  Both longitudinal and transverse coupling impedance for some critical
components need to be measured for accelerator design. The twin wires method is
widely used to measure longitudinal and transverse impedance on the bench. A
mode error is induced when the twin wires method is used with a two-port
network analyzer. Here, the mode error is analyzed theoretically and an example
analysis is given. Moreover, the mode error in the measurement is a few percent
when a hybrid with no less than 25 dB isolation and a splitter with no less
than 20 dB magnitude error are used.
