Study of a Single-Bunch Instability below Transition Energy

In the CERN SPS microwave instability is an important limitation for the single bunch intensity of a proton beam. In the past this instability has been studied at energies above transition energy. In this paper we describe recent observations concerning the instability in the SPS at two different energies below transition. The results are compared with those obtained by the same measurement method above transition as well as with results from numerical simulations.