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. W9XMT

    W9XMT Ham Member QRZ Page

    Please offer the reason for focusing on a "DC Pulse" measurement of a single wire to analyze/predict the performance of that wire when radiating r-f energy.

    The r-f spectrum of a pure d-c waveform is zero hertz, so such a measurement tells nothing about the radiating capabilities a wire may have at radio frequencies.

    I have performed hundreds of d-c and r-f pulse tests of FM/TV broadcast antenna systems where the d-c resistance of the complete load including the antenna at the far end of the transmission line was 100 milliohms or less. Of course that produces a huge reflection in the time domain on the video display of a DC Pulse test set.

    However, when that system is tested at the operating frequency/channel using an r-f pulse having the same width as the d-c pulse, its performance is markedly different (see the pic below).

    These scope pics were made using the demodulated output of a directional coupler located at the source end of the transmission line. The incident pulse in the reflected condition is suppressed by orienting the coupler to its "reflected" setting.
  2. W5LZ

    W5LZ Ham Member QRZ Page

    ... gigo.
  3. AH7I

    AH7I Ham Member QRZ Page

    Thanks Jim,

    For what wavelengths will it look like a high reactance capacitor?
  4. K9AXN

    K9AXN Premium Subscriber QRZ Page

  5. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Don't see what your asking, must have an opinion that is not revealed.

    The equation used to calculate the surge impedance of a wire in it's correct form uses LOG l/d. When you move to the UHF and above it logically becomes
    the fraction Wavelength/d has nothing to do with the earth. I have to use my time for the 12% per wavelength and antenna internals post. CYA there.

    Regards Jim
  6. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Bob, Think I see what your eluding to. Sounds like a microwave frequency --- am I right? Your turn to explain. This is an instructional educational experiment meant for frequencies VHF and below and explained so that anyone could understand the concept.

  7. W9XMT

    W9XMT Ham Member QRZ Page

    As I posted, the pulse waveforms in my graphic show the demodulated r-f envelope. The incident pulse shown there is essentially the sin² shape of the pulse leaving the pulse generator.

    The rise/fall times of that sin² pulse have a half-amplitude duration of about 0.25 µs, and the baseband spectrum it occupies emulates that of the highest video frequencies in an analog broadcast TV signal.

    Also note that the reflection of that pulse from the far end took about 6 µs to return to the source, which indicates that the transmission line at this site was about 1,500 feet in length.

    That r-f pulse signal tested that antenna system over the entire r-f bandwidth it uses, and eliminated off-channel "media" responses such as at d-c/0 Hz, which only pollute the measurement, and are meaningless to the performance of that antenna system.
    Last edited: May 24, 2019
  8. AA5CT

    AA5CT Ham Member QRZ Page

    re: " The best analytical source is Termans chapter "Dampening" "

    Not found in the 1937 edition of Terman, "Radio Engineering", nor is "Dampening" listed as any chapter title in the 1943 edition.

    1943 -

    1937 -

    Would appreciate a pointer where to look for this, in the case where I overlooked something.
  9. K9AXN

    K9AXN Premium Subscriber QRZ Page

    It's in the fourth edition of Termans "Electrical and Electronic Engineering Series. It may not be under a Chapter heading. It should not be difficult to find in any engineering text that speaks to Resonant circuits. If your uncomfortable fingering through the Resonant circuit section just Google Resonant and dampening and be done with it. Don't have the time to walk you through what you must have covered as a highly qualified tek or engineer. You will see the concept when we go through the internals of a 1/2 wave dipole and how it presents a 72 Ohm impedance.

    Regards Jim
  10. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Richard, I have no idea what you are trying to prove! The experiment that I posted, using DC pulse clearly and accurately proved the surge impedance, velocity factor, frequency that the wire would be electrically resonant, and that there was no measurable energy returning to the wire. The reason that experiment was posted is that it is not possible to understand wire antenna theory without understanding Surge impedance and the properties of Surge current.

    The only merit to your post is to verify that no channel interference will occur when multiple frequencies are sent down the transmission line and antenna. The reason is, when a group of frequencies occupy the same transmission line and are modulated, interference can occur because spin up and down of resonant circuits and wire will cause impedance variations and mismatches until steady state is reached. If there is some other reason please explain it. Don't vomit a page of calculations unless you are willing to explain them using words that are appropriate to this list.

    I posted the experiment and explained it precisely using wording that is appropriate to this thread. If you took the time to view the video and read the multiple explanations you must have an opinion as to whether there is some violation of Kirchoff. Ampere, or concept.

    Regards Jim

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