No there's not much gap between. I suppose this could be because 428 khz to 436 khz covers the more hard to kill and larger pathogens so the smaller easier pathogen would be terminated anyway by using this so -called 'widesweep' frequency. It should be named something like narrow selective sweep, but I am not sure I need to read more on this. I would like to know if these frequency can be used higher than 436 khz if this was necessary to kill a stubborn pathogen and is there a Prog. Driver for higher than 436 khz for the Super Zapper, I have not seen one. How higher can a frequency be used before it becomes unsafe for humans to use? I read somewhere recently that a frequency of several hundred thousand (can't remember the exact figures) is lethal to humans.