Wire-wound H-coils exhibit unwanted sensitivity to off-axis field

What is it about?

Wire-wound H-coils exhibit unwanted sensitivity to off-axis field. This sensitivity can be calculated for a given H-coil design, and it is almost independent of the number of turns. Similar unwanted sensitivity can be expected of an H-coil with 20 turns and with 200 turns (of the same physical size).

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Why is it important?

The unwanted off-axis sensitivity is especially important in magnetically open circuits, in which the H component normal to the sample surface can achieve values many times greater than the tangential component which is intended to be measured with an H-coil. The measurement errors caused by this effect are far from being negligible. The unwanted Hz component can reach 30% of the intended Hx component. Hence both the amplitude and the phase of the measured signal can be easily affected. Such effects can be especially important for measurements under rotational magnetisation, and it can lead to additional measurement errors. This can explain why the rotational H-loci can be visibly asymmetrical.