Matching impedance with coax

Hi all,
I'm trying to get an understanding of the MFJ-1800 wifi antenna.
The antenna has a folded loop as the active element.
Is this considered to have a 300 ohm output impedance?
The folded loop is connected to 2.11 inches of 50 ohm coax
that goes to an N connector. The coax has 4 torroids on it.
It looks like a polyethylene core material. So I used .66 as a VF.
With that I get a .66 wavelength of at 2.437 Ghz for the 2.11" coax.
(yes same .66, that's just the way it worked out)
So I think I'm matching 300 ohms to 50 ohms, but I don't
see how .66 wavelength of 50 ohm coax does that.
Fill in the details please.
Here's a picture of the MFJ.
http://www.gigaparts.com/parts/gpcpa...nal/nw0054.jpg

Thanks, Mike

On Tue, 3 Nov 2009 18:24:19 -0800, (E-Mail Removed) (Dave Platt)
wrote:

>In article <8fb2d$4af0dcbd$18ec6dd7$(E-Mail Removed)>,
>amdx <(E-Mail Removed)> wrote:
>
>> I'm trying to get an understanding of the MFJ-1800 wifi antenna.
>>The antenna has a folded loop as the active element.
>>Is this considered to have a 300 ohm output impedance?
>
>Not necessarily.
>
>A folded dipole will have a 300-ohm impedance only under certain
>conditions of design and use. The feedpoint impedance depends on
>several factors, including:
>
>- The ratio of the diameters of the two elements (usually 1:1 in
> common folded dipoles, but not always the case), and
>
>- The ratio between the element diameter(s), and the spacing between
> the two elements, and
>
>- The surrounding environment
>
>The commonest case (of which you're thinking) is a 1:1 ratio of
>element diameters, a relatively small spacing, and a free-space
>environment (i.e. no other conductors nearby). In this case, the
>folded dipole will have a feedpoint impedance of roughly 300 ohms.
>
>However, in the case of the MFJ antenna, the third of these conditions
>is very different. The FD is not in free space - there's a reflector
>on one side of it, and a set of directors on the other.
>
>The presence of these "parasitic" elements will greatly change the
>feedpoint impedance of the FD... typically, to a lower value. Close
>enough spacing of the parasitics can reduce the feedpoint impedance by
>quite a lot.
>
>I suspect that the design of the MFJ antenna was done in a way which
>places the parasitic elements close enough to reduce the folded
>dipole's impedance to somewhere in the neighborhood of 50 ohms. All
>that would be necessary, then, to allow a direct feed from a 50-ohm
>coax, would be a choke balun (to convert the unbalanced coax feed to a
>balanced drive to the folded dipole, without altering the impedance).
>


As you said:

The matching is performed by the cable losses.

Well, its MFJ, isn't it?

w.



>The 4 toroids on the coax stub will serve as a tolerable (less than
>perfect, but probably usable) choke balun.
>
>The FD's impedance probably isn't supremely close to 50 ohms... there
>could be some mismatch and thus an SWR of greater than 1:1. However,
>the losses in the coax stub, and in the main coaxial feedline, are
>going to be high enough to reduce the *effective* SWR (as seen by the
>radio) to a lower value... close enough to 1:1 that the transmitter
>won't be unhappy.
>
>To sum it up: the matching is being performed by the antenna design
>rather than by the coaxial stub or by any separate matching network.
>
>You might want to search for info on the WA5VJB "Cheap Yagi" design.
>Kent Britain figure out a way to make a very simple, effective Yagi
>antenna (out of scrap parts, in effect) with a 50-ohm feedpoint
>impedance and no separate matching network or gamma match. It's done
>by the combination of a "half-folded dipole" driven element, and
>proper choice of the spacing for the reflector and first director.

