Anomalous Negative Group Velocity in Coupled Positive-Index/Negative-Index Guides Supporting Complex Modes

Anomalous negative group velocity (NGV; group velocity antiparallel to the power flow) is reported in a guided-wave structure supporting complex modes. This structure consists of a coupled-line system comprising a positive-index microstrip line edge coupled to a negative-index line. The NGV can be observed on the positive-index line under suitable excitation and termination conditions. What is remarkable about this structure is that the anomalous NGV neither requires any material losses nor any strong reflections on the observed line. This work verifies that absorption or reflection are not necessary conditions for observing NGV, rather NGV can be observed in specific lossless coupled-line structures as well. The general conditions for obtaining NGV in coupled positive-index/negative-index guides are analytically derived and corresponding NGV observations are experimentally reported at microwave frequencies. Specifically, we report the propagation of modulated pulses exhibiting a negative group delay as well as phase shifters maintaining a constant phase over a broad bandwidth. This opens up the possibility of utilizing such coupled-line guides for applications in pulse shaping, delay control, constant phase shifters (vs. frequency) and reducing beam squinting in series-fed antenna arrays.