w are combined as a single capacitance c the resonant frequency of the overall system is given by in order to minimise loss in connecting cables and thus maximise the efficiency of the sensor it may be desirable to minimise the value of the capacitor c ext in a preferred embodiment this may be achieved by increasing the value of c 12 via the use of an inter layer dielectric as described above the invention may be put into effect using either a one or a two layer coil 100 fig9 11 show equivalent circuits representing a two layer coil based antenna used in alternative embodiments of the present invention in fig9 each of the two winding layers is represented by a separate inductance l 1 and l 2 capacitances c w1 and c w2 are respectively equivalent to the sum of the inter turn capacitances of inductors l 1 and l 2 compare with the one layer coil of fig6 the capacitance c 12 is that which appears between the two winding layers the external capacitor c ext is included to modify the resonant frequency of the antenna 300 and appears in parallel with capacitance c 12 in practice the capacitances c w1 and c w2 are small in comparison with c 12 therefore the equivalent circuit may be simplified further by neglecting c w1 and c w2 as shown in fig1 fig1 shows a further simplified equivalent circuit for a two layer coil based antenna the parallel capacitors c ext and c 12.