Matching on the MFJ-1800

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

amdx wrote:
> 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
>
>
Hi,
I used to measure impedance of antenna feed point with home made noise
bridge. I know there is a 300 Ohm to 75 Ohm trnasformer balun.

"Jeff Liebermann" <(E-Mail Removed)> wrote in message
news(E-Mail Removed)...
> On Fri, 13 Nov 2009 07:28:34 -0600, "amdx" <(E-Mail Removed)> 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

"Richard Clark" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On Tue, 17 Nov 2009 18:35:15 -0800, Jeff Liebermann <(E-Mail Removed)>
> wrote:
>
>>On Fri, 13 Nov 2009 07:28:34 -0600, "amdx" <(E-Mail Removed)> 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" <(E-Mail Removed)> wrote in message
news:218da$4b03f193$18ec6dd7$(E-Mail Removed)...
>
> "Richard Clark" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed)...
>> On Tue, 17 Nov 2009 18:35:15 -0800, Jeff Liebermann <(E-Mail Removed)>
>> wrote:
>>
>>>On Fri, 13 Nov 2009 07:28:34 -0600, "amdx" <(E-Mail Removed)> 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
>

"Jeff Liebermann" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On Wed, 18 Nov 2009 07:00:26 -0600, "amdx" <(E-Mail Removed)> wrote:
>
>>The CENTER of the driven element (the plastic screw hole) to the
>>CENTER of the last director is 36.7cm (14-7/16")
>
> Thanks. My numbers came out to 36.52 cm which is close enough.
>
>>ALL directors or the same length (within .003")
>>They are 46.6mm in length.
>
> Amazing...
>
>>> Also, measure the coax balun cable dimensions. Mostly, I'm interested
>>> in the:
>
> Nice. I'll see if I guess(tm) the coax type. It doesn't seem like a
> good fit for any of the common cables as the center conductor is
> somewhat larger diameter than any of these listed. See if you can
> find an exposed center conductor without any tinning or soldering.
> <http://www.epanorama.net/documents/wiring/coaxcable.html>
> One more dimension... the approximate outer jacket diameter of the
> coax (not including the shrink tube).
>
>>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.
>
> Weird(tm). Low loss coax would need to be foam or teflon. Solid
> polyethylene is easier to work with, cheaper, but not the best.
> However, a short piece like this balun would not have much loss, so I
> guess it doesn't matter what flavor is used:
> <http://en.wikipedia.org/wiki/Coaxial_cable>
>
> Gotta do paying work today. I'll play with this more in the next day
> or two (so I don't forget what I'm doing).
> Jeff Liebermann

I have found the need to work for a living, gets in the way of a lot of
fun!

I took the coax loose on the MFJ-1800 and and removed the toroids, I found
the letters found M1Z/111-RG and then the insulation ended. Argh!
Oh, I have a second antenna, so I took that one apart, Eureka! RGS-303
http://wireandcable.thermaxcdt.com/i...d-70-?&seo=110
50 ohm coax. PTFE center insulator, FEP jacket.
Mike

Jeff Liebermann wrote:
> 50 ohm coax does not make it a matching section to a 200 ohm
> folded dipole.

However, a 1/4WL side-by-side balanced run of 50 ohm coax with
a Z0=100 ohms makes an excellent match from a 200 ohm load
to a 50 ohm source.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

"Jeff Liebermann" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On Thu, 19 Nov 2009 06:05:20 -0600, "amdx" <(E-Mail Removed)> wrote:
>
>>I have found the need to work for a living, gets in the way of a lot of
>>fun!
>
> 5 hours on the phone on a slow motion conference call this morning.
> Absolutely nothing useful accomplished. Such online meetings should
> be banned, taxed, or both as a threat to national productivity.
>
>>I took the coax loose on the MFJ-1800 and and removed the toroids, I found
>>the letters found M1Z/111-RG and then the insulation ended. Argh!
>>Oh, I have a second antenna, so I took that one apart, Eureka! RGS-303
>>http://wireandcable.thermaxcdt.com/i...d-70-?&seo=110
>> 50 ohm coax. PTFE center insulator, FEP jacket.
>> Mike
>
> Thanks. 50 ohm coax does not make it a matching section to a 200 ohm
> folded dipole. However, the ferrite beads are a good way to simply
> block the reflected power from the folded dipole so that it looks like
> it's matched. In any case, that reflected power is lost (converted to
> heat) in the ferrite beads. So much for efficiency. When I change
> the characteristic impedance of the model from 200 ohms to 50 ohms,
> the VSWR climbs to 5.5:1. Yech. (Note that the radiation efficiency
> is 75% with or without the mismatch).
>
> I suppose the antenna could be made to function by replacing the coax
> section with a real 1/4 wave 4:1 balun, but I'll leave that to MFJ to
> figure out.
>
> If you need some more entertainment value, it would be interesting to
> actually measure the gain of the antenna. Find a known good reputable
> antenna with similar gain. A panel or patch will work. Find a signal
> source that isn't infested with reflections (including ground
> reflections), Fresnel Zone issues, and is fairly stable (i.e. doesn't
> physically move). Use Netstumbler, WirelessMon, or Kismet to compare
> the signal strengths. For additional accuracy, use a step attenuator
> to adjust the signal levels to a common reference level. Better yet,
> use a spectrum analyzer.
> Jeff Liebermann

