EFHW Antenna: A Detailed Analysis (NEC4.2)

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W9XMT, Mar 29, 2018.

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  1. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    It appears that the source is at the end of the antenna, rather than at the bottom of the coax. If so that's not a realistic situation. And even if it were somehow, the ground connection is very poor for RF so not much current will flow there, as ARK pointed out above.
     
  2. W9XMT

    W9XMT Ham Member QRZ Page

    Average current in top segment of the buried ground rod = -0.1A
    Maximum current at center of horizontal wire = 0.11A
     
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    The graphic you posted does not show that.
     
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    Neither does Nec2. I recreated your example as best as I can with only having a Mininec ground plane with the downwire connected to it. The current profile in my sim matches what you posted, but at 100W in, the peak current in the horizontal wire is 1.66A while the peak current in the downwire (just above the ground plane) is only 0.24A, a ratio of 7 to 1. The current in the downwire right below the feedpoint is only 100mA.
     
  5. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    The NEC2 engine has one major limitation, it doesn't model buried wires. If a wire is connected to Z=0, it will assume it is connected to a perfect ground there. A perfect ground that will not exist in the real world.
    The NEC3 and NEC4 engines does handle buried wires, so if NEC2 and NEC4 disagrees in this case (and they will), I would trust the NEC4 result more. Neither one will be spot on as stray capacitances etc will have a big impact at such high feed point impedance.

    Another limitation of both NEC2 and NEC3 is loss of accuracy when the segment size is very short compared to the wavelength. NEC4 improves this accuracy. To model a "near end fed" antenna, where very short segments are needed to place the source near the end, NEC4 should be used.
     
  6. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    Yes, but just as that low current is transformed by the 1/4 WL of antenna wire to the current maxima point, it will be transformed to another common mode current maxima on the feed line. If the feed line and antenna wire is the same length and no other loading exists, the current maxima on the radiator half and the common mode current maxima on the feed line half will be exactly the same.

    This is not as trivial as it sounds. If the antenna impedance is about 2 kOhm, you need a CM choke impedance substantially higher than that. As a rule of thumb, use 10 x feed point impedance. So aim for 20 kOhm. Can you suggest a CM choke that has at least 20 kOhm CM impedance?
     
  7. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    A coax cable wrapped around a former and with a capacitance from input shield to output shield that tunes to parallel resonance. I made up one for testing the impedance looking into balanced transmission lines with an instrument that is unbalanced.
     
  8. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    That might work for testing on a very narrow frequency range. The Q of such a tuned circuit would need to be very high, so the bandwidth will be very small. Up at the antenna it will probably de-tune easily by rain, snow and ice. Plus, many of these EFHW antennas are marketed as working on "all HF ham bands above 80 meter" without counterpoise.
     
  9. W9XMT

    W9XMT Ham Member QRZ Page

    Applying this good advice to the case at hand ...

    Most NEC software will test the geometry and segmentation of the NEC wire model before running the NEC engine to evaluate that model. Next below is a table showing the segment parameters checked on my NEC model:

    [​IMG]
    In this case, the segment check will produce an error message for lengths shorter than 0.077m (approx).

    Segment lengths for my NEC model are:
    • Horizontal Wire 1m
    • Vertical Wire 0.833m
    • Ground Rod 0.61m
    Below is the NEC Input File data for my EFHW study. Successfully using this file requires the NEC4x engine.

    Running this NEC file on my NEC software produces no warnings or errors when (automatically) testing it for its segmentation and geometry.

    CM NEC4.2 ENGINE
    CM Horizontal Conductor = 36m Length, 4mm OD
    CM Feedpoint = 1.5m From End of Horizontal Conductor
    CM Elevation of Horizontal Conductor = 10m AGL
    CM Coaxial Cable Length/Height = 10m
    CM RF Loss in Matching Network (at Feedpoint) = 10 ohms, assumed
    CM Location of Shield Attachment to Horizontal Conductor = Feedpoint
    CM of 36m Conductor (short side)
    CM Bottom of Coax Shield Connected to 2.44m (8 ft) Buried Ground Rod
    CM Intrinsic Feedpoint Impedance Prior to Matching
    CE
    GW 1 36 18 0 10 -18 0 10 .002
    GW 2 12 17 0 10 17 0 0 .002
    GW 3 4 17 0 0 17 0 -2.44 .002
    GE -1
    LD 0 1 2 2 10
    GN 2 0 0 0 13 0.005
    EK
    EX 0 1 2 0 1 0 0
    FR 0 0 0 0 3.9 0
    EN
     
  10. AI3V

    AI3V Ham Member QRZ Page

    "Without counterpoise"

    That's the problem.

    It's not a issue of matching the antenna wires to the feedline, or, the transmitter itself if no feedline is used (the handi-talkie being the most common)

    The issue is, if you don't provide both halves of your antenna, you might get "the other half of your antenna" in a mighty incovenient place, like the power wire that just happens to be exactly 1/4 wavelength long between your rig, and your next door neighbors tv set.

    Or maybe you will get lucky and the house wireing will work just fine, with no issues.

    Who was it that asked "do you feel lucky" :)

    Rege
     
    KD6RF likes this.

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