"Helmut Wabnig" <hwabnig@ .- --- -. dotat> wrote in message
news:(E-Mail Removed)...
> On Tue, 3 Nov 2009 18:24:19 -0800, (E-Mail Removed) (Dave Platt)
> wrote:
>
>>In article <8fb2d$4af0dcbd$18ec6dd7$(E-Mail Removed)>,
>>amdx <(E-Mail Removed)> wrote:
>>
>>> I'm trying to get an understanding of the MFJ-1800 wifi antenna.
>>>The antenna has a folded loop as the active element.
>>>Is this considered to have a 300 ohm output impedance?
>>
>>Not necessarily.
>>
>>A folded dipole will have a 300-ohm impedance only under certain
>>conditions of design and use. The feedpoint impedance depends on
>>several factors, including:
>>
>>- The ratio of the diameters of the two elements (usually 1:1 in
>> common folded dipoles, but not always the case), and
>>
>>- The ratio between the element diameter(s), and the spacing between
>> the two elements, and
>>
>>- The surrounding environment
>>
>>The commonest case (of which you're thinking) is a 1:1 ratio of
>>element diameters, a relatively small spacing, and a free-space
>>environment (i.e. no other conductors nearby). In this case, the
>>folded dipole will have a feedpoint impedance of roughly 300 ohms.
>>
>>However, in the case of the MFJ antenna, the third of these conditions
>>is very different. The FD is not in free space - there's a reflector
>>on one side of it, and a set of directors on the other.
>>
>>The presence of these "parasitic" elements will greatly change the
>>feedpoint impedance of the FD... typically, to a lower value. Close
>>enough spacing of the parasitics can reduce the feedpoint impedance by
>>quite a lot.
>>
>>I suspect that the design of the MFJ antenna was done in a way which
>>places the parasitic elements close enough to reduce the folded
>>dipole's impedance to somewhere in the neighborhood of 50 ohms. All
>>that would be necessary, then, to allow a direct feed from a 50-ohm
>>coax, would be a choke balun (to convert the unbalanced coax feed to a
>>balanced drive to the folded dipole, without altering the impedance).
>>
>
>
> As you said:
>
> The matching is performed by the cable losses.
>
> Well, its MFJ, isn't it?
>
> w.

How much loss does 2-1/8 inches of rg-58 have at 2.4Ghz?
I calculate it as 0.036db, how does that contribute to cable matching?
Inquiring minds want to know.
Mike

who where wrote:
>> I suspect that the design of the MFJ antenna was done in a way which
>> places the parasitic elements close enough to reduce the folded
>> dipole's impedance to somewhere in the neighborhood of 50 ohms. All
>> that would be necessary, then, to allow a direct feed from a 50-ohm
>> coax, would be a choke balun (to convert the unbalanced coax feed to a
>> balanced drive to the folded dipole, without altering the impedance).
>
> The presense and spacing of the parasitic elements isn't going to
> change the feedpoint impedance that much.

Wrong. It can change it a lot. It can take a 50 ohm DE and move it to
10 ohms or less. And then there's the reactive part.

tom
K0TAR

who where wrote:
> On Wed, 04 Nov 2009 20:10:53 -0600, tom <(E-Mail Removed)> wrote:
>
>> who where wrote:
>>>> I suspect that the design of the MFJ antenna was done in a way which
>>>> places the parasitic elements close enough to reduce the folded
>>>> dipole's impedance to somewhere in the neighborhood of 50 ohms. All
>>>> that would be necessary, then, to allow a direct feed from a 50-ohm
>>>> coax, would be a choke balun (to convert the unbalanced coax feed to a
>>>> balanced drive to the folded dipole, without altering the impedance).
>>> The presense and spacing of the parasitic elements isn't going to
>>> change the feedpoint impedance that much.
>> Wrong. It can change it a lot. It can take a 50 ohm DE and move it to
>> 10 ohms or less. And then there's the reactive part.
>
> Well you can believe what you like.

