  A reconfigurable intelligent surface (RIS) employs an array of
individually-controllable elements to scatter incident signals in a desirable
way; for example, to facilitate links between base stations and mobile stations
that would otherwise be blocked. A principal consideration in the study of
RIS-enabled propagation channels is path loss. This paper presents a simple yet
broadly-applicable method for calculating the path loss of a channel consisting
of a passive reflectarray-type RIS. This model is then used to characterize
path loss as a function of RIS size, link geometry, and the method used to set
the element states. Whereas previous work presumes either (1) an array of
parameterizable element patterns and spacings (most useful for analysis of
specific designs) or (2) a continuous electromagnetic surface (most useful for
determining scaling laws and theoretical limits), this work begins with (1) and
is then shown to be consistent with (2), making it possible to identify
specific practical designs and scenarios that exhibit the performance predicted
using (2). This model is used to further elucidate the matter of path loss of
the RIS-enabled channel relative to that of the free space direct and specular
reflection channels, which is an important consideration in the design of
networks employing RIS technology.
