Novel polypeptides are provided having substantially the same sequence as immunologically significant fragments of AIDS-related viruses. Also disclosed are methods for computing IQ impairments from a sampled complex signal, methods for computing DC properties of a signal path between the transmitter and receiver, and methods for transforming IQ impairments through a linear system. Each of the coupling lateral sides is formed with a coupling groove. Each of the electric conductors has a circumferential width that is smaller than a circumferential distance between a first wall and a second wall defining the slot.