Apparatus, including a probe having a proximal end and a distal end, the probe being configured to be inserted into an organ of a human patient and defining a probe axis. At least two conductors are positioned on the probe at the distal end. The apparatus includes at least two flexible conductive splines, each conductive spline having a first termination and a second termination, the first terminations being electrically connected together at a region on the probe axis beyond the distal end, each second termination being electrically connected to a respective one of the conductors, the splines being configured to bend into respective arcuate forms that encompass a volume. The apparatus also includes a processor configured to receive voltages induced on the splines via the conductors and to calculate a position and orientation of the volume in response to the received voltages.