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Low-Angle Fields from a Vertical Monopole

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W9XMT, Oct 24, 2018.

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  1. W9XMT

    W9XMT Ham Member QRZ Page

    Hams are mostly interested in the space wave radiated by an antenna system. The NEC4.2 plots below show how the space wave and ground wave fields launched by a vertical monopole are related to each other, over a range of r-f loss in the buried radial systems used with that monopole.

    Loss in that r-f ground connection directly affects the fields a vertical monopole radiates at all elevation angles below the zenith, including the horizontal plane. Space wave fields decay at a 1/r rate. Ground wave fields decay at greater than a 1/r rate due to the added propagation loss where the radiated field encounters a lossy ground plane (the Earth).

    The plots below show the fields close to the radiator, where propagation loss for the ground wave is very nearly the same as for the space wave. For long propagation paths, high frequencies and less than perfect Earth conductivity, the magnitude of the ground wave field is so low as to have no practical use. This is the condition shown by NEC and other MoM software when plotting the far-field (only) patterns of a vertical monopole over real earth, for infinitely long paths over a flat ground plane.

    This relationship between the space wave and ground wave fields is the reason that such ground wave field analyses are (or should be) valuable to hams.

    Also note that the greatest field actually launched by each of the systems in the graphic below always occurs in the horizontal plane -- not at some "takeoff angle" above it.

    Last edited: Oct 24, 2018
    WZ7U likes this.
  2. N8CMQ

    N8CMQ XML Subscriber QRZ Page

    I am used to seeing the results in dB, but whatthehay...
    Near field plots are cool, but...
    It is the far field that most antennas are used at, so using the near field plot is almost like using a theoretical model, nice to look at but not practical to use.
    As you say, as you get further from the antenna, the lower the signal at ground level will be, and the pattern then starts to look like it has lobes, rather than the nice doughnut shape of a dipole in free space. Then the antenna has the 'takeoff angles' we all know and love!
    KK5JY and AK5B like this.
  3. W9XMT

    W9XMT Ham Member QRZ Page

    In reality, those plots are in the far field of those monopole systems, not their near field -- no matter the convention to define those fields that is used by NEC software.
  4. N8CMQ

    N8CMQ XML Subscriber QRZ Page

    ?? OK, then why does it show 1KM in the distance field? In my book, that is near field for 7.25 MHz.

    Based on frequency, and distance in wavelength, the lower the frequency, the further in distance you need to be to start having a far field reading.
    But this is also a model, not based on reality, but on theory.
    AK5B likes this.
  5. W9XMT

    W9XMT Ham Member QRZ Page

    Last edited: Oct 24, 2018
  6. N8CMQ

    N8CMQ XML Subscriber QRZ Page

    Never mind, I see what they are doing. ;)
  7. KM1H

    KM1H Ham Member QRZ Page

    That was a standard going back to 1937 for the Brown, et al experiment. The frequency was 3 mc and that 1 km is the criteria for measuring BCB antennas of any dimension since it is a known fact. 350 uV is the perfect result at 1000W.
    The Proof of Performance standards have been relaxed a bit as proposed last year in a copy of the draft below.

    I have performed several of those initial certifications and recertifications as part of a small group over the decades.

  8. KA2RRK

    KA2RRK Platinum Subscriber Platinum Subscriber QRZ Page

    Dang, I had this all wrong then.
    I was always interested in getting my waves to lie down in such a fashionable way
    to be able to stand all your hair folicals upright and flowing backwards slightly waving back and forth also.
    And be able move wheeled chairs, bend spoons, and punch holes thru leaves without a magnified sun beam.

  9. W9XMT

    W9XMT Ham Member QRZ Page

    That "350 uV" number must be a typo, Carl. The correct answer is 313.57 mV/m (units of measure are millivolts per meter).

    The mathematical development leading to the accurate answer for the h-plane field intensity existing 1 km from a 1/4-wave, unloaded, base-driven, perfect monopole radiating 1 kW while using a perfect ground plane is shown in the convenient clip from author Cris Alexander, below.

    Last edited: Oct 25, 2018
  10. N8CMQ

    N8CMQ XML Subscriber QRZ Page

    That is the wonderful thing about standards, there are so many of them!!!
    I was thinking of another operation, not related to BC standards, which is also why I thought a certain automatic used .30 cal ammo, not .50 cal, different situations, similar names. Which is why I have the manual out when I do calibrations, so I don't have to do it twice...
    Another reason getting older is so much fun, things change while others don't, and I get confused.
    KM1H likes this.

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