Observation of the inverse, zero and normal Doppler effect in configurable transmission lines

The inverse Doppler effect has been proposed for over four decades. However, there are only few experiments on it, nor to mention the practical application. The main reason is that it is difficult to build a relative movement between the source and detector in double negative materials (DNG), in which the inverse Doppler effect would happen. Here we report a practical method to realize the inverse Doppler effect based on composite right/left-handed transmission line (CRLH TL). Furthermore, we discuss the Doppler effect at the Dirac point for the first time, which is called zero Doppler effect. The zero Doppler effect is that the frequency keeps invariant no matter how and how fast the detector moves relatively to/away the source since the whole space is equiphase. It is hoped that this work may pave the way to develop the application of the novel Doppler effects.