For DX we are interested in elevation angles from 3-15 degrees. How much error
is there in a NEC model of a monopole at these elevations?
Dave WX7G
Sent from my Verizon Wireless 4GLTE smartphone
----- Reply message -----
From: "Paul Christensen" <w9ac@arrl.net>
To: <topband@contesting.com>
Subject: Topband: Monopole Elev Pattern w.r.t. Earth Conductivity
Date: Wed, Oct 24, 2012 07:56
I have linked to an image of a NEC 4 vertical profile analysis of the WLS-AM
radiator. As most everyone here knows, WLS is a legendary 50KW AM station
in Chicago. WLS uses a 190 degree radiator and is the preferred antenna
height for most AMBC purposes since it's the height that produces the most
gain at zero elevation while simultaneously minimizing skywave lobes.
http://tinyurl.com/9zuqpre
Notice how the pattern is crushed to the horizon with very little of a high
angle lobe produced. I ran a far-field analysis of three hypothetical
models over: (1) average soil conductivity [green]; (2) poor soil
conductivity [red]; and (3) highly conductive salt water [blue]. Notice a
few things from the colored overlay:
1) The WLS radiator over salt water produces a significantly improved field
strength at a distance of 1km over the other two models. No surprise here;
2) The shape of the far-field profiles *appear* to change as a function of
ground conductivity.
However, if we were able to conduct a surface wave analysis and overlay the
three curves (4Nec2 does not appear to allow that in the surface mode, but
I'll check with the author), we would find that the shape of the curves
going up in Z axis vertical distance is the same. I've run the model for
many vertical radiators and that's what NEC 4 reports. This too should be
no surprise as ground wave effects to 3 MHz have been extensively documented
in academia since the 1930s.
Here again, if we were to believe that WLS produces no far field strength at
low elevation over anything but salt water, they would have very limited
daytime listeners - even with 50KW of output power. So, to Rich's past
points, we need to consider both the far field and surface wave components
when computing far field strength over a range of elevations. Don't just
believe that the far field plots we've seen time and again are always
representative of the actual low-angle field strength. According to Jerry
Burke, author of NEC 4, the surface wave component can be a major part of
achieving ionospheric DX reach through at least 3 MHz.
Paul, W9AC
----- Original Message -----
From: "Richard Fry" <rfry@adams.net>
To: <topband@contesting.com>
Sent: Tuesday, October 23, 2012 9:28 AM
Subject: Re: Topband: Monopole Elev Pattern w.r.t. Earth Conductivity
> Cristi YO3FFF wrote:
>>That means, the radiation pattern will be affected too because the
>>electromagnetic wave will be much curved to the ground, so the
>>groundwave intensity will be direct proportional with the conductivity
>> Is it right?
>
> Other things equal, the field values in the vertical plane radiation
> pattern
> first "launched" by a monopole are a function of earth conductivity. But
> the shape of the radiation pattern close to the monopole remains
> essentially the same for all conductivities, as shown in the link below.
>
> The fields in the NEC4 analysis below were calculated along a vertical
> distance of 0-50 meters above the surface of the earth, at a horizontal
> distance of 100 meters from the monopole. A vertical distance of 50 meters
> for this chart is an elevation angle of 26.6 degrees from the monopole.
>
> Note in the chart that the radiated fields at elevation angles below 26.6
> degrees are greater than the field at 26.6 degrees. For the lowest
> elevation angles, those fields are FAR greater than those shown in a NEC
> "far-field" analysis, which for real earth go to zero field in the
> horizontal plane.
>
> At an elevation distance in the chart of of 15 meters (8.5 degrees), the
> difference in the field shown for 1 mS/m conductivity and that for sea
> water
> conductivity is 2.6 dB -- probably less than intuition would expect.
>
> It is the low-angle radiation directed toward the ionosphere from the
> fields
> at ~this electrical distance from the monopole that can produce useful
> skywaves having the greatest single-hop range.
>
> http://i62.photobucket.com/albums/h85/rfry-100/Monopole_Surf_Wv_Compare.jpg
>
> _______________________________________________
> Topband reflector - topband@contesting.com
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