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

    May be, but in the real world, such a gap must always exist. Either in the form of a real gap (with connection to the feed line), or in the form of a transformer with a real physical size. The result will be the same.

    I am much more interested in realising your idea with an "optically fed" OCFD :)
     
    NH7RO likes this.
  2. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Things that I know to be so:

    An antenna can be fed at the very end --- it does not need to be OCF! Proof of concept is available.

    Energy entering an antenna from your transmitter will pass through an approximately 600 ohm port --- center fed or end fed!
    Proof of concept is available.

    An end fed resonant antenna in space has a specific and predictable impedance.

    The electromagnetic waves in an antenna being reflected in both directions DO radiate energy exactly the same as that which enters the antenna from the transmitter.

    A great visual analogy representing the distinction between electron behavior and charge displacement in an antenna or feed line is the Newton's Cradle instrument which is sold as a toy.

    Regards Jim
     
  3. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    Yes the Newton cradle is a pretty good representation of an antenna. It is also very easy to show that a Newton cradle can't be "end fed". If you simply apply a force on one of the balls, the whole cradle (or you, everything is relative) will simply start moving.
    To feed energy into the Newton cradle, opposite and equal force need to be applied to two of the balls, similar to the opposite and equal currents needed to be feed energy into an antenna.
     
  4. AF7TS

    AF7TS Ham Member QRZ Page


    Keep in mind the distinction between a _practical_ antenna and the theoretical model of that antenna.

    I claim that if you look closely enough you will _always_ find the 'other side' of the feedpoint, and therefore there is no true end fed antenna, rather an 'extreme off center fed' antenna. When discussing the _theory_ you _cannot_ have an end fend antenna.

    As a practical matter, if you have a box with a feedline going to it and a single wire coming out of it as an antenna, that is reasonably called 'end fed', and such antennas are known to work. If you want to do a detailed theoretical model you have to recognize that the antenna is fed _close_ to the end, but if you just want to put a wire up and use it is is 'end fed'.

    It is rather like the difference between taking a limit as a function approaches dividing 0 by 0 and actually dividing by zero. For all practical purposes you can get as close as you want to dividing by zero, but actually dividing by zero doesn't work.

    73
    Jon
    AF7TS
     
  5. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Really? http://k9axn.com/attachments/1_bearing_5_total_slowl.avi

    It was used to demonstrate electric field displacement in 1st year Physics classes in the 50's and 60's.

    Regards Jim
     
  6. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    Yes, really:
    https://cobra.e-626.net/Newton-endfed.mp4

    When you pull on one of the balls, you exert an equal and opposite force on the floor, which exert the same force on the table, and then on the cradle (all through friction). By placing the cradle floating on water, that opposite force can (almost) be eliminated, and the effect of "end feeding" the cradle can be shown :)
     
    KX4O likes this.
  7. AF7TS

    AF7TS Ham Member QRZ Page

    Any external force applied to make a ball move must be matched by an equal and opposite force applied to something else. Push on a ball with your finger, the ball pushes back on your finger.

    Even in the 'optically pumped end fed' that I described, where you have a photovoltaic junction on the end of a wire, when you hit it with light and drive electrons toward the wire, the other side of the junction has to become more positive, and the antenna has to be considered the wire on one side and the bit of semiconductor on the other side of the junction feedpoint.

    73
    Jon
    AF7TS
     
  8. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Jon,
    The force is established by the voltage/force that is developed across the output tank circuit. The antenna is attached to the top and ground to the bottom. The antenna has a specific impedance and a voltage/force is applied to END of the wire. In the case where the wire is attached to the transmitter, the transmitter is responsible for providing a pool of electrons to draw from or store to that is substantial enough so that a substantial voltage will not develop on the chassis. If a voltage should appear it will simply reduce the drive to the antenna and burn your lip --- which is not the antennas problem. Remember, a cubic centimeter of copper has hundreds of thousands of Coulombs of electrons living on atoms that share electrons and if you have 5 amps going to the antenna at 14MHz there will only be .000000357 amps vibrating in place.

    Need to stop here and let me prove the concept --- yes from a theoretical point and real world how an end fed antenna actually works.

    This will be in steps so there is no confusion regarding the facts.

    Scenario: I have a 54 foot #14 wire strung horizontally or vertically and apply an 8 volt square wave to it. Tell me exactly what you think will happen during the first 150ns. There is absolutely nothing wrong with saying "Hell if I know"

    Each point must be completely understood before going on to the next. I promise you that it will end with a theoretical and real world logical answer that you won't expect.

    Regards Jim
     
  9. K9AXN

    K9AXN Premium Subscriber QRZ Page

    See my reply to Jon, I explained the force. I don't have the codec to play your video.

    Regards Jim
     
  10. KL7AJ

    KL7AJ Ham Member QRZ Page

    I think it's more useful to talk about an antenna as being either "voltage fed" or "current fed" as opposed to "end fed" or "center fed." While the two types often coincide, they don't always. A good example is the double zepp, which has an impedance almost identical to the "normal" zepp, but it's fed in the center. There are many VERY useful antenna arrays based on voltage feed....the Sterba Curtain, the Bruce array, the W8JK, and more.
     
    NH7RO likes this.

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