Dynamically changing the position and width of the electromagnetic band gap of a periodic microwave structure is investigated. The one-dimensional structure consists of twenty four unit cells. It has two dielectric layers: a relatively thick Alumina bottom layer and a thin Polymide top layer, with a thickness ratio of 25:2. There is a microstrip on the top surface. Metal patches, sandwiched between the two layers, are selectively short circuited to the ground plane on the bottom surface, using vias and FET switches, to change the bandgap and hence the cut-off characteristics of the microwave structure. The results obtained for different switching patterns indicate that this structure can be used in filtering and sensing applications in 2.9 GHz - 3.1 GHz range.