The r-f loss at the operating frequency in a set of buried radials varies
with the conductivity and permittivity of the earth in which they are
buried.
The NEC4.2 study below shows that for poor earth conditions (within about
1/2WL from the base of the monopole), the number and length of buried
radials needed to maintain an r-f loss of a few ohms in the ground return
rises from that needed for more conductive earth.
In the case of AM broadcast stations, the use of 120 buried radials each
1/4-wavelength (in free space) produces a ground system loss of 2 ohms or
less. This is true no matter what are the characteristics of the the earth
in which those 120 radials are buried.
For a 1/4-wave, unloaded monopole with 35 ohms of radiation resistance and 2
ohms of ground system loss, antenna system radiation efficiency is 35/37 =
95% of the applied power (approx).
The FCC requires that a minimum inverse distance groundwave field of 241
mV/m is produced by an applied power of 1 kW at at a distance of 1 km by
even the lowest class of AM station (Class C). A perfect 1/4-wave monopole
driven against a perfect ground plane produces about 313 mV/m for those
conditions.
A typical installation using an unloaded 1/4-wave monopole driven against
120 x 1/4-wave buried radials produces about 306 mV/m for those
conditions -- which field is consistent with a monopole system with a
radiation efficiency of 95%.
The 241 mV/m minimum field required for Class C AM stations could be
produced by a 1/4-wave monopole+ground system with about 59% efficiency.
Class A AM stations such as WLW, WJR, WGN etc are required to generate an
inverse distance groundwave field of 362 mV/m at 1 km for 1 kW of applied
power. This cannot be done with a 1/4-wave monopole. Most of the Class A
stations use monopole heights ranging from 180 to 195 degrees.
WJR, Detroit uses a 195-deg monopole system that produces about 403 mV/m at
1 km for 1 kW of applied power. At their licensed transmitter power of 50
kW, that field becomes 403 x SQRT(50) = 2.85 V/m, approx.
http://i62.photobucket.com/albums/h85/rfry-100/10m_Vert32Buried_Radials.jpg
RF
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