The effect of a periodic reactive load on the phase velocity of a constant-width microstrip transmission line is investigated. The presence of resonant spiral slots etched in the ground plane modifies the electromagnetic response of the structure. It can be quantified by the phase velocity across the structure, the latter being extracted from the dispersion diagram. Because of the presence of the periodic load, the correct characterisation requires the consideration of higher order modes, or analysis in the second Brillouin zone. The dispersion diagram of a loaded width-modulated configuration presents interesting features, such as the presence of phase-independent modes. Combination of these responses with the band-gap structure allows design of devices with interesting and useful electromagnetic properties.