The time constraints in network developments inside
tunnels require a characterisation of canonical tunnels in
terms of propagation path losses and developing a slope model
which is usually represented as a two slope model. One of the
problems arisen when the canonical model tries to predict the
propagation path losses inside the tunnel is to determine the
distance where the slope of the propagation path losses changes.
This paper analyses two of the most common approaches to
determine that distance and their behaviour in different scenarios
in order to clarify the radio propagation mechanisms which
define the break point distance.