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A double periodic composite right/left handed transmission line (DP-CRLH TL) is proposed and analyzed based on transmission line theory. Besides the composite right/left handed property, a new leaky-wave radiation is observed at a lower frequency with narrow bandwidth. Dispersion characteristics of the structure are obtained and the cutoff frequencies are investigated by changing the ratio of inductances (L1/L2) or capacitances (C1/C2) . Experiments are performed using microstrip lines with double periodically loaded shunt shorted-stub inductors and series interdigital capacitors. Frequency response, dispersion behavior and radiation characteristics are measured from the prototype structure and they display the expected novel forward leaky-wave radiation in the direction of θr = +3° and gain of 4.12 dBi at 1.6 GHz, the left-handed backward radiation in the direction of θr = -19° and gain of 8.62 dBi at 3.3 GHz, and the conventional right-handed forward radiation in the direction of θr = +12° and gain of 8.78 dBi at 3.9 GHz.