The fundamental characteristics and structure of a strip transmission line isolator which operates to frequencies as low as 200 Mc are reported. This structure does not utilize dielectric material as a mode distorting material to create the necessary conditions for nonreciprocal wave propagation, but instead depends on the asymmetrical placement of the ferromagnetic material on the center conductor. Isolation-to-insertion loss ratios of better than 36 to 0.4 db have been achieved at 725 Mc, while isolation peaks of 35 db to less than 1 db insertion loss have been obtained at frequencies as low as 375 Mc. The over-all length of the device with the above characteristics was 612 in. and recent evidence indicates that this can be substantially reduced.