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. AF7TS

    AF7TS Ham Member QRZ Page

    Seeing as that is one of the models that I use in my thought experiments to try to understand such antennas, I think it would be a good start.

    I'd like to offer some refinements:

    1) The 'altoids tin transmitter' would need to include some sort of adjustable antenna matching. While we may disagree on the practical functionality of 'end fed antennas', I think everyone agrees that the impedance at the feedpoint changes as the feedpoint is moved toward the end.

    2) Field strength tests need to consider both unmodulated carrier, a frequency sweep and some sort of real world signal. It would seem that the impedance versus frequency of an end fed antenna should show greater variation than that of a center fed antenna; would this 'dynamic match' mean that the antenna works efficiently only over an unacceptably narrow frequency range...or could this be a useful feature?

    3) Is there some way to test the effect of different feedpoint dimensions? It is known that for center fed antennas, the feedpoint dimension can be used to adjust impedance (the 'delta' match). It seems to me that the larger the feedpoint dimension the greater the asymmetry seen in an off center fed dipole, and that an 'extreme off center fed dipole' would require very small feedpoint dimensions.

    For what its worth, the experiment that I have planned (but which I cannot implement until I get past some heath issues) involves putting a battery powered antenna analyzer on a 'clothline' antenna (folded dipole with pulleys at the end, so that the feedpoint can be slid from side to side.)

  2. W9XMT

    W9XMT Ham Member QRZ Page

    Other things equal, it will show how much power those configurations actually radiate for a given transmitter output power -- and that will depend on the loss of the network needed to optimally match their respective feedpoint terminal impedances to that needed for the r-f source (transmitter).

    Given the very high input SWR of the EFHW, its matching loss might predictably be greater than that needed for a λ/2, center-fed dipole (see the graphic below).

  3. AI3V

    AI3V Ham Member QRZ Page

    Of course you will be able to add any matching network you think will help, as long as the network is physically small and completely shielded.

    What say thee "end fed" sellers, want to donate a test antenna?

  4. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Need to clarify some things so you might help. Since the surge impedance of the wire is the building block that all else follows in a free space model; Please tell us what equation is to used to calculate the value, and explain it's derivation --- using words to verify that you understand what you say.

    The Cartoon --- ehh --- video clip that I provided clearly demonstrates that a single connection to the end of a 54 foot wire resulted in .0063A DC passing into that wire for 116ns and it's amplitude was constant --- indication that the stress to the charge pool was almost immeasurable.

    Without regard for what occurred at the function generator --- that should be clear.

    I believe that the real difficulty is when a +4 Volt DC pulse is sent to that wire, you simply cannot comprehend that current could have entered that wire ---Because there is nothing at the other end to satisfy the laws that folks quote. If you choose to quote an equation you have to apply it universally not just to satisfy an agenda --- what's at the other end of that wire?

    Here's a photo of the configuration that I used clearly describing the use of a charge pool that does not request help from the ground connection at this power level. How do I know that? Attach a scope probe to the instrument chassis and another to earth ground.

    What is not shown is a 36 inch circular copper sheet attached to the bottom of the shelf with a short braid attached to the center of the sheet running up to the instrument ground. You will understand this configuration when you explain the equation used to calculate surge impedance. Here's a photo of the physical configuration.

    A great day to you

    Regards Jim
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    Back in the '70s, I used to run the 75m mobile antenna efficiency contest (shoot-out?) at the WIMU Hamfest that was held at West Yellowstone every summer. I used a battery-powered, crystal-controlled 3.9MHz transmitter. To make it "fair", I put a 10db resistive pad between the final stage of the transmitter and the antenna spigot on the test transmitter. This forced the contestants to "adjust" their antenna to accept power from a 50 Ohm source, and it made the power output from the transmitter more-or-less constant, regardless of the actual load or swr.

    The pad wasted about 90% of the power from the transmitter, but made the comparison between antennas easier...
  6. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Are you sure the end feed point Z is 2250-j4000? If the antenna is 1/2 wave long that would mean the input looks like 2250 ohm resistance in series with about 6 pF on 40 meters. I guess I am misunderstanding the data. Thanks! Pete
  7. W9XMT

    W9XMT Ham Member QRZ Page

    That is the feedpoint Z calculated/reported by NEC4.2 for the dimensions and other conditions shown in that graphic. The NEC wire model tests showed no segment errors and no geometry errors.
  8. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Use the tool to calculate the impedance if the antenna is cut to the resonant length at the frequency used.

    Regards Jim
  9. AI3V

    AI3V Ham Member QRZ Page

    We are not interested with what your scope ground lead is attached to.

    We are interested in how a single wire behaves when connected to a real world transmitter.

    Again, the test is simple, a RF source, and a single wire, and then a pair of wires.


    P.S., I am familiar with applying a DC source to a (lumped constant) "transmission line", it's a common circuit in RADAR sets, part of the pulse forming network.

    In short, you store a charge in a length of line, and discharge that line into the magnetron.

    It was an interesting class.

    (Building 520, great lakes naval station)

  10. W9XMT

    W9XMT Ham Member QRZ Page

    Below is a plot of the input SWR vs. frequency for the EFHW configuration/conditions described in the graphic of Reply 162 of this thread. The end-end total length of the two, co-linear conductors in both directions from the EFHW feedpoint is 65.6 feet (which, including end effect, is an electrical half-wavelength at 7.25 MHz).

    In reality, the radiating configuration of an "EFHW antenna" is that of an off-center-fed dipole.

    This configuration is FAR from being self-resonant. For efficient power transfer from the source (transmitter), this EFHW antenna needs a network at its feedpoint to match its input impedance of about 2250 -j4000 Ω to the nominal value of 50 ±j0 Ω expected by most transmitters.

    The r-f loss of that matching network reduces the amount of the available transmitter output power that is actually radiated by an EFHW antenna.

    OTOH, a center-fed dipole has its first self-resonance (zero reactance across its feedpoint terminals) when its free space, end-end length is about 3% less than λ/2.

    Last edited: Mar 13, 2018

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