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The influence of a planar input coil on the V-I characteristics of a dc-SQUID (superconducting quantum interference device) has been analyzed by means of computation and experimental investigation. An extended electrical model of the input coil based on transmission lines has been proposed. The frequency behavior of this model has been validated with an expanded copper model. The V-I characteristics of DC-SQUIDs have been calculated with this extended circuit model using a SPICE-type circuit analysis program. The results have been compared with measurements of niobium dc-SQUIDs. In this paper the relationship between the V-I characteristics of the SQUID and the coupling impedance of the washer is demonstrated.