1/2 Wave center fed resonant dipole --- why is the impedance 72 Ohms

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K9AXN, May 25, 2019.

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  1. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Ted, I would like you to first review the technique and schematic that was presented to measure the Z0 or surge impedance of a wire, coax, or twin line. It's imperative to understand the 600 Ohm concept. The 600 Ohm Z0 of a stand alone wire is approximately 600 Ohms but is influenced by diameter, length and wavelength of the signal applied.

    The schematic for the technique is at http://k9axn.com/attachments/Finished_3_jpg_final_for_web.jpg

    The video describing the technique is at http://k9axn.com/attachments/Single_wire_good.avi Make sure you have both.

    The setup uses a 54 foot length of bare wire and is represented by the red antenna in the schematic. You see a voltage divider that consists of the instruments 50 Ohm impedance and a 1000 ohm carbon noninductive variable resistor rendering a 50 to 1050 Ohm range. The other half of the voltage divider is the 54' wire.
    The signal is a 1MHz 8.0 Volt square wave. The center of the voltage divider is the junction of the antenna and everything else.

    Have to break off until tomorrow.

    Regards Jim
  2. W4KJG

    W4KJG Subscriber QRZ Page

    About 50 years ago I was taught 1/48th scale brass antenna modeling on big turntables by some pretty well respected individuals. This work led to what became computerized antenna modeling over about the next 15-20 years. We were taught to start with the basic assumption that we were transforming our designs to 377 Ohms of free space impedance and its purtubations. We designed some pretty crazy configurations that worked incredibly well in very special situations.

    I just can't get excited about abstracts. Broadband matching networks are pretty easy to design.
  3. W5DXP

    W5DXP Ham Member QRZ Page

    Ted, the "surge impedance" is the same characteristic impedance seen by a traveling wave such as exists on terminated long wire antennas, rhombic antennas, and terminated Beverage antennas. Consider the following quotes from The ARRL Antenna Book chapter on traveling wave antennas. "The terminating resistor R is also equal to the Zant of the wire ... The rearward pattern when the wire is terminated with a 600 ohm resistor is reduced about 15 dB ..." and "The characteristic impedance of an ordinary rhombic antenna ... is in the order of 700 to 800 ohms..." and "Zant = 138*log(4h/d) where Zant = characteristic impedance of the Beverage = terminating resistance needed". All of these traveling wave antennas have a Zant characteristic (surge) impedance close to 600 ohms.
  4. SM0AOM

    SM0AOM Ham Member QRZ Page

    The whole discussion here is about an 1/2 wavelength resonant dipole in free space.

    The concept of a constant "surge impedance" of a wire in free space is strictly valid
    only for terminated or infinitely long wires, on which there are no standing waves.

    For unterminated finite length and resonant antennas, this is be modified by the standing wave pattern, which led to the development of the Schelkunoff "mode theory".

    This model adequately explains why "fatter" antennas have both shorter fundamental resonant lengths and wider bandwidths due to their lower Q which goes contrary to W9AXN:s statements.

  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    A "terminated" wire can only exist in close proximity to the surface of the earth, such as a Beverage antenna.

    An infinitely long wire in free space is of little use to the average ham...

    'Axn's original experiment is measuring nothing but the RC time constant created by the source's internal resistance and the capacitance of a wire relative to its surroundings. He could get the same waveforms by replacing the "wire" with a capacitor.
    Last edited: Jun 13, 2019 at 3:14 PM
  6. K9AXN

    K9AXN Premium Subscriber QRZ Page

  7. KX4OM

    KX4OM Ham Member QRZ Page

    Okay, then. Thanks to K9AXN, SM0AON, W5DXP and WA7ARK for those explanations. I also read Tom, W8JI's page last night on three measurement techniques to determine the proper termination of the Beverage antenna, which I figured would be an appropriate use case.

    When I originally read Jim's work, I was thinking that the 12% per cycle reduction was similar to what happens with an R-C time constant.

    Ted, KX4OM
  8. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Ahhh, the second voice of reason in the wild

    We have motion Thanks and a GDay to you

    I'll get started with the full explanation of the experiment if there is a break in the Flack field.

    Regards Jim
  9. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Cecil, you are the first person on this thread that has the courage to respond that actually understands antenna logic. You to must have been educated before the CAD tools replaced the Applied Physics and logic classes.

    Kindest regards Jim
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    Ah, but a Rhombic is two wires, which form a complete circuit loop, where Kirchoff's Current Law holds at every point around the loop.

    Ditto for a Beverage antenna, which is two conductors, a wire parallel to the earth and current that returns through the earth...

    All you showed in your original experiment is that a piece of wire can be one plate of a parallel-plate capacitor relative to the earth (maybe house wiring?). You observed that when driven with a square wave, it takes a transient current to charge and discharge that capacitance. You then launched off formulate a new "antenna theory", overlooking the obvious fundamental principles, which, by-the-way, the CAD tools are based on...

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