The Wheeler equation has an extension in the Kerr background. It has an exact solution representing a running harmonic wave in the tortoise coordinate. The wave models astrophysical jets in a new light because it collimates along the axis of spacetime rotation, increasingly tightly when rotation quickens. The model endorses a previous numerical work suggesting intrinsic collimation of jet fuelled by a Penrose-like process. It also endorses a previous theoretical work to crop one-way running waves from the exact solutions to Heun's equation.