Complex sentence: The advantage is that multiple frequencies can be jammed simultaneously however the jamming effect can be limited because this requires the jammer to spread his full power between these frequencies, therefore the more frequencies being jammed, the less effectively each is jammed.
Split into simpler sentences: The advantage is that multiple frequencies can be jammed simultaneously; however, the jamming effect can be limited because this requires the jammer to spread its full power between these frequencies. So the more frequencies being jammed, the less effectively each is jammed.