It's true that higher freqs are required to excite a non-magnetic low-conductor. The FBS utilizes a high freq of about 25 kHz and a low freq of about 3.125 kHz. This is the high and low freq channels. The FBS units compare the returned responses at several different times. If a target is more resistive (to eddy flow) then the time constant will be short but produce a high freq resistive signal. If the target is conductive with a longer Tc, then the detector gets a longer but lower frequency resistive signal. Sample the low and high freqs received and compare the ratio of low to high freq component to get an idea of the time constant of the target.
When the detector searches the channels for the least EMI it is only able to adjust the xmit freq a minor amount. Thus, the high freq of 25 kHz,which would normally be Channel 6 would be pushed to 27.78 kHz if Channel 11 were selected or dropped to 22.2 kHz if Channel 1 were used.
In theory, Channel 1's lower freq set would be better for exciting higher conductors (that require less pulse repetition to form eddies, like silver, copper, and larger gold rings). Since iron target are high resistive the lowered pulse repetition would make the iron less reactive and less likely to produce those resistive high tone chirps from eddies.
Channel 11's slightly higher freq set would excite high conductors too, but not as deeply (skin effect) and would have more impact on low conductors that require more pulse repetitions to sustain eddy development. Not ideal if you want to keep iron quiet but better for high resistive targets (low conductivity) like lead, steel, stainless steel, foil, cupro-nickel cents, thin rings, and some pull-tabs, etc.
In a single freq VLF machine raising the operating freq might give an advantage to lower conductors but with the FBS system, the net effect is still a determination of target time constants. The VLF machine is more interested in phase, comparing R to X, while the FBS is a hybrid that measures signal decay at two frequencies. So, I think the gain or loss in frequency shifting is not that important as the ratio of Tc is not as affected.
Of course, if you override the preferred auto setting and dial in Channel 11 because you are after thin rings along the beach - and you think this is going to give an advantage, what if local EMI would otherwise make Channel 1 the preferred channel. EMI can ruin your sensitivity, make the detector chatty and unstable - all the things you don't want. It can ruin your depth without any noticeable audio affect - you wouldn't even know it. So, I'd still recommend to let the machine determine the best setting.
