Question How can I avoid the reflections at the end of ground wires due to an injection of a surge at the top of a tower?
Answer The best way to avoid those reflections is to add a lumped impedance that matches the impedance of the medium at the end of the ground wires. This approach has been used successfully several times with HIFREQ.
The problem with this approach is to find the appropriate impedance to use to minimize the reflections. For conductors located in the air, you can simply use sqrt(L/C), where L is the inductance per unit length of the conductor and C its capacitance per unit length. For a buried conductor, the same formula should give a pretty good approximation as well. Also, you could experiment by changing the value of the impedance and comparing the magnitude of the reflected wave for different impedances. This would require several HIFREQ runs, but it could be done on a smaller case: once the correct value of the impedance is known, it could then be inserted in your original scenario.