  Wakefields in a rectangular accelerating structure can be calculated in time
domain by directly solving Maxwell's equations by a 3D code. In this paper, we
will give analytical formulae to calculate the synchronous modes' loss factors.
From these analytical formulae on can get the delta function wakefields. The
relations between the loss factors (wakefields) and the structure geometrical
dimensions are well established. These analytical expressions of loss factors
can be used also in a single rectangular resonant cavity. It is shown that the
potential application of a rectangular accelerating structure is to accelerate
a flat beam in a linear collider.
