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Reduced height waveguide arrays are commonly used in fusion plasma radio frequency heating in the lower hybrid frequency range. In order to distinguish the incident wave and the reflected wave, power detection with high directivity in each reduced height waveguide is required. Due to the confined space in a stack of reduced height waveguides, power detection with high directivity in the reduced height waveguide is extremely difficult, especially from the wide side of the waveguide by using a conventional E probe. A new compact probe, which employs current loops, to monitor the incident and reflected wave from the narrow side of the reduced height waveguide has been developed. According to the test data, greater than 35 dB of directivity was measured with this compact probe at 4.6 GHz.