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" <[email protected]> To: <[email protected]> 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 > > > _______________________________________________ > UR RST IS ... ... ..9 QSB QSB - hw? BK > > > ----- > No virus found in this message. > Checked by AVG - www.avg.com > Version: 10.0.1424 / Virus Database: 2411/4978 - Release Date: 05/04/12 > _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
