Curiosity About Antenna Connections to Coaxial Cable (And one about a balun)

Discussion in 'Wireless & RF Design' started by PGB1, Jan 7, 2019.

  1. PGB1

    Thread Starter Active Member

    Jan 15, 2013
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    Hi All!
    I made a quick prototype of a fractal type antenna to receive digital broadcast television form one of many designs found on line & in text books. I am curious about the wiring:

    In all of the designs I viewed, one of the lead wires is connected to the center wire of the coaxial cable and the other is connected to the shield. (With or without passing through a balun, depending on the particular author's description.)

    The left column of the attached photo (of a really quick prototype) is attached to the center wire and the right to the shield.
    I took apart a television and saw that the shield connector is connected to the case of the tuner.

    The Main Question-
    I am wondering if the right column (attached to the shield) is actually doing any work receiving broadcast signals, or if I would have even better reception if the two columns were connected together to the center coax wire so both columns capture signals. The coax's shield would then be simply dead ended.

    A Balun Question-
    I think I still remember the purpose of a balun from the olden days of parallel cable lead in wires. As I remember, hopefully correctly- if not balanced, one of the conductors will radiate signal away from the receiving components, hence the need for a 75-300 ohm balun.
    In some of the homemade designs, a balun is used to connect coaxial cable to the lead wires on the antenna. In this one, I tried with and without the balun. In each, I received the same channels and the same signal strength bar rating on the weaker stations. (Actually lots more stations than I expected.)
    Is a balun actually helpful in such a short lead wire setup?

    Thanks Very Much for explaining & educating! It's always good to learn & understand new things.
    Enjoy This Day!
    Paul
     
  2. SLK001

    Senior Member

    Nov 29, 2011
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    You need a 300Ω-75Ω (4:1) balun to properly work. The antenna may still work without one (like you have set up), but your signal losses will be high. It's always a good idea to properly match components in a receiver string.
     
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  3. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    There is a possible 'third' wire effect when connecting single-ended coax directly (no balun) to a balanced type antenna. The outside surface of the ground shield is insulated by skin effect from the inside of the ground shield causing a third RF current conductor that can also radiate but it's not something we normally worry about with receive antennas and short transmissions lines.

    If your receive transmission line is short, proper matching does little to help reception since the receiver is really a voltage sensitive input instead of power transfer device. If your receive transmission line is long then proper impedance matching reduces losses from possible signal re-radiation but the actual transmission line loss is fairly fixed even with a total receive impedance mismatch. 300 ohm balanced lines typically have lower loss than coax and the voltage to current ratio of the received signal power is higher.
     
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  4. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    I look at it in a different way. I believe that a wave is a turned inside out, field.

    A wave is what you get when you reverse the natural superposition between the electric and magnetic fields, by 180 degrees. It's anti-resonance. Or reverse resonance. Or out-of-phase resonance. What it physically does....is reverse the handedness of the EM superposition. This is the emission mode.
    TWO perpendicular repellents. An area emission.

    And an area absorption. In absorption mode.....it reverses the handedness of the wave.....back into the handedness of the field.

    If you put a field in the feedpoint, the dipole will emit a wave. If you put a dipole in a wave.....the dipole will give you a small field at the feedpoint.

    The mis-match for emission can be taken in a different context to a mis-match for reception.

    Because an anti-resonance load is different than a resonant source.

    You may think of an antenna as a continuous field to discreet wave transducer(emission), and a discreet wave to continuous field transducer(absorption).

    But for the mean-time, read and study all you can and learn to match all your circuits.

    Look at it as conformity and apprenticeship.

    Because everyone believes Maxwell's interpretation of his EM field equations.

    But I believe a EM emission is discreet, like drops. Not continuous, like a stream.
     
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  5. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Found some people who might agree with you.

    https://www.bbc.com/news/world-asia-india-46778879

    • Also in 2017, the education minister for the western state of Rajasthan said it was important to "understand the scientific significance" of the cow, claiming it was the only animal in the world to both inhale and exhale oxygen
     
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  6. PGB1

    Thread Starter Active Member

    Jan 15, 2013
    91
    5
    Thank You all for taking time to reply and help me (and others) learn. I appreciate your help & thought provoking comments.

    As is the usual case when I visit this group, I have many new curiosities & learning opportunities presented. I began studying more; with a starting point of Maxwell's studies on the origin of EM waves.
    To me it is quite interesting- fascinating actually- although most is still very much over my head. But with more study it will be in my head instead of over it!

    Thanks Again All!
    Enjoy Today!
    Paul
     
  7. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    While Maxwell's equations are the gold standard and a rite of passage in most electromagnetic studies they sometimes don't provide much intuitive power to understanding the origin of EM waves in practical devices like complex antennas and transmission line without using massive computer power for the needed microscopic calculations.

    https://unlcms.unl.edu/cas/physics/tsymbal/teaching/EM-914/section6-Electromagnetic_Radiation.pdf
    One alternative method is retarded time as the charges move (current) in response to the applied potential.
    http://physics.weber.edu/schroeder/mrr/MRRtalk.html

    [​IMG][​IMG]

    accelerating charge

     
    Last edited: Jan 10, 2019
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  8. PGB1

    Thread Starter Active Member

    Jan 15, 2013
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    Thank You All! for the education you are providing. I apologize for my delayed reply. I wanted to study more before responding.

    The videos & reference you linked Nsasppok were fascinating and very easy for this novice to understand.
    Doc Schuster's video and explanation turned out to be 'edge of me seat' viewing. What a great explanation and fun to watch (repeatedly)!

    As is the norm when I read on this forum, I have even more interest in the topic than I had when starting out. I guess there still is room in my brain after all.

    Thank You All again for helping me understand and learn.

    I Hope You Each Enjoy This Day!
    Paul
     
  9. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Glad you enjoyed the post.

    It's also important to note that just changing fields don't mean EM radiation. Maxwell's equations give a wave equation (the propagation of waves in the x-direction at the speed c) that requires a changing flux across space at a instant in time.

    This means that space becomes a part of the physical source antenna in the near-field because there is a phase shift as the surrounding changing fields move outward at c in respect to the changing fields generated from the physical antenna. This phase shift due to retarded action at a distance increases as the length of the antenna increases until there are min/max field strength points along the antenna and surrounding space.

    Retarded action is one key to understanding EM field theory in general.
    From the book: https://www.abebooks.com/book-searc...lines-antennas-and-wave-guides/first-edition/

    [​IMG]
    and is obviously important to understand how antennas work.
    [​IMG]


     
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