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During the last decade orthogonal fluxgates have been mainly manufactured using magnetic micro wire surrounded by a pick-up coil. This structure has several advantages, however it employs a finite length core as well as traditional orthogonal fluxgates based on cylindrical bulk core do. The measured field on axial direction is applied on a magnetic core with open magnetic circuit, which is therefore affected by the length of the wire. In this paper we compare sensor heads with wire-core having different length: the resulting noise is significantly lower for longer core (about 4 pT/√Hz at 1 Hz vs. 7 ÷ 8 for short wire). In fact, the terminations appear less saturable than the central part of the wire. Moreover, we show how the ends contribute less to the total sensitivity. Finally we model by FEM simulation the structure of the wire and we show that the different response of the wire to the axial field is fully described by demagnetizing effect. Therefore, we suggest to use a long wire keeping the termination if possible out of the pick-up coil in order to obtain the output signal from the more uniform part of the wire.