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
_______________________________________________
Topband reflector - topband@contesting.com
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