There is a difference between expertise and experience and there is a difference between how an engineer and a technician go about things. Several people have given you the answer but bypassed the working out (i.e. what rather than why). To see the why you need to finish your analysis of what is going on at the driver surface, exit of different sized slots and on a sphere in the far field. Even if someone told you why you would still need to take it on trust to some degree. If it is important to you then working it through yourself is the reliable way to make sure you unearth all the things you do not understand and work away at them until you do.
Of course, very few people on this forum approach things in this way which is fine for a hobby since it certainly helps keep the possibilities alive and reduce the time to cutting metal (well wood). For something a bit more serious like a business the engineer's approach is likely to be more effective in picking the good possibilities from the less worthwhile.