Whites TDI SL Owner's Manual page 19

literally gives you two different size coils running simultane-
ously: a larger outer coil for large target depth, and a smaller
inner coil for small target sensitivity. As such, even the 12" DF
coil, with its 6" inner coil, can pick up nuggets of only a few
grains. The 7" DF coil, with its 3.5" inner coil, is even better at
picking up grain-sized nuggets, with little compromise on large
target depth.
Proper sweep technique is important with any detector, and
the DF coil adds a slight twist. Round coils tend to have a coni-
cal pattern of sensitivity as illustrated below. As such, coverage
at maximum depth is less than the size of the coil, so it is always
recommended that successive sweeps are overlapped by about
50%.
The DF coil has an additional inner coil which has its own
conical detection pattern. Since this coil is most sensitive to
shallower tiny targets that the larger coil may miss, in order to
get the best ground coverage on these targets it is recommended
that the 50% overlap be applied to the inner coil, as illustrated
below.
TDI SL
Owner's Manual
Page 19