Measurements of quadrupole frequency shift below transition

Measurements of the quadrupole frequency shift done at 14 GeV/c with a single bunch for different bunch lengths and intensities are compared with estimations based on the nonlinear dependence of frequency on the amplitude of oscillation and the present model of the longitudinal SPS impedance. Analysis of the measurements shows that the quadrupole frequency shift is mainly related to bunch length variation. Due to the relatively low energy of the beam the contribution coming from the space charge was expected to be non-negligible.