Such characteristics are described in Answer 1 below, as sourced in a study of antenna engineering textbooks, and by generating NEC4.2 studies.
VERTICAL MONOPOLE, EARTH, and BURIED RADIALS as FACTORS in ANTENNA SYSTEM RADIATION EFFICIENCY
The components shown in the graphic below, and the soil where they are located/buried are elements of the complete antenna system.
R-F currents flowing on/in the earth within a radius of 1/2 wavelength from the monopole as a result of its radiation must be collected and returned to the 2nd terminal of the transmit/antenna system. The sum of those currents is equal to the current flowing along the monopole, itself.
The r-f resistance of the Earth+radial path through which those currents travel is a series element of the transmit system. Reducing the r-f resistance of that path increases the radiation efficiency (gain) of the antenna system.
R-F currents flowing in the earth beyond a radius of 1/2 wavelength from the monopole are not collected by buried radials of any length(s),and so do not affect the radiation efficiency of the antenna system.
The e-m fields existing just beyond 1/2-wavelength from the monopole are fully formed, and will propagate outward into space.
Fields radiated by the monopole toward higher* elevation angles decay at an inverse distance (1/r) rate.
Fields radiated by the monopole toward lower* elevation angles decay at a rate greater than 1/r, due to the effects of their (lossy) interface with the earth.
The radiation efficiency/gain of a monopole antenna system is independent of the conductivity values of the earth existing in the areas beyond 1/2 wavelength from the radiator.
Received fields beyond a 1/2-wavelength radius of the monopole do vary as a functions of frequency, length of the propagation paths, and the physical conditions along and near those paths (earth conductivity, ionospheric reflection, obstructions etc).
*The elevation angle separating lower from higher elevation angle radiation is arbitrary, but generally thought of as ranging from 5 to 10 degrees.
Author: Richard Fry - March 2020