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. AA7EJ

    AA7EJ Ham Member QRZ Page

    Please note / read the title and STAY with the subject if possible.

    Here is a starting point of reference :

    Here is a direct quote form the link above

    That is, the radiated electric field strength E[​IMG] is proportional to the integral of the current distribution along the antenna. The current at the center is just the driving current I=I0e−i[​IMG]t and the current must drop to zero at the ends of the antenna, where the conductivity goes to zero. For a short antenna, we can make the approximation that the current declines linearly from the driving current at the center to zero at the ends:

    I have taken the liberty to highlight what I feel is a statement to agree or dispute with.

    IMHO it is NOT necessary to add any other antenna parameters / variables etc.
    I am talking about feed line , feed point impedance , radiation pattern, common mode current etc.
    And especially any "periodical" variables such as frequency , wire size, wire color etc.

    This should be purely theoretical discussion about how the current is distributed in a radiator which IS NOT fed at /in center.

    I think the starting argument point could be - if the antenna is end fed AND per quote above " current must drop to zero at the ends of the antenna" - how does that jive ?

    If despite my suggestion there is / will be an (friendly) arguments about "counterpoise" or "no such thing as end fed antenna" - try using Windom instead.

    73 Shirley
  2. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Try this for a starting point. It describes a pulse sent to a wire and should be relevant to what you ask. is the circuit used and the video clip.

    The firsts is a schematic of a technique using a voltage divider to measure the surge impedance and traveling wave on a WIRE, coax, or twin transmission line. The second, a video clip demonstrating the measurement technique. The video is of an 8 Volt 1Mhz square wave sent to a 54' wire. You can use a dual trace probe and to measure the wave at any point on the wire providing unambiguous logic points. The video logic is to adjust the variable resistor to the 1/2 voltage point (4 Volts) which is the balance point then read the resistance. This determines the surge impedance, Z0 of the wire.

    What you see:
    For 57ns the wave is traveling outward at 4 Volts @ 6.666mA representing a Z0 of 600 Ohms. The first 57ns you will see a pure unmolested traveling wave. At 57ns the wave reaches the end and is reflected toward the feed point and the outbound and inbound will add to create an 8 Volt wave. When reaching the feed point, the incoming will pass into the 600 Ohm resistance which is a perfect load and current ceases. The surge impedance of the wire is the measurement that represents the anchor for other measurements in an antenna.

    This was presented in another thread and shouted down in the past. Anything that you find puzzling, please don't hesitate.

    If we can get past this I'll explain the 12% calculation and how it fits.

    Regards Jim
  3. AA7QQ

    AA7QQ Ham Member QRZ Page

    How can you figure an antenna's performance without the frequency in the equation?

  4. AA7EJ

    AA7EJ Ham Member QRZ Page

    That is what I was trying to get across.
    I very vaguely recall , it has been few years, that "pulse" is commonly used to demonstrate the physics of TL.

    I do not care for the trend the other thread took, but somehow I feel that it is painful for some to imagine that antenna can accept something beside pure sine wave (@27 MHZ HI HI ) .

    So professor - tell me about the 12% , I am ready.

    73 Shirley
  5. AG6QR

    AG6QR Subscriber 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.

    In any antenna, at any point where the conductivity goes to zero, the current must drop to zero.

    At the "fed" end of an end-fed antenna, does the conductivity go to zero? If not, there's no reason to assume that the current would go to zero (it might go to zero for some other reason, like because it's an integral number of half-wavelengths from a non-conductive end, but you said you didn't want to bring in frequency). If so, how is the antenna being fed, if not by a conductor?
    AI3V and KD6RF like this.
  6. KD6RF

    KD6RF XML Subscriber QRZ Page

    It doesn't jive.

    For center or off-center-fed antenna physical current = 0 at the ends as you state. The role of "displacement current" is fun to talk about but does not add to the basic understanding.

    For end-fed, where the "end" is defined as the tip of the 1/2 wave "intentional radiator", current is NOT zero as we have discussed several times, and depends upon the length of the counterpoise (often the just coax shield).

    In addition, the current standing wave on the counterpoise (often the just coax shield) depends upon it's length and termination - just as it does for the "radiator". "Radiator" is in quotes because the counterpoise ALSO radiates in proportion to the length and current standing wave - also just exactly like the "radiator".
    Last edited: May 12, 2019
    AI3V likes this.
  7. SM0AOM

    SM0AOM Ham Member QRZ Page

    Perhaps the most convincing argument for a non-zero current at the fed end of the EFHW antenna is the fact that it actually is supposed to have power delivered to it.

    Power transfer when the current is = 0 is a contradiction...

    KX4O, AI3V, AG6QR and 1 other person like this.
  8. KD6RF

    KD6RF XML Subscriber QRZ Page

    That would be great Karl-Arne, but conservation-of-energy and Kirchoff are not well understood on the forums :rolleyes:
  9. SM0AOM

    SM0AOM Ham Member QRZ Page

    There are very few amateurs with proper engineering backgrounds around nowadays...

  10. KD6RF

    KD6RF XML Subscriber QRZ Page

    True - and that's just fine. I've always believed that one doesn't really need to be an engineer to understand that electrons aren't conjured up from unicorn dust !!! ... But sometimes it's even manufacturers and other weird agenda dorks - which really confuses the issue for folks who are honestly trying to understand.

    Anyway, GL Shirley!
    Last edited: May 12, 2019

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