FEKO mathematical time signal

Does anyone know how to define a pulse mathematically in POSTFEKO? I'm trying to simulate a lightning strike but am unable to find any guidance how to define this as a function of time as I keep getting an error message when I do.

Listen to an AM radio when a lightning storm is in the area. A strike is not always a single click, but often a scratchy sound lasting maybe 1/3 of a second. Therefore try defining a pulse train of 20-40 Hz, which lasts that length of time.

Also try a forum search on lightning. By opening one likely thread, you see a list of similar threads at the bottom of the page, which might lead to further information.

I know what the mathematical formula is from BS EN 62305, but I don't know what format to use so FEKO recognises it.

As a last resort, try a forum search on FEKO and POSTFEKO.