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We investigate microwave active metamaterial transmission lines incorporating gain devices to provide effective negative resistance. Our simulation and experimental results show that by inserting semiconductor tunnel diode at the unit-cell level, the negative phase constant of this kind of transmission line is maintained (i.e., left-handedness kept) while gain can be obtained (negative attenuation constant of transmission line) simultaneously. Important design aspects of the active metamaterial transmission lines are discussed. As a potential application, the performances of passive and active versions of a single unit-cell zeroth-order resonator antenna based on composite right-handed/left-handed transmission line (CRLH-TL) is also compared.