Bob,I think you have some valid(not annal)questions (I would also like to know the answers to),but youll probably have to call Minelab and hope someone there can answer them.I had asked one of their service techs a while back ,what the 28 specific operating frequencies of Explorer were and he couldnt answer..I was told just whats in the book--28 frequencies from/between 1.5 khz and 100khz......So,if its using 28 channels(frequencies)why does it only select 11 for noise cancelling??.Well, in the SE manual,Noise cancel section(pg.56)it states the noise cancel will listen to EVERY channel for the quietest one....is that every channel of 28??? or 11???..You see,transmitting even on a single frequency gets complicated.Thats why most manufactures only use one frequency for their machines..Even though Minelab states 28 frequencies,chances are their not all putting out the same power..If you take for example,transmitting the 1.5khz signal it will inherantly also transmit a harmonic signal multiple (x2,3,4 etc)to that freq..so youll also get 3khz,4.5khz,etc signal that the power will decreasing as the multiple increases..so techenacally,(for example)transmitting 1.5khz at a specific power and allowing the 2nd and 3rd harmonic(3khz@50% power + 4.5khz@25%power) to pass(filtering),you can say its a 3 frequency machine even though the power is not the same on all channels...maybe the explorer only has 11 primary frequencies and the other 17 are harmonics...????..just my thoughts...Ralph