There may be several reasons for what you observed. Was you in AM? You my have silent EMI. Thou high freq may be more sensitive to smaller items the problem is its affected more by minerals in the ground. Low freq deals with minerals better, but .... is affected by EMI. Take a look at this:
The 28 frequencies transmitted by the Explorer are all produced from one variable frequency oscillator, and all 28 are multiples or sub multiples of this oscillator frequency. When noise cancel is initiated, it steps the variable frequency oscillator through its range of 11 frequency steps (thus slightly changing all 28 frequencies by the same ratio). It records the received electrical noise at each step. At the end of this procedure, it selects the quietest frequency (ie the one that received the least noise). It also remembers this frequency when you switch the detector off and sets it to that particular frequency when you turn it back on again. The receiver is designed to properly receive the whole range of possible transmitted frequencies, so neither depth nor discrimination will be affected by which frequency the noise cancel circuit finally selects.
I dont disagree with what you found ive manually changed mine as well and it seems to work better in channel 6. Our problem is no one can tell us what these 28 freq/harmonics are randomly set at per channel. If we knew this we could better use this manual option. As it is now.... id let the machine do its thing since you can test it every 10 feet and you might get a different response especially if the machine is GB.
Dew
