The dielectric dispersion in the ferroelecric and antiferroelectric phases of a new liquid crystal (S)-4-(1-methylheptyloxycarbonyl)-phenyl 4-(4-dodecyloxybenzoyloxy)benzoate has been investigated in the frequency range from 10 Hz to 10 MHz. The behaviour of the soft mode in SmA and the Goldstone mode in SmC* have been determined. Two dielectric relaxations have been recognized in the antiferroelectric SmC*Aphase : the first one between 5 kHz and 50 kHz and the second one, also observable in the ferrielectric SmC*FIphase, around 1 MHz. In SmC*FI, a drastic change in the behaviour of the low frequency response has been found depending on whether the sample was heated or cooled. A sign reversal of the helical pitch has been determined inside the SmC*FIphase, leading to low pitch values of opposite sign in SmC*Aand SmC*.