I believe what occurs and is measurable.

tom
K0TAR

Richard Clark wrote:
> On Thu, 05 Nov 2009 21:52:18 -0600, tom <(E-Mail Removed)> wrote:
>
>>> Well you can believe what you like.
>> I believe what occurs and is measurable.
>
> Hi Tom,
>
> It's amazing how after a period of silence, BOTH Art and Jaro pop up
> at the same time.
>
> Does Art have an antipodes sock-puppet?
>
> 73's
> Richard Clark, KB7QHC

Well, I've been silent also. And for almost the same time period. I
could be both of them. I do have 2 feet.

tom
K0TAR

who where wrote:
>
> Whatever - and whoever Art is. I wonder why people like you carry on
> at a personal level towards posters whose views you don't share. And
> you seem to need the limelight, posting a name and callsign.
>
> I'm describing how the matching IS done commercially. You can crap on
> forever if you wish about how you might do it. Fini.

The "ways it's done commercially" depends a lot on the desired result.

A choked line into a 50 ohm DE is an easy to do but not optimal method.

It doesn't give best gain or BW or best F/B or best noise temperature
and never ever gives the best combination of them for weak signal work.

But it IS easy.

And it's not always what the commercial antenna builders use. It's what
you have noticed that they sell. Or you might be pushing how much it's
used just a bit.

tom
K0TAR

"Jeff Liebermann" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On Thu, 05 Nov 2009 08:06:30 +0800, who where <(E-Mail Removed)> wrote:
>
>>The presense and spacing of the parasitic elements isn't going to
>>change the feedpoint impedance that much.
>
> Ummm... try again. If you simplify the antenna, the feed point
> impedance of a 2 element yagi (just driven element and reflector) is
> roughly the same as a dipole over a perfect ground. There are plenty
> of graphs online to show how the impedance varies:
> <http://extranosjacal.blogspot.com/2009/06/antenna-z-and-height-above-ground.html>
> Note that the impedance of a simple dipole varies from nearly zero for
> very close to the ground, to a max of about 100 ohms at about 0.35
> wavelengths. A folded dipole will behave similarly, with a nominal
> impedance of about 277 ohms, but varying over roughly zero to perhaps
> 450 ohms impedance. Add a director (or more director elements) and
> available range of impedances changes even more.
>
>>Mike needs to check out - and understand - how a side-mount folded
>>dipole is matched to a 50 ohm line.
>
> Assuming the folded dipole is the traditional 300 ohms, I usually
> match it with a 1/2 wavelength 4:1 balun. At 2.4 GHz, it would look
> something like:
> <http://pe2er.nl/wifisector/>
> Scroll down to the diagram and photo of the coax balun. I use
> semi-rigid 0.141 coax, not RG-316.
>
>>I'm sure this yagi will simply
>>use a similar series coax balun.
>
> I'm not so sure. The photos on the MFJ web pile seem to be
> intentionally tiny and obscure. Looks like considerable effort was
> made to hide the method of construction. This is the best photo I
> could find:
> <http://www.permo.no/MFJ/MFJ-1800.jpg>
> I can't really see what's going on, much less have enough info to
> create an NEC2 model. In addition, I'm more than a little suspicious
> of what looks like about 1/2" of exposed coax center conductor at the
> center pin of the "N"(?) connector. That's an inductor and radiator,
> which are not very clever design or construction.
>

I'll try to get a better picture of the feedpoint for you.
Is there a way to work the .66 wavelength of 50 ohm cable backwards ie.
What impedance would be transformed to 50 ohms with .66 wavelength of
50 ohm coax?
(this assumes the little knowledge I have about impedance transformation
using
coax is correct.)
Mike

