The results I gave were intended only as a brief summary of a series of experiments I did. Those experiments looked at oscillator frequency (28 to 60KHz), inductance, wire (Litz vz. magnet wire), and free-air resonance frequencies of the coils. At the frequency ranges tested, Litz wire made no difference. I also tried a few pot cores and got virtually the same results.
Thus, using small coils of the size described and 3/16" wire (hard-drawn, high-carbon music wire) and under the conditions of my experiments, the TDA0161 can be depended to provide good sensitivity up to 1 cm, and usable sensitivity up to almost 2 cm; however, to get above 1.5 cm, the settings are a bit "touchy."
On the other hand, with a two-coil detector (basically a Zircon metal detector -- a reverse-engineered schematic is available on the internet), I got sensitivities of 5 cm or more.
John