  Diffuse scattering of electromagnetic waves from natural and artificial
surfaces has been extensively studied in various disciplines, including radio
wave propagation, and several diffuse scattering models based on different
approaches have been proposed over the years, two of the most popular ones
being Kirchhoff Theory and the so-called Effective Roughness heuristic model.
The latter, although less rigorous than the former, is more flexible and
applicable to a wider range of real-world cases, including non-Gaussian
surfaces, surfaces with electrically small correlation lengths and scattering
from material inhomogeneities that are often present under the surface.
Unfortunately, the Effective Roughness model, with the exception of its
Lambertian version, does not satisfy reciprocity, which is an important
physical-soundness requirement for any propagation model. In the present work,
without compromising its effectiveness and its simple and yet sound
power-balance approach, we propose a reciprocal version of the Effective
Roughness model, which can be easily implemented and replaced to the old
version in ray-based propagation models. The new model is analyzed and compared
to the old one and to other popular models. Once properly calibrated, it is
shown to yield similar - if not better - performance with respect to the old
one when checked vs. measurements.
