I didn't
really understand the design of the balun and the purpose of the
ferrite beads. My best guess was a 1/4 wave (or 3/4 wave) coax
matching contrivance made from RG-62/u (93ohm) to match to 173 ohms.
If the feed impedance came anywhere near 173 ohms, and the coax turns
out to be 93 ohms, then it has a chance. However, even if it met all
the criteria for an effective 1/4 wave match, the sloppy construction
and exposed center conductors will certainly have detrimental effects.
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.
Well, the impedance is going to be somewhere between a circular full
wave loop (about 100 ohms) and that of a folded dipole (about 288
ohms). The problem is that this folded dipole isn't folded very much
and more closely resembles a loop. In addition, the reflector and 1st
director are not particularly close to the driven element. That makes
me suspect (i.e. guess) that their effect on reducing the feed
impedance is going to be minimal.
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
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558