Sure, as soon as I try to make you King you find work for me :-)

Ya, I have quad panel on the boat, when I get some time I'll try the
comparision.
BTW, I had a friend purchase an Alfa-Awus-036 Wifi Adapter.
He said it worked very well! So I ordered one, I replaced a TP-Link
TL-WN321G
with the Alfa-Awus-036. I had 23 signals received with only about 4 usable
signals on the
TL-WN321G. After I installed the Alfa I received 36 signals and all of them
have a signal
strength that would make them usable. (Of course some are encrypted)
This thing screams! I had used the TL-WN321G for several years and thought
it was good
until I tried the Alfa.
http://www.amazon.com/Adapter-Wardri...7004388&sr=8-4
I'm using it with a 19dbi* panel antenna aimed a a 7 story condo.
http://www.fab-corp.com/product.php?...cat=255&page=1
Thanks, for all the input guys.
* advertised

"Jeff Liebermann" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On Thu, 19 Nov 2009 18:46:59 -0600, "amdx" <(E-Mail Removed)> wrote:
>
>> Sure, as soon as I try to make you King you find work for me :-)
>
> I hate to tell you this, but that's what kings are paid to do. They
> give orders. Do you require a public proclamation, executive order,
> or pontifical bull (in Latin), in order to be properly inspired? Now,
> pleae do some testing. After all this is your antenna, your question
> and your problem.
>
>>Ya, I have quad panel on the boat, when I get some time I'll try the
>>comparision.
>
> Doing it on a boat might be a problem. You're too close to the water
> which will probably be inside the Fresnel Zone. The water also acts
> as a great reflector. Thinks also move on a vessel, making stable
> readings difficult. I do my testing across a valley, where there's
> little chance of ground reflections (and there's a convenient 2.4GHz
> RF source from the local mountain top WISP). The path also has a wall
> of 40 meter high trees on both side to attenuate any interference.

>Jeff Liebermann

All that may be true, sorry King, is true.
My antennas are 14ft above the water and the signal only goes over water for
about 30ft
before a concrete parking lot fills the rest of the distance to my targets.
Total target distance
is about 600ft. The antennas are tipped upward about 10 degrees.
Here's the test I performed, I aligned the panel and the yagi on a pole
with about the 2ft between the yagi and the center of the panel. I aimed at
the target as
accurately a I could see. (Center of condo building) During testing I did
not terminate the unused
antenna.
I used net stumbler for signal strength numbers, I received 42 signals with
the Panel,
and 44 signals with the yagi.
The one signal I usually use went from -48db (panel) to -50db (yagi),
That's +2db for the Panel.
Then I did a Netstumbler screen print of the first 35 signals using the yagi
and then the panel.
I added all 35 signal strengths and divided by the 35, to get an average
signal.
(35 is all that fit the screen without scrolling)
The Panel AVE = -58.11db, the Yagi AVE = 59.9db that is + 1.79db for the
Panel.
So if you have any faith in my method, it looks like the yagi is down about
2db from the Panel.
I'm impressed, the MFJ-1800 yagi is advertised as a 15dbi antenna, the
panel
as 19dbi antenna.
Here's the Panel seller for the 19dbi panel antenna.
http://www.fab-corp.com/product.php?...cat=255&page=1
At one time I found antenna plots for it, but can't locate them now.
Here's the yagi seller.
http://www.mfjenterprises.com/Produc...uctid=MFJ-1800
Thanks, Mike
PS.
Previously a saw 33 signals, I had never tried to optimize by adjusting the
antenna, 33 was
good enough. While mounting the yagi I moved the position of the panel,
looks like
I found a better position. :-)

Dave Platt wrote:
> A piece of coax, with several ferrite beads or cores around it, *IS* a
> balun. It's a 1:1 "current balun".

According to The ARRL Handbook, a "w2du balun",
popularized by Walter Maxwell.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com