Modulational instability of two counterpropagating waves in an experimental transmission line

We present experiments on the modulational instability of two counterpropagating waves, observed in an electrical transmission line. Our theoretical analysis leads to a set of two coupled nonlinear Schr\"odinger equations and predicts a partition of the linear spectrum into several regions with different behaviors. The unstable region, found in our previous study [Phys. Rev. E 49, 828 (1994)] for a single wave, remains here unstable, but with a much larger growth rate. Moreover, two new unstable regions appear. Our experiments on the dynamics of the waves are in good agreement with the theoretical predictions.