In a sheet stacking apparatus, in particular spiral slot stacker, one determines the relative position of a group of sheets 7A, 7B, for example distance ta between the sheets or total length tL of overlapping sheets. Each implantation rod is secured by at least one fastener element having a head. The converter unit receives the first summed component signal and the second summed component signal and compensates for polarization skew between the antenna array and a source.