In this work, backward (or left handed (LH)) transmission lines in microstrip technology are designed and fabricated. To this end, the line is periodically loaded with square shaped split ring resonators (SRR), etched in close proximity to the conductor strip, and via holes. The SRR provide the negative effective permeability, /spl mu/, to the medium in a narrow band above their resonant frequency, whereas the metallic vias act as shunt connected inductors that make the structure behave as a microwave plasma with negative permittivity, /spl epsiv/, up to the plasma frequency. It is shown that in that region where both /spl mu/ and /spl epsiv/ are simultaneously negative, left handed wave propagation is allowed. Since this occurs in a narrow band above SRR resonance, and the period of the structure is electrically small, these metamaterial transmission lines can be of interest for filter applications where miniaturization and narrow pass bands are pursued.