The dispersion relation of spin waves can vary monotonically about the center of the Brillouin zone, allowing zero-momentum wavepackets to flow unidirectionally, which is of interest for applications. Techniques such as propagating spin wave spectroscopy are inoperative in such cases because of the difficulty to identify the spin wave wavevector at a particular frequency within a spectrum. Here we present a method to analyse this case and apply it to acoustic spin waves in a synthetic antiferromagnet in the scissors state, in which we confirm that propagation parallel to the applied fields is unidirectional. Interestingly, we find that in this unidirectional situation, the phase accumulated by the spin waves propagating between two antenna is not proportional to the antenna spacing. It is also a function of the two other lengths of the problem: the antenna width and the spin wave decay length. Accounting for them is required to avoid wavevector errors in the dispersion relations.