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An analysis of the microwave magnetic fields associated with a dominant TEM mode propagating in a coaxial line reveals only a linear sense of microwave H-vector polarization. As a consequence, this transmission line structure does not inherently lend itself for use with ferrites to obtain nonreciprocal propagation characteristics. A means for obtaining microwave H-vector circular polarization in coaxial line is described in this paper. This technique consists of partially filling a coaxial line cross section with a low-loss dielectric. This structure, in conjunction with certain broad-banding techniques, has been utilized in the development of an octave bandwidth coaxial line isolator. A treatment of the parameters, with associated experimental verification, is presented which affects the operation of the isolator. Also included in this paper are the design and experimental characteristics of this isolator. An anticipated mode configuration in the dielectric and ferrite loaded coaxial line is derived on the basis of the experimental results presented in this paper.