  The operation and parameters of Bunch Shape Monitors using coherent
transformation of time structure of an analyzed beam into a spatial one of low
energy secondary electrons emitted from a wire target is influenced by the
characteristics of a beam under study. The electromagnetic field of a bunch
disturbs the trajectories of secondary electrons, thus resulting in a
degradation of phase resolution and in errors of phase reading. Another effect
is the perturbation of the target potential due to the current in the wire
induced by a bunch as well as due to current compensating emission of the
secondary electrons. The methods, the models and the results of simulations are
presented.
