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This paper reports an experimental study of microwave diagostic techniques as applied to electrodeless discharge plasmas. Here relatively dense plasmas (~1012. electrons/cm3) are created by inductive coupling from a radio frequency (40 mc) coil using only a one kilowatt generator. A microwave diagnostic system has been developed which permits X-band (8840 mc) waveguide transmission in the TM01 mode through this plasma without appreciably loading the RF excitation coil. Simple slotted line probing techniques have been developed which are capable of measuring the point by point plasma densities with a high degree of accuracy in relatively dense plasmas using only a single microwave frequency. The techniques described here are to be applied to a large 100 kilowatt plama generator now nearing completion.