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

    Yes and no. No, if it is one wire or two wires in parallel (the feed line) doesn't mater. But yes, the resistive losses in that counterpoise will generally be much lower than the alternative, the soil nearest to the fed end / feed line / rig.
    Try to look at how the antenna functions in free space, without any feed line (that is not part of the antenna as in the case of the Zepp), and with an infinitely small transmitter. If you remove the matching section from the Zepp, and the physical size of the transformer and rig, where is the current in the feed point (even if it is small) going to go?
     
    Last edited: Feb 28, 2018
  2. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Only with the source on the antenna. See what happens in your model when you feed it with a transmission line as in real life. Substantial unbalanced currents will exist.
     
  3. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    Plus the added benefit of that controlled counterpoise, being further away from the house or any equipment, lessen the chance of inducing and picking up interference :)
     
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    Therein lies the problem with end-fed antennas. I have been baiting you guys, and one of you finally bit...

    It is not that a half-wave end-fed antenna needs a counterpoise to work, it is difficult to connect a feedline to it without the feedline becoming an un-intended counterpoise which greatly influences the radiation pattern of the antenna, modifies the antenna's resonance and feedpoint impedance. Because the feedline is asymmetric with respect to the center of the antenna, common-mode currents may flow on the feedline, causing the problems above.
     
    KX4O likes this.
  5. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Well, how nice.

    I hope you didn't also seriously mislead some people reading the thread too.
     
    NH7RO likes this.
  6. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Better to make the factual point than to drag the un-schooled around the yard waiting for someone to correct you. sigh.
     
    N0TZU and NH7RO like this.
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    Nothing I have posted should mislead anybody. I have supported it all as I went along.

    There is a tremendous amount of incorrect info already on these forums, especially about end-fed antennas "needing" counterpoises. The reality is that end- and off-center fed antennas need special treatment of their feedlines so that they do not inadvertently become a counterpoise.
     
    KX4O likes this.
  8. AI3V

    AI3V Ham Member QRZ Page

    Yep, it's there... somewhere.


    And exactly what magic matching device is going to send equal power into the two wildly different impedances?

    You got 2 choices, either equal voltage, or equal current.

    Rege
     
    NH7RO likes this.
  9. AI3V

    AI3V Ham Member QRZ Page

    I think you will find the real counterpoise is the Zeppelin itself.

    Parasitic coupling from the tx chassis to the Zeppelin superstructure. If a j pole worked like you think, nobody would ever have issues with his working.

    Rege
     
    NH7RO likes this.
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    What does equal power have to do with it? What does different impedances have to do with it?

    The only requirement is (you said it yourself) is that the current in the left wire be the same as the current in the right wire. A transformer will do exactly that. An isolated, balanced transformer winding must have the same current flowing in one end of a winding that flows out the other end of the same winding.
     
    KX4O likes this.

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