Current distribution on / in NOT in center fed antena

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AA7EJ, May 12, 2019.

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  1. AI3V

    AI3V Ham Member QRZ Page

    The problem as I see it is that almost any length of wire, installed almost any way imaginable WILL radiate all the power applied to it.

    Somewhere.

    And the amount of power radiated (read maybe somewhere other than where you want to talk) might be $ubstantially less than you are paying for due to unpredictable matching circuit losses.

    Rege
     
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    ^This is the essence.

    Another problem is, although "Shirley" wrote "not center fed antenna," he did not state how long it was electrically.

    There's always no current at the far "end" of any wire antenna, but the current at the feedpoint can be anything from almost zero to "maximum," depending on the electrical length. In a 1/2-WL or multiple 1/2-WL antenna, FP current is very low; but if the same wire is 1/4-WL or odd multiple of 1/4-WL, it's maximum, and as high as it will and can be.

    The only constant is what happens at the far end.

    An "EFHW" is only a half-wave at its fundamental frequency, and sorta close to that at harmonic frequencies. At all other frequencies, it's not and doesn't resemble anything like one.
     
  3. W5DXP

    W5DXP Ham Member QRZ Page

    Speaking of Windoms, here are the currents for a 130 ft Windom used on 40m.

    Wndm40m.png
     
  4. K9AXN

    K9AXN Premium Subscriber QRZ Page

    For Shirley: Will answer in blue to separate our discussion and answer all but the folks who want to poison this thread with vitriolic BS about vendors and theories that they can't explain. If you don't mind I would like to attempt to answer the valid questions above because surge impedance and current must be understood in order to pursue this thing.

    For Ed AA7QQ: Everything that I post on this list can be assumed to be in space conditions except the experiment. The experiment was tailored to mimic in space conditions as it radiates little or no energy that can be reflected or returned. Did you review the schematic and watch the video? The experiment is used to measure the velocity factor, surge impedance and defects in the integrity of the wire. The velocity factor and length tell me what frequency the wire will be electrically resonant. The surge impedance will be used to calculate the radiation resistance.

    Valid statements and answers:
    Answered several times in past posts. The current that reaches the end of the wire does go to zero because of the collapsing mag field --- for the very end atom so to speak but 1 atom before and after that event the total unsigned combined current will be the same as anywhere in the wire where the incident and reflect have met. It's a form of wave guide ---- simply carries on just in the opposite direction. Put one scope probe on the feed point and the second further down the wire or the end. You will see the voltage double at the end to 8 volts where the incident and reflected meet and remain at 8 Volts as the reflected moves down the wire. The current will continue until the return pulse reaches the feed point. You may think about theory as you please but as seen in the video of the experiment, the current ceases when the pulse returns and is quenched by the resistance at the voltage divider.

    If you haven't referred to the schematic and watched the video please don't pose a question regarding this material.

    Regards Jim
     
  5. AA7EJ

    AA7EJ Ham Member QRZ Page

    "You left out part of the sentence you quoted. Note they say that the current must drop to zero at the ends of the antenna, where the conductivity goes to zero."

    No, I did not.

    But I am glad you have pointed out such gross omission up front.
    Apparently some need to be told that end of wire has no conductivity to accept the fact that is what causes the TRAVELING WAVE to reflect back to the source.

    If it was commonly accepted we would not have been subject of many pictures, some animated, of STANDING WAVE distribution on RESONANT antenna when MY goal is to see forward TRAVELING / SINGLE shot current distribution.

    Just friendly reminder - the quoted resources , or guide for THIS thread, specifically states "SHORT ANTENNA" and has no mention about frequency (in use).

    It is my believe that single shot (pulse) current distribution should be independent of frequency.

    As far as "no current "at the END of wire goes - there are TWO terms - source and load.

    The "BEGINNING / start " of end fed wire antenna is SOURCE and NOT an " end" it real sense - initial traveling wave / pulse will travel towards the REAL end - load - NOT towards the source .

    Also the physical source - coax / twin lead etc. is immaterial.

    I am strictly interested in current distribution on (SHORT) antenna - not on "source" .

    I would recommend to anybody aspiring to discuss counterpoise to go to the other thread where the real knowledge about counterpoise can be found.

    73 Shirley
     
  6. KX4OM

    KX4OM Ham Member QRZ Page

    Just a couple of comments.

    It is useful to think in terms of electrical charge and the coulomb field around the antenna. Accelerating electrons (vibrating at the AC feed frequency) produce charge.

    The feed point and ends of a center fed resonant dipole have something in common. The radiated power is minimum at those points along the length. Not equal, but minimum. At the center, feed current at about 72 ohms resistive. At the ends, vanishingly small current, and high impedance.

    As to Shirley's short dipole, the Maxwell equations applied to antenna analysis (as well as modeling) consider the wire dipole as a string of short dipoles.

    Ted, KX4OM
     
    Last edited: May 13, 2019
  7. WB5WPA

    WB5WPA Ham Member QRZ Page

    re: " current must drop to zero at the ends of the antenna"

    In case no one has added: Displacement Current picks up where electron current stops.

    References upon request.

    References as to the Confirmation of the existence of Displacement Current also upon request.

    PS Displacement current is what "connects" one end of the dipole to the other. A crude example below showing electric field circuit (first image (a))


    electricdipoleoscillating_642x781.gif
     
    Last edited: May 15, 2019
    W0RIO likes this.
  8. AH7I

    AH7I Ham Member QRZ Page

    Dear Jim-

    I'm puzzled.

    With a short wire how do things look:
    When the rise time of the pulse is the same as the length of the wire;
    When the wire is as long as it takes the pulse to rise to ~70% of maximum (~3 volts in your demo);
    When the length of the wire is < 1% of the rise time (getting close to Shirley's "short")?

    73, -bob

     
  9. AH7I

    AH7I Ham Member QRZ Page

    Dear Karl-Arne,

    Of course there is current at the feed point when energy is being transferred there.

    Whether or not we call it the end has nothing to do with what is happening there.

    There is a lot of conceptual difference in the EFHW discussion.

    No current at the end vs current at the end. When not discussing the same 'end'!
    Current vs current distribution.
    Voltage vs voltage distribution.
    Charge vs charge distribution.
    Voltage reference to St Elmo, another point in the wire, another point in time, the other side of the feed...
    Radiating vs reactive fields.
    B field, H field, E field, EM field.
    Superposition applies and then it doesn't.
    Antennas modeled with different parameters.

    As a practical matter, an important thing to keep in mind when deciding on EFHW or another antenna with a high impedance feed point, is the potential difference across the feed point. My full wave 40m center fed antenna would arc across it's center insulator at 500 watts. I changed the length of the wire and lengthened the insulator. With lowered impedance and a longer path, it's been good. I hope to try it with 1200 watts come winter.

    73, -Bob
     
  10. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Bob, It will look like a very high reactance capacitor. A great RF ground is a circular sheet of metal with a ribbon connected at the center. Use l/d not h/d in the equation. The thickness will represent the l, and diameter the d --- a very high Q capacitor with no distributed properties.

    Regards Jim
     

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