Folded Dipole Calculator Help

Hi All, I'm having a disagreement about the use of this folded dipole calculator impedance calculator.

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We are using it to calculate the impedance of folded dipoles to drive a yagi. When entering data it asks for "Simple dipole feed impedance (ohms)" Default is 72 ohms. This assumes a 1/2 wave dipole. The fellow I'm disagreeing with say's, "the reason that the variable in the calculator for the "dipole feed impedance" could be if you want to transform the z to something else, such as 50 ohms." I don't even understand what he means. I can only think of one reason to alter the 72 ohms, that would be if you use a a different length folded dipole. I'll readily admit the other fellow knows more about this than me, but I need more convincing :-) Can anyone explain this to me. Thanks, Mike

Reply to
amdx
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Hi, The eq. seems right. Folded dipole is basically a loop. Usually we use 300 Ohm feeder. Tony VE6CGX

Reply to
Tony Hwang

Hi, And you can use a tranformer 300 Ohm to 75 Ohm to use coax if need. It becomes a from 300 Ohm balaanced to 75 Ohm unbalanced feed.

Reply to
Tony Hwang

folded_dipole.html

The way I read it is that you put in the feed impedance that a similar- length plain-ol' dipole in the same situation would have. In a Yagi this can be significantly lower than 72 ohms, which is the whole reason you're considering a folded dipole.

Does the author have contact information? Perhaps you could ask the source?

--
www.wescottdesign.com
Reply to
Tim Wescott

In what case would I alter the 72 ohms? Mike

Reply to
amdx

In what case would I alter the 72 ohms? Mike

Reply to
amdx

folded_dipole.html

I've seen older Yagis that use folded dipoles, but the newer ones seem to lean more toward gamma or T matches. You get more latitude for adjusting impedance, in the gamma case you get a kinda sorta good match to coax without a balun, and if you're employing plumber's delight construction it's no more difficult to fabricate than a folded dipole.

So why a folded dipole, pray tell?

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www.wescottdesign.com
Reply to
Tim Wescott

Excuse me, but what does such simple dipole calculator have to do with the calculations for a Yagi feed element ?

Even assuming a single element feed element antenna, playing with the tube diameter (upper and lower bars) or the number of bars in a folded dipole will radically change the feed impedance.

Paul

Reply to
Paul Keinanen

I think it all started with the MFJ-1800 that uses a folded dipole, but there are many yagis that use a folded dipole as the driven element. But, I think (now) I understand enough to know why you ask the question. I think your point is the impedance of dipole feed on a yagi is highly modified by the reflector and directors.

I excerpted this from my latest email to my friend I'm having the disagreement with. (Might not even be a disagreement, it could be a miscommunication.) Excerpt; "The online calculator takes the (Impedance of a dipole x Ratio). This gives you the impedance after folding the dipole. We still have no idea what the impedance of a dipole is when surrounded by the reflector and director. We do know how to change the transformation ratio once we know what the impedance of dipole would be in that surrounding." Mike

Reply to
amdx

The nominal impedance of a dipole in free space is approximately 72 ohms. If it is part of a system its impedance will be influenced by that system causing it to change from nominal.

Folding the dipole allows it to behave as a transformer ! Thus the nominal dipole impedance can be altered to a value that can more easily be matched to the feed line.

Commonly the impedance of a dipole that is part of a system. ie "Yagi" falls to a much lower value. Under these conditions the folded dipole can be used to raise the impedance seen at the feed point to a more usable value.

Someone mentioned "Gama match". Whilst symmetrical gamma matches can be used, single ended is popular because it is inherently unbalanced and can be easily matched to a co-axial feedline.

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Best Regards:
                     Baron.
Reply to
Baron

Hi, Feed point impedance becomes near that when it is just a dipole(not folded). On a haf wave element at center feed point the current and voltage phase is such that the impedance is near that figure. If it is vertical uater wave whip, the other half mirror image is prjected into the ground forming dipole and in that case feed point impedance at the bottom is around 50 Ohm. I am old dinosaur EE(class of

60), my brain is not as bright as it used to, LOL!
Reply to
Tony Hwang

Hi, Also folded dipole tends to have broader b/w compared to dipole.

Reply to
Tony Hwang

Hi, Even now Yagi-Uda antenna design is empirical business tinkering with it in the antenna range based on theory plus actual field result.

Reply to
Tony Hwang

Due to the transformer effect of folding it.

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Best Regards:
                     Baron.
Reply to
Baron

Ahh, someone who has read Kraus on antennas.

:-)

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"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
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Reply to
Fred Abse

Ummm... I'm looking at "Antennas" by John D. Kraus, first edition

1950, and find that the Yagi antenna entry in the index is just "Yagi" with no mention of Yagi-Uda anywhere in the TOC or text. Perhaps in later editions?

Extra credit. Which was the teacher and which was the student?

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Reply to
Jeff Liebermann

Ok guys, back on the online calculator.

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If I set d1 at 4mm and d2 at 2mm and S at 12.5 mm Would I need to adjust the input data labeled "Simple Dipole Feed Impedance (Ohms)" to a value other than 72 ohms? In other words does the "Simple Dipole Feed Impedance" change when the d1 and d2 are altered by these amounts. (I'm asking about larger amounts not just 1 or 2 ohms) Mike

Reply to
amdx

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Fundamentally, No !

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Best Regards:
                     Baron.
Reply to
Baron

Second edition, 1988

"Shintaro Uda, an assistant professor at Tohoku University"

"Hidetsugu Yagi, professor of electrical engineering at Tohoku University and 10 years Uda's senior"

[Antennas, John D. Kraus, second edition, 1988, 11-9b ]
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"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
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Reply to
Fred Abse

From the above URL:

"The folded dipole multiplies the normal feed impedance of a simple dipole. For a 1/2 wave dipole, in free space, this is approximately 72 Ohms. You may not be dealing with a dipole in free space, but 72 Ohms is close enough to start with. You can adjust it to other impedances in the appropriate text box below."

The nub is "in free space". Coupling to other conducting objects (parasitic elements, even support structures) alters the feedpoint impedance. There are published curves on this, I'll have to see if I've got them.

The figure of 72 ohms is the feedpoint impedance of a (theoretical), infinitely thin, dipole. You don't need to alter this to take account of element diameters, the function shown does this for you. You will have to alter it if there is coupling to other conducting structures, for example, parasitic elements. How much to alter it depends on other variables.

The (then) National Bureau of Standards did a lot of work on yagis in the

1980s. They published results that may be around on the Web.
--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
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Reply to
Fred Abse

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