Coil device and antenna

A coil device comprising a coil, and a ferrite core arranged in a magnetic path of the coil, at least part of which is coated with a resin;

• said ferrite core being a Ni ferrite core having an average crystal grain size of 5-9 μm;
• and
• said Ni ferrite core having
• (a) initial permeability μi of 450 or more at a frequency of 100 kHz and a temperature of 20° C.,
• (b) temperature-dependent inductance change ratios TLa and TLb of −0.6% to +0.6%, wherein TLa is represented by the formula 1 of TLa=[(LT2−LT1)/LT1]×100(%), and TLb is represented by the formula 2 of TLb=[(LT3−LT1)/LT1]×100(%), wherein LT1 is inductance in a compression-free state at a frequency of 100 kHz and 20° C., LT2 is inductance in a compression-free state at a frequency of 100 kHz and −40° C., and LT3 is inductance in a compression-free state at a frequency of 100 kHz and 80° C., and
• (c) stress-dependent inductance change ratios PLa and PLb of −0.6% to +0.6%, wherein PLa is represented by the formula 3 of PLa=[(LP2−LP1)/LP1]×100(%), and PLb is represented by the formula 4 of PLb=[(LT1−LP1)/LP1]×100(%), wherein LP1 is inductance when monoaxially compressed with a surface pressure of 10 MPa at a frequency of 100 kHz and 20° C., LP2 is inductance when monoaxially compressed with a surface pressure of 20 MPa at a frequency of 100 kHz and 20° C., and LT1 is inductance in a compression-free state at a frequency of 100 kHz and 20° C.,
• (d) both of the sum of TLa and PLa and the sum of TLb and PLb being more than −1.0% and less than +1.0%.