Realization of the inverse Doppler effect in tunable transmission lines

According to the Doppler Effect, the frequency of the incident wave changes when there is a relative motion between the source and the detector in normal substances with both permeability and permittivity positive. While in metamaterials with both permeability and permittivity negative, the inverse Doppler Effect would occur and the variation of the frequency is opposite to the normal Doppler Effect. Here we demonstrate the inverse Doppler Effect in a one-dimensional tunable composite right/left-handed transmission line loaded with varactors in series, which can be tuned by the external bias voltage supplied to the units of the transmission line. The characteristic of the transmission line can set in left-handed passband, right-handed passband and band gap under different bias voltages. A reflective surface is built at the interface between units set in passbands and units set in the band gap. With suitable switching method of the bias voltages, the reflective surface can move at tunable velocities. The inverse Doppler Effect is realized at 1.1GHz which is in the left-handed passband, and the normal Doppler Effect is achieved at 2.75GHz which is in the right-handed passband.