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voltage and current in dipoles

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AF7TS, May 19, 2016.

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  1. AF7TS

    AF7TS Ham Member QRZ Page

    The discussion here:
    https://forums.qrz.com/index.php?threads/75-ohm-cable-rg6.521587/page-2#post-3847308
    got me thinking, and I realize that I don't really understand how voltage and current are related in a simple resonant dipole.

    As I understand things:
    A simple resonant circuit is an inductor and capacitor in parallel, at a frequency where the reactance of the inductor and capacitor are equal and opposite.
    The voltage across the capacitor and the current through the inductor are out of phase. Energy is shifting between the inductor and the capacitor.
    An external feed across both inductor and capacitor sees a _resistive_ load, equal to any losses in the resonant circuit.

    The feed into a resonant dipole is resistive.
    The impedance of the feedpoint depends on where it is selected on the dipole, but is is always resistive.
    This implies that the current at any point on the dipole and the voltage between two points (close to each other??) on the dipole are in phase.

    In the reactive near field of the antenna, the electric field and the magnetic field are 90 degrees out of phase, as energy oscillates between the electric and magnetic field.
    In the far field of the antenna, the electric field and the magnetic field are in phase.

    Okay, the above is _not_ self consistent, and I know that there are errors, but I need help figuring out what I am missing.

    Thanks and 73
    Jon
    AF7TS
     
  2. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    First of all, don't be mislead by the animation, compelling though it is. It's wrong! The voltage and current in a resonant dipole are, as you observe, theoretically always in phase. If you already knew that the animation was wrong, great, carry on, and sorry I can't be of more help; but if you missed it, read Steve's post #39 for an explanation.
     
    Last edited: May 19, 2016
    W0BTU, AF7TS and K7TRF like this.
  3. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yes, a dipole is typically designed to be resonant at some operating frequency. That just means the feedpoint impedance does not have a reactive term but that does not make the dipole a parallel resonant or tank circuit. There are many forms of resonant structures and the tank circuit is one but it is not the only one.

    IMO, the best way to understand voltage vs current relationships on a dipole is to think in terms of wave theory, reflections and standing waves rather than trying to think of it as a particular RCL circuit. This vintage film does a great job of introducing wave theory, standing waves, what impedance means in this context, SWR, etc.



    In terms of the thread you linked, also consider Steve's (G3TXQ's) comment that there is the spatial relationship between voltage and current distributed across the dipole which establishes the impedance at any chosen feedpoint. But there's also the temporal relationship which is what the cute animation messed up a bit, IOW how the voltage vs current relate to one another at a given time in each RF cycle.
     
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  4. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    I like the guitar string analogy: The string is plucked at its center.
    This pressure on the string represents voltage, its vibratory motion
    is current--maximum of which occurs at the center. The pressure
    (voltage) transfers to each end of the string (the bottom end of
    which transfers the sound energy to the bridge).
     
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  5. AF7TS

    AF7TS Ham Member QRZ Page

    Thanks.

    But here is what I don't get: I had thought that the whole point of differentiating between the reactive near field and the far field was that in the near field the electric and magnetic fields were not in phase. If the current in the dipole is in phase with the voltage, then wouldn't that put the electric and magnetic fields in phase?

    -Jon
     
  6. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    Hi Jon:
    In a dipole operating at the resonant frequency, the voltage and current are exactly in phase in TIME, but displaced by 90 degrees in POSITION. This is why the thing radiates!
    Because the voltage and current are in phase with respect to time, the power is REAL.

    Hope this helps a bit.
    Eric
     
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  7. AF7TS

    AF7TS Ham Member QRZ Page

    I love that standing wave video, and pretty well get what standing waves in a transmission line are.

    Here is what I am currently pondering:

    If I have a 5m long dipole, and I feed it at any location via a 2cm gap, the voltage across that gap and the current coming out of the feed line will be in phase. That is what is meant by a resistive load.

    But if I measure the voltage from one end of the dipole to the other, what will that phase relation be to the current flowing at the center of the dipole.

    What is the meaning of a voltage measurement across 1/2 wavelength at the frequency of interest? Is it meaningful to take the integral of the electric field between two points that are far enough apart that we care about speed of light between them?

    73
    Jon
    AF7TS
     
  8. AF7TS

    AF7TS Ham Member QRZ Page

    Thanks; as I understand that point, you are saying that the voltage distribution and the current distribution have different shapes in space, but that these different shapes are expanding and contracting synchronously in time.

    That sounds reasonable to me, but then I have trouble fitting that point in to the difference between the near field and the far field of an antenna.

    73
    Jon
    AF7TS
     
  9. G3TXQ

    G3TXQ Ham Member QRZ Page

    Jon,

    There's not a simple relationship between the relative phases of the antenna voltage & current on the one hand, and the relative phases of the E and H fields on the other.

    For example, an antenna could have a very reactive feedpoint impedance - voltage and current close to phase quadrature - whilst in the Far Field the E & H fields are in-phase. On the other hand the feedpoint impedance can be resistive and yet the E & H fields be in quadrature in the Near Field.

    Steve G3TXQ
     
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  10. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Steve's answer above is succinct and gets right to the point, but I started the following reply this morning before getting caught up in work stuff.

    There's some academic work looking into the reactive near field region around radiating structures and some good insight into how it applies to things like capacitive body sensors. But realistically I've never seen a really clear model or even a good theoretical discussion of the subject beyond the statement that there is a region in close to the antenna where fields are very complex, typically reactive, changing in reactance across short distances and generally hard to model. There's far more information on Fresnel zone behaviors but even that isn't as easy to model as the far field region.

    Perhaps someone who's delved into the theoretical physics of the reactive near field region will have a better explanation but to the best of my knowledge it's one of those things that's not all that well understood even though some of the basic characteristics have been described.

    For instance I've generally used the rule of thumb that the reactive near field region extends to roughly 1/(2*pi) wavelengths from the radiating structure, the radiating near field extending up to roughly one wavelength and the Fresnel region extending to roughly 2 wavelengths. I suspect those are just rough estimates but that's the sort of guidelines I've used in past projects to estimate when I need to think in near field or far field terms.

    As to the wave physics that create that near field reactive region I've never heard a solid explanation but perhaps that's something physicists discuss even if engineers tend to gloss it over. From a comms systems perspective we just don't spend much time worrying about what's happening within a small fraction of a wavelength of the antenna structure.

    I'm not sure what value there is in comparing the phase of voltage measured across the total length of a dipole to the current at the feedpoint. Seems like kind of an apples to oranges measurement of dipole characteristics. Maybe there's something there, but it's way too theoretical for this old ham :)

    Good luck,
    -Dave
     

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