J-Pole effective height above ground

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by ZS6XOX, May 8, 2016.

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  1. W9XMT

    W9XMT Ham Member QRZ Page

    For "calibration," below is a graphic showing the NEC4.2 comparison of the elevation pattern and gain of a 1/4-wave ground plane with a 1/2-WL, center-fed, vertical dipole.
    • Both configurations do not consider radiation from a transmission line or any other conductor (support pole, etc.) — only the radiation launched into free space by the conductors of the antennas, themselves
    • The maximum gain ("takeoff angle") of both antennas lies the horizontal plane
    • When mounted near the surface of the earth, these radiation patterns will be equally affected by its reflections, and those from other, nearby re-radiators (support pole, etc)
    • Comparison is valid for other frequencies, when using properly scaled conductors
    For these conditions, a properly constructed J-pole also should radiate maximum relative field in the horizontal plane, but it may not reach the same peak gain values in dBi as those shown in the graphic below.

    [​IMG]
     
    Last edited: Feb 11, 2019
  2. W9XMT

    W9XMT Ham Member QRZ Page

    As a followup to my post above, below is another NEC4.2 graphic showing the results calculated for the 6-m transmit system defined there.

    Of interest here is that the far-field NEC radiation patterns many hams refer to for the "takeoff angle" of an antenna over real Earth invariably will have significant errors for the fields/gains shown at low elevation angles. The reason for that is due to the exclusion of surface wave radiation when running the NEC far-field analysis.

    Without including the surface wave, NEC always shows zero to very little gain/relative field toward and near zero degrees elevation for the elevation pattern of an antenna sited above a ground plane. But clearly that is untrue. If it was true, then (for example), FM broadcast stations would radiate zero power in the horizontal plane, and not much more at small angles above and below it.

    Experience (and theory) shows that this is not the case.

    [​IMG]
     
  3. WB5WPA

    WB5WPA Ham Member QRZ Page

    re: " The reason for that is due to the exclusion of surface wave radiation when running the NEC far-field analysis."

    That 'surface wave' is pretty much killed in an urban area due to buildings whose height easily exceeds 1/4 Lambda thereby forming RF lossy obstructions at 2 meters and higher frequencies. Even suburban housing sub divisions will have this effect on account of the brick work, foil-backed insulation/foam board, and stucco on a metal lath. One encounters this environment in the course of doing cellular propagation testing and working to solve 'coverage' issues.

    A well-known feature on RF/antenna test ranges are a series of screens or 'reflectors' located midway between the RF source and the test tower with the receiver and antenna positioner that are intended to kill ground reflections.
     
    Last edited: Feb 11, 2019
  4. W9XMT

    W9XMT Ham Member QRZ Page

    The surface wave is not confined to the radiated energy following the surface of the earth, but continues to significant heights above it -- as shown in the field intensity vs. height AGL plotted at a range of 30 miles from the transmit antenna, on the right side of my NEC graphic in Reply 42 above.
     
  5. WB5WPA

    WB5WPA Ham Member QRZ Page

    if it "continues to significant heights above it" its not a surface wave any more.

    I think perhaps you are either mis-terming things or don't understand propagation near the earth?

    Have you read the texts and seen the depictions of a wavefront propagating over earth, i.e., groundwave in particular?

    Please don't conflate what NEC engines "show" for propagation versus what a true RF propagation program or what reality (physics) demonstrates RF propagation to be ...
    .
     
    Last edited: Feb 11, 2019
  6. W9XMT

    W9XMT Ham Member QRZ Page

    Actually, NEC is capable of calculating surface-wave fields including propagation losses due to earth conductivity with very good accuracy, compared to those fields measured for the same system/conditions by a broadcast consulting engineer using a calibrated field intensity meter (see the graphic below).

    In fact, the FCC accepts the results of M-o-M software such as NEC in the application data and licensing processes needed for their acceptance of the antenna arrays used by directional AM broadcast stations.

    [​IMG]
     
  7. WB5WPA

    WB5WPA Ham Member QRZ Page

    You're jumping into a new area where some assumptions are being made, to wit, GW (ground wave) propagation. You've mixed in prior to this the mythical "take-off angle" as well.

    I still stand on what I've posted.

    I further take it you are unfamiliar with the TIREM RF propagation models or techniques, as well as the Okumura propagation model?

    What is valid for AM broadcast frequencies can be inappropriate for the higher frequencies, such as 2 meters.

    We just went through all this a few weeks back, but I don't recall which thread it was.
     
  8. W9XMT

    W9XMT Ham Member QRZ Page

    I am familiar with them, and use whatever process best serves the purposes of the evaluation I am making (see below, for one example).

    [​IMG]
     
  9. K8JD

    K8JD Ham Member QRZ Page

    First, an antenna with more gain hears weaker signals, (ignoing capture area and local noise) it's reciprocal RX gain = TX gain ! o_O

    There is a problem about increasing power on the system and doing nothing else. :eek:
    More people will hear you, at greater distance, but you will not hear them because it's the same receiver !~
    Using a better antenna system, with some gain , better coax with less feedline loss and radiation, will improve , BOTH, reception and transmssion reach ! :rolleyes:
     
    Last edited: Feb 11, 2019
    NH7RO and WB5WPA like this.
  10. N8GDX

    N8GDX Ham Member QRZ Page

    As others have said, when limited to "Line of Sight" Height is everything
     

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