# Folded Dipole Calculator Help

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

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

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

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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.

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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?

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In what case would I alter the 72 ohms? Mike

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In what case would I alter the 72 ohms? Mike

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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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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

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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

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

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

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Due to the transformer effect of folding it.

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Ahh, someone who has read Kraus on antennas.

:-)

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