Hi Bobby,
Pretty normal F5 operation in my opinion. First, when you notched out the trash ranges (foil, tabs, zinc) you also notched out where most of your EMI noise hits on the conductive scale. Second, the F5 also uses the ground signal to cancel out EMI noise as you sweep the coil. Your medium ground bar reading tells me that feature is in use and is also using that feature to cancel EMI noise. So between those two functions, you are going to get a pretty stable detector.
Lastly, and Bob, mentioned this, if the Threshold dial is turned too hard into the dail stop, it will also put you into a reduced operating mode, simular to the ground signal effect, and will offer a quieter operation but with reduced performance.
There is a point where the threshold setting stops giving you a depth increase and changes over to a modification of target response. Threshold settings close to 0 make fringe depth targets sound as "whispers". When the threshold is moved out toward the edges (-9 or +9) then the fringe depth targets start to sound more like faint "ticks". Since it is often difficult to get a +9 threshold setting in urban environments it is not general knowledge, and I may be the only person that hunts with a -9 threshold setting on a regular basis and aware of the differences. In fact, although I prefer the audio rubustness that positive threshold setting give, at sites where my ground minerals are low enough, I like coin hunting with very low (-9 to -5) threshold settings and very high gain settings. For gold jewelry hunting I use very high (+5 or higher) threshold settings with stable gain settings.
HH
Mike
