Abstract: A technique for compensating for bias errors that are inevitably introduced in an N-port analog modeformer (14) of the Butler matrix type, used to transform antenna arm signals obtained from a cylindrically symmetrical or spiral antenna (10), to an equal number of more useful mode signals. Corrupted mode signals from the analog modeformer (14) are downshifted in frequency in a coherent receiver processor (18), converted to digital corrupted mode signals in an analog-to-digital converter (22), and then further processed in a bias error reduction processor (26) to produce output signals that are a close approximation of true, uncorrupted mode signals. The bias error reduction processor 26 uses a memory (28) to store matrix quantities obtained from measurements previously made of the analog modeformer (14), and performs an error reduction function by simple matrix manipulations of the digital corrupted mode signals and the matrix quantities stored in the memory (28).