Hi All,
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.


Here is a drawing and some more pics.
http://i395.photobucket.com/albums/p...intFileJPG.jpg
http://i395.photobucket.com/albums/p...connection.jpg
http://i395.photobucket.com/albums/p...MFJRuledDE.jpg
http://i395.photobucket.com/albums/p...Jruledcoax.jpg
Hope I covered everything,
Mike

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

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

A few changes. I changed the simulated folded dipole into something
more closely resembling the original MFJ folded dipole. I also
changed the characteristic impedance to 200 ohms so that more closely
resembles a 4:1 match to 50 ohms. Of course, the VSWR went from
tolerable to horrible (10:1). I suspect I did something wrong in
locating the feed point, but I'll fix that later. When using a
geometry editor/generator, I turned my nice neat NEC macros into
unreadable gibberish. Sigh... I'll also fix that later.

Gain is still 14.1dBi.

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

On Tue, 17 Nov 2009 20:29:08 -0800, Jeff Liebermann
wrote:

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

A few changes. I changed the simulated folded dipole into something
more closely resembling the original MFJ folded dipole. I also
changed the characteristic impedance to 200 ohms so that more closely
resembles a 4:1 match to 50 ohms. Of course, the VSWR went from
tolerable to horrible (10:1). I suspect I did something wrong in
locating the feed point, but I'll fix that later. When using a
geometry editor/generator, I turned my nice neat NEC macros into
unreadable gibberish. Sigh... I'll also fix that later.

Gain is still 14.1dBi.

More changes(tm). I fixed the feed point on the flattened folded
dipole and the VSWR magically came down to normal (1.47:1 into 200
ohms). Vertical gain dropped to 13.1dBi, but horizontal gain dropped
to 6.35dBi. Something (else) is wrong.

I'll fix the macros so that the NEC2 file is readable, later.


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

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

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

"Richard Clark" wrote in message
...
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.

Yes it is just a stamped piece of aluminum with rough edges and
little nibs here and there that were never cleaned off.


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).

Understood.


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.

By this statement, do you mean the spacing between elements is large
so it looses it's folded loop characteristics?

Mike

"amdx" wrote in message
...

"Richard Clark" wrote in message
...
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.

Yes it is just a stamped piece of aluminum with rough edges and
little nibs here and there that were never cleaned off.


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).

Understood.


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.

By this statement, do you mean the spacing between elements is large
so it looses it's folded loop characteristics?


I meant folded dipole characteristics. (not folded loop)

Mike

On Wed, 18 Nov 2009 07:07:21 -0600, "amdx" wrote:

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.

By this statement, do you mean the spacing between elements is large
so it looses it's folded loop characteristics?

Hi Mike,

I was a bit obscure on that last point. YOUR antenna's looped dipole
has rather large turns for a folded loop. A folded quarterwave (for
verticals) or folded halfwaves (for dipoles) generally show 4X
impedance boost for same sized wire/element in the loop. Some designs
use different sized halves of the loop to change the multiplication
factor as this is the primary determinant with closely spaced folds.

However, your antenna has rather larger spacing which may lower the
multiplication factor IF my model conforms to actual. This choice of
larger looping may show MFJ's engineering talent in achieving a
natural match. I don't think the rest of the array is going to matter
much in getting the "most" gain it could, but it is probably good.

73's
Richard Clark, KB7QHC

"Jeff Liebermann" wrote in message
news
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.


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.

The CENTER of the driven element (the plastic screw hole) to the
CENTER of the last director is 36.7cm (14-7/16")


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.

ALL directors or the same length (within .003")
They are 46.6mm in length.

Also, measure the coax balun cable dimensions. Mostly, I'm interested
in the:

1. Shield to shield length.

54.2mm (2.134")

2. Center pin to start of folded dipole length. (i.e. exposed center wire
length).

61.75mm (2.431)

3. OD of center conductor wire.

0.94mm (0.037")

4. ID of shield.

3.3mm (.130)


5. A good guess as to the type of dielectric (foam, solid, or PTFE).

My guess is solid poyethylene, if you have a test other than poking it
with a pin to get feel of it, which I did.

6. Any markings that might identify the coax.

No help there.


That thing doesn't look anything like a
4:1 balun (unless there's something we missed under all that shrink tube).

Looks like 4 toroids that slide over the coax, you can see the outline of
the edges of each toroid.

Mike