  The Wake Field Monitor (WFM) system installed on the CLIC prototype
accelerating structure in CERN Linear Accelerator for Research (CLEAR) has two
channels for each horizontal/vertical plane, operating at different
frequencies. When moving the beam relative to the aperture of the structure, a
disagreement is observed between the center position of the structure as
measured with the two channels in each plane. This is a challenge for the
planned use of WFMs in the Compact Linear Collider (CLIC), where they will be
used to measure the center offset between the accelerating structures and the
beam. Through a mixture of simulations and measurements, we have discovered a
potential mechanism for this, which is discussed along with implications for
improving position resolution near the structure center, and the possibility
determination of the sign of the beam offset.
