Well, if you have the transfer function of the antenna (S21 mag and phase to a receiving antenna), you can just do a fourier transform of that to see the time response. Of course, the bigger the simulation bandwidth, the more accurate the result will be.
In other words, if you know the transfer function H(F) of the antenna, you can compute the impulse response h(t), and then you can input all sorts of waveforms to the h(t)--such as a unit step, ramp, gated sine wave, etc.