Topband: Question - optimum number of radials

[Richard Fry]

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