"amdx" <(E-Mail Removed)> wrote in message
news:60c03$4af6e83f$d8baf3ed$(E-Mail Removed)...
>
> "Jeff Liebermann" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed)...
>> On Thu, 05 Nov 2009 08:06:30 +0800, who where <(E-Mail Removed)> wrote:
>>
>>>The presense and spacing of the parasitic elements isn't going to
>>>change the feedpoint impedance that much.
>>
>> Ummm... try again. If you simplify the antenna, the feed point
>> impedance of a 2 element yagi (just driven element and reflector) is
>> roughly the same as a dipole over a perfect ground. There are plenty
>> of graphs online to show how the impedance varies:
>> <http://extranosjacal.blogspot.com/2009/06/antenna-z-and-height-above-ground.html>
>> Note that the impedance of a simple dipole varies from nearly zero for
>> very close to the ground, to a max of about 100 ohms at about 0.35
>> wavelengths. A folded dipole will behave similarly, with a nominal
>> impedance of about 277 ohms, but varying over roughly zero to perhaps
>> 450 ohms impedance. Add a director (or more director elements) and
>> available range of impedances changes even more.
>>
>>>Mike needs to check out - and understand - how a side-mount folded
>>>dipole is matched to a 50 ohm line.
>>
>> Assuming the folded dipole is the traditional 300 ohms, I usually
>> match it with a 1/2 wavelength 4:1 balun. At 2.4 GHz, it would look
>> something like:
>> <http://pe2er.nl/wifisector/>
>> Scroll down to the diagram and photo of the coax balun. I use
>> semi-rigid 0.141 coax, not RG-316.
>>
>>>I'm sure this yagi will simply
>>>use a similar series coax balun.
>>
>> I'm not so sure. The photos on the MFJ web pile seem to be
>> intentionally tiny and obscure. Looks like considerable effort was
>> made to hide the method of construction. This is the best photo I
>> could find:
>> <http://www.permo.no/MFJ/MFJ-1800.jpg>
>> I can't really see what's going on, much less have enough info to
>> create an NEC2 model. In addition, I'm more than a little suspicious
>> of what looks like about 1/2" of exposed coax center conductor at the
>> center pin of the "N"(?) connector. That's an inductor and radiator,
>> which are not very clever design or construction.
>>
>
> I'll try to get a better picture of the feedpoint for you.
> Is there a way to work the .66 wavelength of 50 ohm cable backwards ie.
> What impedance would be transformed to 50 ohms with .66 wavelength of
> 50 ohm coax?
> (this assumes the little knowledge I have about impedance transformation
> using
> coax is correct.)
> Mike
>
> Here's a link to a picture.
http://i395.photobucket.com/albums/p...MFJCollage.jpg
Mike

"Richard Clark" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On Sun, 8 Nov 2009 11:25:01 -0600, "amdx" <(E-Mail Removed)> wrote:
>
>>> I'll try to get a better picture of the feedpoint for you.
>>> Is there a way to work the .66 wavelength of 50 ohm cable backwards
>>> ie.
>>> What impedance would be transformed to 50 ohms with .66 wavelength of
>>> 50 ohm coax?
>>> (this assumes the little knowledge I have about impedance transformation
>>> using
>>> coax is correct.)
>>> Mike
>>>
>>> Here's a link to a picture.
>>http://i395.photobucket.com/albums/p...MFJCollage.jpg
>> Mike
>>
>
> Hi Mike,
>
> That is a pretty good rendering given the other pix. Have you any
> experience with Smith Charts? Still, and all, you need to know the Z
> of at least one point to transform to another.
>
> 73's
> Richard Clark, KB7QHC

They should have been better, those are pictures I took a couple of years
ago.
I didn't blowup someone elses pictures.
Re: "you need to know the Z of at least one point to transform to another."
I would be happy with the assumption the the impedance at the N connector
is 50 ohms. But I think I have a misunderstanding because, in use you would
add more 50 ohm coax to run from the N connector to the transmiter. Soo,
that .66
wavelength section on the antenna becomes anything you add to it.
AT this point, I have to think the folded loop is forced down to 50 ohms by
it's
surrounding structures and there is no impedance transformation betwen the
loop
and the N connector.
Mike