We present a transmission line model for the lightning return stroke. This model incorporates a model for the corona surrounding the charged lightning channel as well as a model for the evolution of the channel incorporating hydrodynamics, energy losses, joule heating. We find that the model reproduces the “double exponential” shape of the observed field in the radiation zone, without imposing a priori the time behavior of the current pulse. The decrease in field at late times precedes any decrease in current, and is due to a lengthening of time scale. The current pulse is substantially altered as it progresses up the lightning channel, and the radiation zone signal is not of the same shape as the current pulse at any position or time.