Experimental results obtained for a binary distributed phase shifter based on RF MEMS coplanar shunt switches are presented. A new approach based on the image parameter representation of two-port networks is proposed for the modelling of this structure. Vector network analyser measurements have been performed by recording the scattering parameters of the device, obtaining a differential phase shift of 180° at a frequency f0 = 13.7 GHz, very close to the theoretical one ftheo=14 GHz. A very low number of switches has been used for the realisation of the component.