Instruction manual
83Understanding your X-TERRA - Technology
How coils identify targets in very close proximity to the windings
It is known that continuous wave metal detectors can be unbalanced (detuned) by targets, in
particular higher conductivity and larger mass coins, that are in close proximity to the coil and
this can change the target phase angle of the TID suciently to push it into the next “bin”. Also,
close to the windings, the target is seen more like a collection of parts than a whole and this also
aects the TID.
Now that we have covered the background theory, the answer in this case is: the recalculated TID
of the U.S. silver dollar is closer to the centre of “bin” 48 for the HF coil than it is for the LF or MF
coils. Small changes due to proximity to the windings are not enough to make it ip into “bin” 46
for the HF coil, but they do so for LF and MF coils.
Normally, changing the frequency of the coil
by a large amount (e.g. from 7.5 kHz to 3 kHz or
18.75 kHz) would drastically change the TID of a
target. However, because we wanted the TID’s to
be the same regardless of the coil frequency, the
correspondence between the phase angles and
TID’s for the LF and HF coils has been modied
(see illustration on pg. 82) to be similar to those
for the standard MF coil. This further distorts the
TID “bin” size for LF and HF coils.
+
-
+
+
-
+
+
+
+
-
+