What's the story with end-fed antenna's?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KI7QVR, Feb 27, 2018.

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  1. G3TXQ

    G3TXQ Ham Member QRZ Page

    How can a voltage difference between two points produce differing E-Field magnitudes at those two points?

    Steve G3TXQ
     
  2. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    There is no point in looking at how each individual electron behaves. The point is that the electrons as a group is being displaced on each side of the capacitor, thereby changing the electric field between the sides.
     
  3. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    What do you mean? That the E-field is stronger around the short end? Yes, because that side is much shorter than the other side and the electric field therefore much more concentrated there.
     
  4. G3TXQ

    G3TXQ Ham Member QRZ Page

    Are you saying that the voltage distribution along the dipole is different in the near-end-fed case compared with the centre-fed case?

    Steve G3TXQ
     
  5. W5DXP

    W5DXP Ham Member QRZ Page

    I didn't introduce the capacitor discussion and only responded to it. Whoever made this posting is the one who initiated the capacitor discussion.

    "Displacement current is nothing magical, it is simply the current that seem to pass through a capacitor (through the means of varying electrical fields). A capacitor requires two sides, two plates, to terminals (otherwise no varying electrical field can exist)."

    Only the person who made that posting can say what it had to do with end fed antennas?:)

    If Maxwell had known about photons, he wouldn't have invented displacement current to explain the energy transfer through capacitors. Here's an example of how far an electron moves driven by one amp of current.

    https://en.wikipedia.org/wiki/Drift_velocity

    "Therefore in this wire the electrons are flowing at the rate of 23 μm/s. At 60 Hz alternating current, this means that within half a cycle the electrons drift less than 0.2 μm. In other words, electrons flowing across the contact point in a switch will never actually leave the switch."

    That's at 60 Hz. At 60 MHz, the electrons drift less than 0.2 pm, i.e. less than 0.000000000002 meters. That's not a whole lot of displacement.
     
    Last edited: Mar 1, 2018
  6. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    Capacitor discussion, yes. That was needed to explain how the (so called) end fed antenna works. But there is no need for using quantum mechanics to describe how a capacitor works, or these antennas for that mater. Nor does it provide a way for a capacitor/inductor/resistor to only have one lead.
     
  7. AF7TS

    AF7TS Ham Member QRZ Page

    For a dipole in free space, and for a feedpoint of negligible length, under steady state excitation at the resonant frequency, the voltage and current distributions are the same center fed or near end fed.

    Introduce a feedline and you can't consider the dipole to be in free space.

    Once you introduce a feedline near the end of the antenna (or the entire radio setup if you simply run the wire out of the shack), you have to change things.

    IMHO considering the extreme/ideal cases makes good sense to help understand a real world antenna, but we have to remember that these extreme/ideal cases are _not_ the real world.

    Oh, and I've been pondering just how closely we could approximate a near end fed antenna in free space, using real world components that can actually be purchased. My current though is using a 'plastic optical fiber' photodiode as the 'final', with one terminal cut off and the other terminal soldered to a long wire. Support with insulating ropes a couple of wavelengths long and a couple of wavelengths above the ground. Use a modulated laser to supply signal to a fiber that reaches the 2-3 wavelengths from the signal source to the photodiode..... probably only useful as a thought experiment. :)

    73
    Jon
    AF7TS
     
    KX4O likes this.
  8. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    Yes, absolutely, as can be seen in the simulations in the article. You can very clearly see where the feed point is, and that the electric field is much more concentrated on the small side, compared to the large side:
    [​IMG]

    The sum of all fields will be the same on both sides though.
     
    N0TZU likes this.
  9. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    A very interesting idea :)
    The equipment for such an experiment is easy to come by, I have everything needed at home already. But getting it up in free air is one big problem. The other problem would be the very low efficiency of such a setup. A couple of wavelength away, the signal received by any reference antenna would likely be way below the noise floor.
     
    Last edited: Mar 1, 2018
    KX4O likes this.
  10. AF7TS

    AF7TS Ham Member QRZ Page

    I recall seeing that picture in the original article, but don't remember a link to it.

    I _think_ that the simulation included a finite gap for the feedpoint, which does change things especially when the feedpoint is near the end.

    I believe that in the limit as the feedpoint dimension goes to zero the voltage distribution becomes smooth.

    73
    Jon
    AF7TS
     

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