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. N8CMQ

    N8CMQ XML Subscriber QRZ Page

    An end fed antenna is a Zeppelin antenna, or as they are called now, a J-pole, unless you try to replace the open wire transmission line section with an unun. Then they are magic!
     
    NH7RO likes this.
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    When you talk about Zeppelin, or J-Pole, you are talking about one of the possible ways (a shorted quarter-wave feedline stub) to feed an (nearly) end-fed wire antenna, while simultaneously transforming the very high impedance I showed in post #29 to something friendlier to a transmitter and a feedline.

    Are you saying that a magic transformer with a 1:6, 1:7, 1:8 or even 1:9 winding ratio cannot do the same as the feedline stub mentioned above?
    Guess what, it can, and it can do it over a wide range of frequencies, while the stub can only do it for frequencies where it is an odd number of quarter wavelength long.
     
    KX4O and KP4SX like this.
  3. W9XMT

    W9XMT Ham Member QRZ Page

    Can you/anyone post the radiation patterns that this EFHW produces on all of those bands that correspond with the frequencies in that chart ?

    If they are accurate, they will show many pattern nulls on the higher frequencies/bands.
     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Of course they will.

    That's the advantage of installing one as a sloper and not a flattop wire: If you slope the same wire at 45 degrees, while all the same nulls still exist, they now exist at various elevation angles instead of azimuths and when you "light up the sky" in that manner (e.g., having peaks at 10-20-30-40-50-60 degrees, with nulls between) the antenna behaves in a much more omnidirectional fashion which is usually better than having deep nulls at several azimuth angles.

    That's why I wrote about having one end at my shack window with the other end up in a 120' tall tree. "Sloping" really makes an obvious difference.
     
    NH7RO likes this.
  5. KP4SX

    KP4SX XML Subscriber QRZ Page

    Exactly. Its only "half wave" at one frequency.
    As it gets longer or the wavelength gets shorter it will approach the behaviour of a true "longwire" and the pattern will become somewhat like a cone off the end with the strongest lobes 20-some degrees off from the direction its pointing.
     
  6. W5DXP

    W5DXP Ham Member QRZ Page

    It IS lossless in the thought experiment in which I define the conditions. You are free to define your own thought experiment. We talk about SWR circles on a Smith chart but, because of losses, there is no such thing as an SWR circle on a Smith chart for a real world transmission line. We talk about conjugate matches but the conjugate matching theorem assumes a lossless system.

    Of course there is, Michael, in virtually every college textbook dealing with transmission lines. In Ramo and Whinnery, there is a section titled, "SINUSOIDAL WAVES ON IDEAL TRANSMISSION LINES". (emphasis is not mine.)

    We give the gain of an antenna in dBi. Do you object to that because an isotropic antenna cannot exist in the real world?
     
    Last edited: Feb 28, 2018
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    Assuming that the transmitter has a 50Ohm output impedance, then the 50 Ohm load dissipates the transmitter power minus what is not lost either in the coax or the 300Ohm feedline. If you can come up with lossless coax and lossless Ladderline, then 100%
     
  8. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    Now this has been discussed endlessly in other threads but no, they can't. The quarter wave matching stub of a Zepp is performing double duty as counterpoise (quite a good one too, as it is about a quarter wavelength long), something the transformer cannot do with it's small physical size.
     
  9. W5DXP

    W5DXP Ham Member QRZ Page

    Conclusion: Assuming ideal transmission lines, even with highly mismatched loads, 100% of the source power can be delivered to the load, i.e. with a steady state conjugate match, reflected power is not lost but is instead delivered back to the load. Given real world, relatively short, low loss transmission lines, most of the power reflected by a reasonably mismatched antenna is eventually radiated by the antenna because at the match point, reflected power is redistributed back toward the antenna as part of the forward power. Again, assuming lossless transmission lines:

    100w XMTR---50 ohm coax---+===1/2WL 300 ohm twinlead===50 ohm load

    What is the forward and reflected power on the 50 ohm coax and on the 300 ohm twinlead? Where is the match point?
     
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    Ok, but I just demonstrated that a half-wave end-fed antenna provides its own counterpoise, radiates the same as if it were center-fed, and by implication, does not require a separate counterpoise.

    Are you saying that adding 100% more wire to the existing antenna (i.e. two quarter-wave parallel wires coming off the fed-end of the dipole, either co-linear or at right angles) will make the antenna radiate more power? Change the directivity?
     

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