Topband: Fwd: radals fer 160m vertcal

[Charlie Young]

Dick, thanks very much for posting the link for the Beverage elevated counterpoise article.   Very interesting reading. 
 
Also thanks for the surface wave vs skywave graphic.  
 
 
73 Chas N8RR 
 

> From: rfry at adams.net
> To: topband at contesting.com
> Date: Sat, 5 May 2012 07:07:11 -0500
> 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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