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Skywave Coverage: Low-Angle Radiation from Vertical Monopoles

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W9XMT, Jan 14, 2021 at 7:57 AM.

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  1. W9XMT

    W9XMT Ham Member QRZ Page

    At various times and on various websites, a well-known ham has claimed that the low-angle, v-pol radiation from a vertical monopole is not "contributing to global communication" (quote below):

    Quote from: WA---K on January 09, 2021
    Furthermore, that well-known ham claims that the radiation efficiency of a vertical monopole is severely reduced by the effects of the conductivity/permittivity of the soil at, and near the site of that monopole antenna system.
    Below, using FCC data and methodology is a comparison of the 1-hop skip angle and received, nighttime field intensities at a great-circle path distance of ~895 miles from two AM broadcast stations. Both are 24/7, 50 kW, omnidirectional stations using very similar tower radiator, (v-pol) antenna systems.

    Notice in the graphic below that:
    • The "takeoff angle" for 1-hop skip to reach the Earth ~895 miles downrange from each station is on the order of 2.45 degrees, and
    • The "Received Field" of WSB at ~895 miles is greater than that of WGN at ~895 miles.
    ► Earth conductivity at/near the transmit site of WSB is about 1 mS/m (poor), compared to 8 mS/m (average) soil at/near the WGN site. ◄

    These realities exist even though (according to the theses/posts of WA---K):
    • Far-field radiation from a takeoff angle of ~2.45 degrees from a vertical monopole would not provide useful "global communication," and
    • The nighttime Received Field of WSB at ~895 miles should be LESS than that of WGN at ~895 miles, due to the relatively lower Earth conductivity at/near the WSB transmitter site.
    However, and as shown in well-respected textbooks on antenna engineering by Kraus, Terman, Balanis, Laport etc, ALL vertical monopoles of 5/8WL and less vertical height radiate/launch their maximum relative field (E/Emax) in the horizontal plane, and not much less than that at low angles above the horizontal plane.

    Also notice in the graphic below that the h-plane field produced by WSB and WGN 1 km away from their radiator is comparable, at about 390 mV/m for 1 kW of applied power.

    It would be useful to his readers if this well-known ham considered the documentation above, before continuing to post his present beliefs on these topics as proven facts.

  2. AH7I

    AH7I Ham Member QRZ Page

  3. WA4SIX

    WA4SIX Ham Member QRZ Page

    For giggles, check WBZ.

  4. W9XMT

    W9XMT Ham Member QRZ Page

    Yes, when a shopping center was constructed adjacent to and around their tower location. To make that possible, a network of elevated wires was installed around the tower base and over the buildings and parking lots, serving as a counterpoise to drive their tower against.
  5. AI3V

    AI3V Ham Member QRZ Page

    And this is oversimplified to the point of absurdity.

    You are making a mistake that the ionesphere reflects radio signals with a sudden turn.

    And that waves bounce abruptly between the ionesphere and earth.

    Nothing of the sort happens.

    E/M waves are very gradually turned in the ionesphere.

    Exactly the same way cathode rays are turned by a magnetic field in a tube.

    To give a angle to the hundreth of a degree...:rolleyes:

    In addition understand that the degree of bending is never constant, even at a particular instant over a given distance.

    This idea of abrupt bouncing is a bad idea from 100 years ago.

    AK5B and W4HWD like this.
  6. AE1N

    AE1N Ham Member QRZ Page

    Angle vs Distance (2020_11_06 06_40_22 UTC).jpg
    Ht vs Gain (2020_11_06 06_40_22 UTC).jpg

    Do whatever you can to lower your radiation angle...
    There is a lot less loss in a vertical ground plane with four equi-angular radials above ground and buried or lay-on-the-ground radials. The ground plane radials need only be inches above the ground.
    Note the peak gain of 3 db in a vertical monopole at 5/8 wavelength... thus the popularity of the 43 foot vertical.
    ---Layne AE1N
    WA7ARK likes this.
  7. AK5B

    AK5B Ham Member QRZ Page

    Layne; I presume you meant to say, "There is a lot less loss in a vertical ground plane with four equi-angular radials above ground than buried or lay-on-the-ground radials," correct?


  8. W9XMT

    W9XMT Ham Member QRZ Page

    Just noting that there is a fairly large difference in the 1-hop elevation angles shown in Figure 1 of Reply 6 (repeated at the right side of the graphic below), and the FCC chart showing them on the left side of the graphic.

  9. W6RZ

    W6RZ Premium Subscriber QRZ Page

    The angle depends on the height of the ionosphere. Here's the math (which assumes reflection, not refraction).

    At an F2 layer height of 250 km, the take-off angle for a 1000 mile geometry is 13.3 degrees.
    Computes distance for a take-off angle (250 km F2-layer height)
    #include <stdio.h>
    #include <stdlib.h>
    #include <math.h>
    int main(int argc, char **argv)
        double    a, d;
        double    r = 6371.0;
        double    h = 250.0;
        double    n = (90 * 3.14159256) / 180;
        if (argc != 2) {
            fprintf(stderr, "usage: f2angle <angle>\n");
        a = atof(argv[1]);
        a = (a * 3.14159256) / 180;
        d = 2 * r * (n - a - asin(r * sin(a + n) / (r + h)));
        printf("distance = %.2f km, %.2f miles\n", d, d / 1.609344);
        return 0;
    KX4O and AI3V like this.
  10. W6RZ

    W6RZ Premium Subscriber QRZ Page

    Note that the code in my previous post can replicate the FCC chart pretty much exactly. I can't replicate the other chart. A 1000 mile skip at 24 degrees would indicate an ionosphere height of 436 km (which is huge), but I can't match the rest of the plot to that point.

    W9XMT likes this.

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