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Folded Dipole Calculator Help

Discussion in 'Electronic Design' started by amdx, Nov 14, 2009.

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  1. amdx

    amdx Guest

    Hi All,
    I'm having a disagreement about the use of this folded dipole calculator
    impedance calculator.
    http://www.k7mem.150m.com/Electronic_Notebook/antennas/folded_dipole.html
    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
     
  2. Tony Hwang

    Tony Hwang Guest

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

    Tony Hwang Guest

    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.
     
  4. amdx

    amdx Guest

    In what case would I alter the 72 ohms?
    Mike
     
  5. amdx

    amdx Guest

    In what case would I alter the 72 ohms?
    Mike
     
  6. 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
     
  7. amdx

    amdx Guest

    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
     
  8. Baron

    Baron Guest

    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.
     
  9. Tony Hwang

    Tony Hwang Guest

    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!
     
  10. Tony Hwang

    Tony Hwang Guest

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

    Tony Hwang Guest

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

    Baron Guest

    Due to the transformer effect of folding it.
     
  13. Fred Abse

    Fred Abse Guest

    Ahh, someone who has read Kraus on antennas.

    :)
     
  14. amdx

    amdx Guest

    Ok guys, back on the online calculator.
    http://www.k7mem.150m.com/Electronic_Notebook/antennas/folded_dipole.html

    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
     
  15. Baron

    Baron Guest

    Fundamentally, No !
     
  16. Fred Abse

    Fred Abse Guest

    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 ]
     
  17. Fred Abse

    Fred Abse Guest

    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.
     
  18. amdx

    amdx Guest

    Yes. but for a simple dipole in free space we can leave it at 72 ohms.
    Yes, agreed, coupling structures will alter the impedance of a dipole and
    also a folded dipole.
    As I see it the changing element diameter alters the transforming ratio
    NOT the 72 ohms that is feedpoint impedance of a (theoretical), infinitely
    thin, dipole.

    ( this is an important point and may be the nub of the disagreement)

    Using the calculator you will find altering d1 and d2 will NOT alter the
    The 72 ohms in the data line "Simple Dipole Feed Impedance (ohms)".
    (I don't know, maybe it should)


    Ok, I think I getting close to being able argue my point, but I'm looking
    for some agreement
    here for the statements I made above.
    Thanksfor your help, Mike
     
  19. Phil Allison

    Phil Allison Guest

    "amdx"
    ** You are so dumb you keep inventing non existent complications.

    1. The calculator assumes the folded dipole is in free space.

    2. It then calculates the impedance of the design for you, long as you use
    72 ohms in the data.

    3. IF you KNOW the impedance value for some dipole that is part of a more
    complex antenna - then you use that value in the data instead, to find the
    value for a folded version.




    ..... Phil
     
  20. amdx

    amdx Guest

    Those "non existent compications" are what I'm trying to eliminate so
    I can make a better arguement for my position.
    Yes, I understand that
    Would 72 ohms still be the correct number if d1 is twice the diameter of d2?
    I would think that is the reason the author put that variable in the
    program.
    Thanks, Mike
     
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