A square waveguide carries TE11 mode whose axial magnetic field is given by Hz = H0, where the guide dimensions are in cms. The cut off frequency of the mode is
5. An unmodulated voltage 10 V effective is applied to a dioxide detector in which load resistance is 4 x 10-5 ohms. A micrometer shows that the rectified DC current in this resistance is 30 A. The efficiency of detection is