Question When modelling a lightning channel in HIFREQ, is it preferable to energize at the top or at the bottom of the channel?
Answer It can be interesting to model both types of lightning energization, i.e. from the bottom of the lightning channel as well as from the top: the energization from the top can be used to model a lead stroke while the energization from the bottom can be used to model a return stroke. Typically, the latter have a larger magnitude and are therefore more interesting. There is, however, a problem regarding the termination of the channel when using a "bottom-type" energization: what should be done with the current as it reaches the top of the channel?
There is no completely satisfactory solution to this problem. What is normally done is to energize at the top of the channel for both types of stroke, simply reversing the sign of the current for the return stroke. This is not quite correct, since it doesn't account completely for the loss of charge that occurs along the channel, but is consistent with the general level of approximations used to solve such problems.
Another approach is to explicitly impose the current on every segment in the lightning channel; this makes it possible to specify any desired decay curve for the current as a function of elevation (an exponential with a decay factor of 10 km is typically used). Aside from being much more difficult to model, this kind of lightning channel tends to generate abnormally large electric fields.
Either way, the termination of the channel in the ground should be done, of course, by connecting the channel to the structure that is hit by lightning. As far as the properties of the conducting material used to model the channel are concerned, a good conductor should be used (the default conductor type in HIFREQ is fine), with a radius of the same size as the lightning channel (up to 5 centimeters). As a rule, these properties do not have a very strong effect on the surge voltages, especially when a distant strike is considered.