Don't Earth a counterpoise ?

Discussion in 'General Technical Questions and Answers' started by M6LVC, Dec 1, 2016.

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  1. KK4YWN

    KK4YWN Ham Member QRZ Page

    the currents should be equal and the voltage opposite. elsewise the transformer / tank is incorrect.

    if you seek absolute perfection i recomend the Sports Illustrated Swimsuit edition. Thats as close as you're going to get. Propagation delays cause errors. I can't bend space-time.
     
  2. AA7EJ

    AA7EJ Ham Member QRZ Page

    OK, making progress.
    I still think that there is a small disconnect in currents representation. .
    The current distribution on radiator shows current flow in "one direction" / in phase and current flowing thru transmission line is / must be 180 out of phase.
    Also the way you describe the DC path thru the part of the center fed with coax antenna could probably be refereed to as "unbalanced " system.
    Unfortunately we usually say that (only) coaxial line is unbalanced, which is little misleading.
    Thanks for such down to earth presentation.
    I sure would like to see similar analysis on full wavelength loop.
    73 Shirley
     
  3. KK4YWN

    KK4YWN Ham Member QRZ Page

    yeah thats a good point. we tend to look at balanced hi-z lines as perfectly balanced. and, in a vacuum, they are. but when connected to a half wave resonant body, the external bodies impose asymmetry.

    an example: a 75m antenna mounted close to ground, with a portion of the radiator in proximity of a truck body, can not be symmetrical.

    in some link-coupled tuners, we can address this imbalance with imbalanced reactance.

    therefore:
    no free lunches.
    no perfect lunches.
    no free perfect lunches.
    no perfect free lunches.
    [if we endeavor to account for every variable logically]

    room still exists for 'art' despite what maths say.
     
  4. AH7I

    AH7I Ham Member QRZ Page

    Try it on your bench. Maybe on 2m band. Tune the tank to resonance. Add the antenna. Is the tank still resonant? What does it take to bring it back to resonance? Are the currents still at 180 degrees?
     
  5. KK4YWN

    KK4YWN Ham Member QRZ Page

    yeah you caught me being lazy. sorry about that. i should not have responded as i did about tuning the tank.

    i discovered the exact same problem last year while trying to build that exact antenna: a near-end-fed halfwave vertical for 2m. i was shooting for a radiation resistance of 4.7k but couldn't quite get the damned thing to work using a fixed-value capacitor. i set that project aside and let it "marinate" for awhile.

    typically, end-fed matching units are built using variable capacitors. i don't think anyone has demonstrated the maths required to solve the power factor problem using a calculator, so tuning capacitors are used to sneak-up on the correct value.

    basically, the resonant tank has no power-factor problem (V leads I as it should). therefore, resonance doesn't solve the off-center feed problem, it puts us back at square one.

    but as we sweep freq through a parallel RLC circuit near resonance, we find that below resonance the power factor lags. above resonance power factor leads.

    so the final solution, while not wholly clear to me mathematically, is to detune the tank. i don't have any variable capacitors to tinker with so i can't put this information to work.
     
  6. AH7I

    AH7I Ham Member QRZ Page

    For that 1/2 wave end fed antenna, we can reduce that common mode current(related to matching the antenna) on the feed line by adding a lower impedance source(sink). This is our counterpoise(in the sense used by the original poster). The 1/4 wave(low impedance) wire used by the original poster, along with a common mode choke(high impedance) on the feed line divides the 'balance' current, with most of it to the 1/4 wave wire. Why was the 1/4 wave wire better than earth? We are not putting as much energy into moving the charge on and off that wire. Like bouncing on a diving board as opposed to bouncing in the sand.

    73, -Bob ah7i/w4
     
    M6LVC likes this.
  7. KK4YWN

    KK4YWN Ham Member QRZ Page

    you chose to solve the equation by throwing out the old antenna and replacing it with something that gave you a simpler match.

    you can deduce very little about a 1/2 wave radiator by studying a 3/4 wave.
     
  8. AH7I

    AH7I Ham Member QRZ Page

    No. I did not throw out the 1/2 wave antenna. Adding the 1/4 wave is not the only way to make it work. Anything that can supply balancing AC of the appropriate phase will work. The 1/4 wave wire is an example. What else works? A ground plane works. What else?
     
  9. KK4YWN

    KK4YWN Ham Member QRZ Page

    you're focusing on the needs of the generator and ignore the behavior of the half wave structure.

    draw a center fed dipole. draw the current distribution. draw the voltage distribution.

    notice anything? I and V are in phase at center.

    what about at the end? power-factor problem. this is why there is no radiation off the ends. i and v are out of phase: there is no real work being done. however, since there is a value of V, we know that the charge is still propagated: we're simply not getting any more radiation because current was consumed along the along the way.

    suppose you move the feed point, isolated it from the feedline, and attempt to satisfy the needs of the radiator at that particular point.

    you would induce a large voltage and a smaller curent that are not in phase

    as the charge travels down the short line v increases and i decreases. in the longer line, the i increases as v decreases, until the problem of phase is settled, where it begins to move into the other half of the dipole and i/v begin to go out of phase again.

    so the claim that the radiating components need to be the same length, or some multiple of 1/4 wavelength to maintain balance is wholly untrue. we only need to isolate the structure and induce a phase error in proportion to the distance to center of the overall body.

    the reason why you can't make it work on your bench is because you're shooting at a moving target. i've been working on the equations to prove this out for some time. i'm not a mathmetician so its going to take some time, but the whole problem relates to dipole impedance when in relation to earth, and which "polarity" you've decided to point toward earth.
     
  10. AH7I

    AH7I Ham Member QRZ Page

    No where have I required 1/4 wave multiples...
    Radiating components MAY be the same length. That works. I^2R losses and radiation losses are equal on the sides of our dipole, in free space. It does not have to be a resonant dipole.

    As soon as the feed is moved from center, I^2R losses and radiative losses are higher on one side of the feed than the other. To maintain a balance of energy into and out of the antenna, we must feed more energy into one side than the other. We do this by accelerating more charge on that side. This requires more work. A phase difference is NOT a work difference!

    There is no way to do more work on one side of the antenna then the other without a balancing force.
     
    Last edited: Dec 17, 2016

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