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. K9AXN

    K9AXN Premium Subscriber QRZ Page

    If the antenna is a RESONANT and fed at the end of the wire, not some distance down line it will be 4900 ohms with little if any reactance.

    Regards Jim
     
  2. W9XMT

    W9XMT Ham Member QRZ Page

    Kindly post the conductor length(s) and OD(s), their physical orientations with respect to each other, and the operating frequency for the configuration of an end-fed half-wave antenna that you believe will have a steady-state input terminal impedance of 4900 ± " little if any reactance" Ω.
     
  3. W9XMT

    W9XMT Ham Member QRZ Page

    A simple analogy -- if significant steady-state power can be delivered by the "charge pool" of a power source to a load connected to it by a single conductor, then it would be impossible to turn off most flashlights. The on/off button of a flashlight interrupts only one of the two paths leading from the two ends of the bulb filament to the batteries.

    All steady-state electrical loads (both a-c and d-c) need simultaneous and continuous connection to the power source using two conducting paths. Otherwise there is no path for current flow.
     
  4. AF7TS

    AF7TS Ham Member QRZ Page

    I believe that this statement is strictly true only as the feedpoint dimensions approach zero. With finite feedpoint dimensions (eg. as modeled by W9XMT) the feedpoint will have reactance.

    An interesting question is how sensitive the feedpoint impedance will be to feedpoint size, and how sensitive the impedance is to changes in frequency.

    73
    Jon
    AF7TS
     
  5. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    4900 sounds a bit high. Many are being successful with 1:49 Z step up ratios correlating to about 2450 ohms with a low SWR. I am looking forward to testing one of these antennas/transformers in about a month when the snow is gone and it isn't too cold:)
     
  6. KD9BYI

    KD9BYI Ham Member QRZ Page

    maybe this would be a good time to ask about the end fed setup I have been planning. I skimmed over this https://www.w8ji.com/long_wire_antenna.htm

    I was considering to run a wire from my deck angled upwards to the wooded area in the back of my lot that gets progressively taller with tree's. The counterpoise wire I was going to run straight out underneath. Maybe 6 feet off the ground due to the raised deck. I was going to connect a good 1:1 current balun directly to the antenna / counterpoise and plug the manual tuner in right there. If I like it maybe a radial field later on but then I think the balun would have to be on the ground and there would be more vertical section of antenna.

    I've found it very difficult to raise dipole feed points into tree's and then hope they are in just the right place to correctly route the coax away from the antenna. Putting up even hundreds of feet of wire going for the sky being fed from the end? No problem! Current dipole is about 20 feet up. Would I do better with the end fed considering I can angle it up much higher? I'm also not sure what length I should go for.

    I was hoping for 40- 15 meters without the pattern getting weird but would be even happier if I could do 80-20 with a usable pattern. Or later on I could add another wire for 80/160 and swap the connection to the radial field etc.

    thanks!
     
  7. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Good morning Richard,

    As for the length of a truly resonant wire, I have to defer to the measurement that I collected from the circuit in the video and photo. I used a 54 foot #14 copper wire. +4V @ .0063A was sent to the wire and returned 115.75ns later. The velocity factor was .917 as measured total length 2 X 54=108 feet/(115.75/.9836). For the length it would be resonant at .917 X 1/2 wl.

    Best thing to do is add a cycle by cycle trip through the feed sequence to the end of a resonant 1/2 wave wire.

    I have to put it on the web site --- too difficult to key up here. I'll reply when it is finished.

    BTW, this will represent the wire alone without regard for the configuration or method of coupling. The coupling is a total other subject. Suggest not coming to conclusions before reading the script.

    Have a god day

    Regards Jim
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    No excuse.

    +3 dB if it's below freezing.
    +6 dB if it's below zero.
    +9 dB if it's below zero with 40+ mph wind.
    +12 dB if it's below zero with 40+ mph wind and you have to stand on a steeply sloped, ice-covered roof.
     
    WB2UAQ, K9AXN, W8IXI and 2 others like this.
  9. W9XMT

    W9XMT Ham Member QRZ Page

    Q: How do you define resonance for this example?

    A: ?

    Q: At what frequency do you consider that 54 foot of #14 copper wire to be resonant?

    A: ?

    Q: What is the steady-state value of the impedance (R ± jX Ω) you ascribe to/at the end feedpoint of this single wire at the operating frequency?

    A: ?

    Q: How does the use of a pulse generator having most of its energy contained in a spectrum lying well outside the resonant frequency of an antenna show whether or not that antenna is resonant ?

    A: ?
     
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    I am posting this to further the discussion about end-fed antennas.

    I recently stood up a 40ft vertical antenna made from 2" aluminum irrigation pipe on a large steel roof 20ft above earth. The base of the vertical is about 20ft6in above ground, while the tip is at 60ft6in. The antenna stands on an insulated base, where the there is a gap of about 1.5" between the bottom of the vertical, and a support pipe that is electrically bonded to the steel roof and projects up 4.5" through the roof.

    I am using the pipe as a non-resonant vertical antenna, where I feed it via a remote L-C tuner network on 60m, 40m, 20m, 17m and in the future I plan to expand the tuner to cover 15m, 12m and 10m.

    To come up with the required matching networks for the various bands, I measured the feedpoint impedance at various frequencies. The antenna length is not a pure quarter wavelength (or odd multiples) in any ham band by design; neither is it a pure half-wavelength (or integer multiples) in any ham band (except by accident on 15m), but it might be instructive to show how R and jX vary as a function of frequency, because this antenna could easily be scaled (cut down or length added) so that the quarter wave resonances, or the half-wave resonances would fall into (or avoid) the ham bands...

    So the first plot is the SWR of the 40ft vertical against the metal roof ground plane sweeping frequency from 3 to 33MHz. Note the three frequencies at the SWR minima, 5.8MHz, 16.9Mhz, and 28.4 Mhz (close to 10m). These three "resonances" occur where the antenna is ~1/4, 3/4, and 5/4 wavelengths, respectively. All these measurements were made using a RigExpert AA-600. Click on any of these images to enlarge them:

    SWRWide.png

    Next is a plot R and jX over the same frequency range. If you consider an antenna to be "resonant" when jX = 0, note the green trace, X crosses the zero line in six places; three going from - to +, and three others going from + to -. Note that three upward X zero-crossings approximatedly correspond to the three SWR minima, where the R term (the red trace) is reasonably close to 50 Ohms, while the three downward zero crossings correspond to places where the R term hits a maximum, namely at 9.75MHz, 21.375MHz, and 32.625 MHz. This is where the antenna is physically 1/2, 1, and 3/2 wavelengths long.

    RjXwide.png

    In order to look at the details of the half-wave resonances, here are three more plots, each showing more detail near the respective half-wave resonances:

    1HW.png 2HW.png 3HW.png

    The center frequency of each of the preceeding plots is where the X term (green trace) is close to Zero.


    It would be practical to drive this antenna from 50 Ohm coax at 9.725MHz with just this simple network:

    network.png smith.png

    Note that I calculate that the voltage across C1 in the network above is only about 4X the voltage across the coax, so this could work even at a power level of about a hundred W without requiring an exotic capacitor. Now this network would only work at one frequency. It would take a broadband transformer based matcher or a motorized remote tuner like I am building to work multiple bands .

    Operating a half-wave vertical antenna by driving it with coax while using an acre of steel as a counterpoise is much different than driving a half-wave wire with no counterpoise. However, I wanted to show some measured data for a "real" half-wave antenna...
     
    Last edited: Mar 15, 2018
    K9AXN and WB5YUZ like this.

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