James Rodenkirch wrote:
>What about radials above the ground?
This link http://www.commtechrf.com/documents/nab1995.pdf leads to a paper
by Clarence Beverage with some real-world results for monopoles with
elevated wires used as a counterpoise. Here is a quote from it:
\ \The antenna system consisted of a lightweight, 15 inch face tower, 120
feet in height, with a base insulator at the 15 foot elevation and six
elevated radials, a quarter wave in length, spaced evenly around the tower
and elevated 15 feet above the ground. The radials were fully insulated from
ground and supported at the ends by wooden tripods.
Power was fed to the system through a 200 foot length of coaxial cable with
the cable shield connected to the shunt element of the T network and to the
elevated radials. A balun or RF choke on the feedline was not employed and
the feedline was isolated from the lower section of the tower. The system
operated on 1580 kHz at a power of 750 watts.
The efficiency of the antenna was determined by radial field intensity
measurements along 12 radials extending out to a distance of up to 85
kilometers. The measured RMS efficiency was 287 mV/m for 1 kW, at one
kilometer, which is the same measured value as would be expected for a 0.17
wave tower above 120 buried radials. / /
So while such "elevated" installations are rare for AM broadcast stations,
their performance has been measured to be about the same as when using an
r-f ground consisting of 120 buried wires, each 1/4-wave long (free space
length).
These elevated systems are readily modeled using NEC-2. However the
radiation patterns shown by a typical NEC far-field analysis do not
accurately show the fields actually "launched" by them, or by any vertical
radiator with its base near the earth, because they do not include the
surface wave.
The fields radiated in and near the horizontal plane by any vertical
monopole of 5/8 wavelength height and less are the greatest fields it
radiates in the entire elevation plane, regardless of earth conductivity.
Those fields from very low elevation angles (say, less then 5 degrees) can
reach the ionosphere, and under the right conditions return to the earth as
a useful skywave.
The link below illustrates this concept.
http://i62.photobucket.com/albums/h85/rfry-100/Space_Surface_Wave_Compare.gif
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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