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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. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    To create a varying E-field, you must apply a varying voltage difference somehow, and that requires two sides. You cannot remove half of the capacitor and still have a capacitor. You cannot create a varying electric field using one plate/electrode only.
     
    N0TZU likes this.
  2. G3TXQ

    G3TXQ Ham Member QRZ Page

    What creates the high E-field at the tip of the dipole if it is centre-fed?

    Steve G3TXQ
     
  3. AF7TS

    AF7TS Ham Member QRZ Page

    It really is a misnomer to call the thing an end fed antenna. It is far more accurate to call it an 'extreme off center fed' antenna.

    It is a valid argument to say that you _can't_ feed an antenna exactly at the end, exactly as SM0XHJ notes. But it is certainly possible to have an antenna with vastly different lengths on either side of the feedpoint.

    73
    Jon
    AF7TS
     
  4. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    The voltage difference between the two ends of the dipole. Since the voltage difference between the halves is the greatest at the tips, there is where the E-field will be strongest. For the same reason the H-field is the strongest where the current is the largest, normally the centre of the dipole (unless larger than 1/2 wavelength).
     
  5. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    You can try to feed an antenna at the very end, but nature will force the actual end of the antenna to move elsewhere, such as to your feedline.
     
    NH7RO likes this.
  6. G3TXQ

    G3TXQ Ham Member QRZ Page

    So, what accounts for the much higher E-Field at the fed end of the wire compared to the other end, in Fig 14 of that article?

    Steve G3TXQ
     
  7. W5DXP

    W5DXP Ham Member QRZ Page

    What if the displacement path takes more time than the measured propagation time through the capacitor? Sometimes we forget that the lumped circuit model ignores the speed of light limit.

    We now know that those "varying electrical fields" are quantized, i.e. the fields consist of photons. Photons are emitted by electrons on one side of the capacitor, flow through the capacitor's dielectric, and are absorbed by electrons on the other side of the capacitor. Given the drift velocity of electrons and the frequency of RF, the electrons move hardly at all. They essentially vibrate in place and become a bucket brigade for the photons.

    The electrons in the middle of the capacitor leads never leave the capacitor leads at RF. Photons are performing the energy transfer - traveling through the capacitor leads and through the capacitor dielectric at the speed of light in the medium. Photons don't care whether it is a wire or capacitor dielectric. One might say that photons are radiating through the capacitor.

    The "other end" of the wire is a point where the current goes to zero and all of the RF energy is transferred to the electric field. At the feedpoint, the average current never goes to zero so some of the energy is in the magnetic field, i.e. not in the electric field. Since it is a standing wave antenna, the wave reflection model explains what happens. At the "other end" of the wire, the forward wave sees a (nearly) infinite impedance and is 100% reflected back toward the feedpoint. When that reflected wave reaches the feedpoint, it doesn't see an infinite impedance so some of the energy is not re-reflected. Since energy cannot disappear the question becomes, what happens to the rest of the energy that is not re-reflected at the feedpoint impedance discontinuity? Where does it go?
     
    Last edited: Mar 1, 2018
    NH7RO, N0TZU and KA0HCP like this.
  8. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    The voltage difference between the tip (mainly) of the 2% long side and the tip (mainly) of the 98% long side. It is still not fed at the end.
     
  9. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Great description and mental picture!
     
    NH7RO likes this.
  10. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    What does this have to do with end fed antennas? Electromagnetic waves have properties that can only be described using the concept of photons, yes. But those properties does not take away the fact that they are still electromagnetic waves. And on these frequencies the quantum properties can pretty much be ignored.
     

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