Matching impedance with coax

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).

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.

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

(snip)

The presense and spacing of the parasitic elements isn't going to change the feedpoint impedance that much.

Mike needs to check out - and understand - how a side-mount folded dipole is matched to a 50 ohm line. I'm sure this yagi will simply use a similar series coax balun.

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

Well you can believe what you like.

I believe what occurs and is measurable.

tom K0TAR

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

But not one of them in Perth.

No relation to anyone you are thinking_of/describing/etc, sorry to ruin your conspiracy theory.

If you want to try and achieve a match to 50 ohms by moving the adjacent parasitic elements seriously close to the driven folded dipole, go for it. (I could dust off trusty Elnec and get a result.) But I'd be surprised if anyone who gives a rats about the consistency of the result would go down that path.

I am very familair with how the commercial side-mounted dipoles and yagis are manufactured here in Australia, and I doubt that the rest of the world is dramatically different. In three simple words - series coax transformer. Let's agree that with an SMD you don't have parasitics to play around with - except for tower spacing (which has an impact on pattern, and variations are used for that end.) The Aussie manufacturers use eaxactly the same method on the FD on their yagis. That is why I suggested the O/P look into that approach.

Your confirmation here doesn't ruin anything. Art would hug you no matter how you sign. Those he does have a remarkable need for retaining anonymity. He would have us believe it's because his supporters are easily bruised in the jostle. The following comment would support that:

which is another but perhaps left-handed confirmation.

73's Richard Clark, KB7QHC

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.

Yeah, as a longstanding convention for thousands of posters here, it is a strange thing about being public and open in this world isn't it?

If you can't put your name to it, then any posting is only vacant spam. "No one at home" informs us all about quality.

On the other hand, you choosing to be anonymous means you could have us believe you are writing from a cave on the Afghan/Pakistan border while waiting for your dialysis treatment to finish. Only Ossama and vampires avoid the limelight - as you call it.

Your painted-into-the-corner explanation has nothing to do with the correlation between exhibited low feedpoint R and the proximity of passive elements to what would have ordinarily been a very HiZ folded element.

We shall await your next post as snipped-for-privacy@onefloorshortofthetop.org

73's Richard Clark, KB7QHC

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

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:

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.

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:

Scroll down to the diagram and photo of the coax balun. I use semi-rigid 0.141 coax, not RG-316.

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:

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.

If you re-read what I posted, you will notice I stated "series coax transformer". An in-line impedance transforming section is totally different to simply stuffing RF choking on the line.

It is the method the three major manufacturers here in Australia employ on their SMD's and the driven FD's on their yagis.

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

Mike