6M Wire Dipole - Electrically Short, Physically Long?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KV9Y, Jun 14, 2019.

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  1. AI3V

    AI3V Ham Member QRZ Page

    What impedance value does your computer use for the shield of rg8?

    And why?

    Keep in mind you are claiming that by raising and lowering a horizontal dipole, that at certain heights you will see vertical radiation.

    Actual on the air practice does not bear this out.

    Last edited: Jun 15, 2019
  2. KV9Y

    KV9Y Ham Member QRZ Page

    Thanks to everyone who replied. I don't have an antenna analyzer but nearly bought one to figure this out. Here was the problem... The antenna was too long to begin with. :confused:

    My dad always told me to measure twice, cut once. I should have measured ten times and cut once. After trimming it down I managed to get about about 1:1. Now, at 60W the reflected power meter on my MFJ-822 doesn't move at all!

    At least I got in my exercise this morning. :p

    73s and thanks all!
    NH7RO and KA0HCP like this.
  3. KD8DEY

    KD8DEY Ham Member QRZ Page

    Did you use 468/F or 475/F ......
  4. KV9Y

    KV9Y Ham Member QRZ Page

    I used 468/f
    Here's problem #1 - I used 50MHz, and should have used 51.
    Here's problem #2 - I made it 6" longer to allow for pruning. That's what I do for my HF antennas. I didn't take into consideration the shorter wavelength and really should have only left an inch or two for pruning.

    Ah well. It's a good lesson learned.
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    I specify the diameter of the wire 4 conductor representing the coax shield; in this case I used 0.25" like it was RG58 and that the conductor is Copper. The other wires are #14.

    The question you should have asked is: "What is the input impedance at the top end of a 1 wave-length (at 51.0MHz), 0.25"dia, vertical wire that is connected to ground at its bottom?"

    The answer, even without resorting to NEC, is that it will be very low, because the impedance where the other end of the wire ( 2ea 1/2 wavelengths away) is connected to the dirt is low (I guessed 35 Ohms). NEC confirms this, and because there is no balun at the top, wire 3 (antenna leg) and wire 4 (coax shield) are effectively in parallel. Because Kirchoff's Current Law applies to the electrical node where wire 2, 3, and 4 join, the current into the node (from the 1A source in wire 2) must equal the current out of the node (the sum of current in wire 3 and wire 4).

    As the simulation clearly shows, as the length of wire 4 changes (thereby changing its input impedance), at certain heights, more current flows in wire 4 than does in wire 3, which just says that wire 4 presents a lower impedance at that node than does wire 3... If you watch the animation carefully, you can see that the "missing" current in wire 3 goes into wire 4 instead, and that the sum of the two currents always adds to equal the current in wire 2.

    You just haven't looked!

    Raising and lowering the dipole was just a convenience to change the length of wire 4 (to observe the common-mode current on wire 4, the coax shield). Exactly the same behavior occurs if you keep the dipole height fixed, and you vary just the length of the coax. This makes the model more complicated because you have to "bend" the coax to make it shorter and longer...

    This effect is real, and measurable in any antenna where the constructor is inexperienced, (or too cheap) to put a balun at the dipole feed-point!

    Of course the coax will radiate (a lot) if you are unlucky to pick a length of coax that "resonates" in its own right. Even worse, the coax also "receives". If the coax runs into a RF noisy house, then it can pick up that noise and conduct it back up the coax shield to the antenna.

    For a lot of coax lengths, the current is there, but is small enough so that it might not get noticed.

    The OP had some unexpected SWR measurements that didn't appear to make sense. I posted this to show that there could well be more to his simplistic view of his dipole than meets the eye...

    Oh, and by the way, any antenna, be it vertical, dipole, OCF or EF is subject to common-mode currents on its coax. The causes are always the same, the mitigation is the same.

    If I get some time today, I will repeat the simulation of post #6 with a proper common-mode choke at the coax-to-antenna junction.
  6. KD8DEY

    KD8DEY Ham Member QRZ Page

    As others have said, 468/F is just a starting point, and is used for HF
    6m is considered low VHF where the formula is 475/F.....
    N5CM likes this.
  7. AI3V

    AI3V Ham Member QRZ Page

    So nec cannot model coaxial cable.

    Got it.

    And its laughable that you see vertical polarized radiation, that simply is not whats observed in the field, millions of gamma matched Yagi-Uda antennas out there with nary a balun, and without a doubt switching from the vertical to the "flat side" shows the expected cross polarization loss.

    This is something any cb'er with a "moonraker" has observed.

    What do you suppose would happen with a longer feedline?


    P.S. when you add a "choke" , that is , a series resonant coil, the coil is the "lumped constant" version of a half wavelength of wire.

    Whatever current you have one one end of the coil you have the exact same current on the other.

    Now lets talk about the bulk absorption of ferrite, and how a external magnetic bias can make that property unidirectional, and you might be getting somewhere.

    Then again, you might have a bit of a problem buying a hunk of ferrite big enough to observe this effect on 160 meters...
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    Of course it can. I just showed you how.

    Apples and oranges, Rege. What is the center of a driven element of a Yagi connected to? (I think it is called the boom)
    What is the boom of a Yagi connected to? (I think it is called a tower)
    What is tower connected to? (I think they are called ground rods)
    Where is the coax shield connected if using a Gamma match? (usually to the boom, near where the driven element is).

    Now do you still want to claim that a Gamma-matched Yagi teaches us anything about the dipole-coax-no balun example we are discussing here?

    By the way, modern Yagis do not use Gamma matches any more, most have gone to balanced feed precisely because the Gamma match causes unbalanced currents in the driven element and Common mode current on the feed-line and/or the tower itself.

    There is so much wrong with this statement that I dont know where to start :confused:
    First, a coil is just an inductor. Resonance is not wanted; it only happens due to some parasitics, and if it is a proper choke, the resonance happens at a frequency 3 to 10 times higher than the frequency you want to choke.
    There is no similarity between an inductor used as a choke and a half-wave wire. The impedance of an inductor is proportional to frequency (X=2*pi*f*L, where L is inductance). A half-wave wire carrying current is a radiator, a part of an antenna, doesn't do anything to block current.

    You have described any two terminal circuit element. Same is true for a resistor, a capacitor, a coil.

    You seem to have trouble understanding how a simple series circuit and Ohm's Law works. If you insert a series impedance (the CM Choke) into a circuit, and that impedance is much higher than what was there before, the current is decreased by an amount inversely proportional to the impedance of the choke. We are trying to block the flow of the Common-mode current by introducing a series impedance onto the conductor that was formerly carrying the common mode current!
  9. KD0CAC

    KD0CAC Ham Member QRZ Page

    I do not know if I have it right , but from multiple threads with AI3V I have read , I'm starting to get a theme [ lots of room for mistaken impression ;)
    Also lots of stimulation to look into ham radio issue's .
    A thought I had midway in this discussion was , Rege seems to have a understanding that is not matching others , and seems to take the position that if he does not understand it , other people's understanding / views are wrong , until he does ?
    Maybe this comes from my own way of thinking , with the exception that I want to find the correct understanding , in his way I think Rege is trying ?
    This gets real difficult when I read posts / threads between multiple engineers , with similar backgrounds - disagreeing , then they get deeper than I can , but I still try to read & follow , at some point I may pick a side / perspective / view ;)
    NH7RO likes this.

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