There have been several reports of established AM stations that the FCC gave
permission to replace a decayed or destroyed inground radial system with
elevated radials or an elevated mesh/radial arrangement.
In all the cases I read the FS measurements exceed the original and power
had to be reduced to the original level.
Carl
KM1H
----- Original Message -----
From: "Richard Fry" <rfry@adams.net>
To: <topband@contesting.com>
Sent: Saturday, May 05, 2012 8:07 AM
Subject: Re: Topband: Fwd: radals fer 160m vertcal
> 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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>
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