Investigation of Microwave Negative Refractive Index (NRI) Transmission Lines Incorporating Tunnel Diodes

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.