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

    All very interesting - but Cecil never mentioned a frequency, so we don't know the length of the 300 Ohm line; neither do we know its loss characteristics nor the length of the 50 Ohm coax.

    I think you'll find Cecil was trying to get folk to think about the problem rather than come up with example numbers. So I'll quote his question again:

    Steve G3TXQ
     
    KX4O likes this.
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    When it comes to baluns, feedlines, impedance matching, antennas there is no such thing as a lossless system...
     
  3. G3TXQ

    G3TXQ Ham Member QRZ Page

    Agreed, but assuming that a system is lossless often helps with an understanding of basic principles; once they are understood "real life" factors can then be introduced.

    But, that aside, what then is your answer to Cecil's question?



    Steve G3TXQ
     
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  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Well of course I understand this; what I was getting at is in either case, we are tuning for the best possible power transfer from source to load.

    The internal pi-net or whatever the tube transmitter has by design isn't "fooling the tubes," it's resonating a tank circuit while providing maximum power transfer to the load for best transmitter efficiency. An external tuner is dealing with a 50 Ohm source and an unknown (usually) load, but the goal is the same.
     
  5. ND5Y

    ND5Y Ham Member QRZ Page

    I'd like to know how you do that with a 10 meter EFHW that is only 16' long without carrying the radio 104' up the tree.
     
  6. NH7RO

    NH7RO Ham Member QRZ Page

    Simple; he moves up to the penthouse on the 10th floor.:p
     
  7. AI3V

    AI3V Ham Member QRZ Page

    The very first thing you have to understand about antennas is there is no such beast as a end fed.

    Half wave, or any other wavelength.

    All antennas are center fed. They all consist of two electrically identical halves. This is a must to satisfy Kirchoff. :)

    All the confusion comes from the incorrect thought that you can describe the transmission (rig-feedline-radiator) system by describing 1/2 of the radiator part.

    Rege
     
    NH7RO likes this.
  8. N8CMQ

    N8CMQ XML Subscriber QRZ Page


    Ghost counterpoise system!
     
  9. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Well tree-topping with the radio sounds like fun but I was referring to the conventional 80m EFHW, which with the right unun can actually cover 8 HF bands, like the image shows for one well-designed 120' long model. EFHW-8010.png
     
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    This is just plain wrong.

    Here is a horizontal, 67ft (half-wave) long, center-fed wire, 66ft above poor soil. Note the current distribution (the height of the purple line relative to the green line). Note the feed point impedance of ~71 Ohms at resonance at 7.18MHz. Note that the current in the wire just to the left of the red circle is the same as the current to the right of the red circle, as required by Kirchoff. You happy? You should be, because this is text book...
    cf.png

    Exactly the same wire. The only difference is that the feedpoint was moved to 2.5% from the left end.

    This drastically changed the feedpoint impedance to much different numbers than before (2003 - j2819 Ohms), but we can still feed a current into the wire. Now it only takes 0.188A, but a much higher voltage to get the same ~71W of power into the antenna.

    Current distribution along the wire hasn't changed. The current in the wire just to the left of the feedpoint is the same as the current to the right of the feedpoint, so Kirchoff is happy. Why aren't you?

    ef.png

    Oh, and by the way, I overlayed the predicted pattern of the end-fed with the center-fed. Do you see any difference?

    pt.png

    The "counterpoise" is the part of the antenna to the left of the feedpoint.
     
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