Low‐field 10‐GHz noise radiation and attendant rf current oscillations are generated in 10‐mm‐long specimens of n‐type InSb at 77°K. Using a special experimental arrangement, both the azimuthal and the polar orientation of the applied magnetic field can be varied and the spatial distribution of the radiation along the length of the InSb specimen can also be measured. Two distinct modes of low‐field radiation are found in InSb specimens. One mode of radiation (B∥ mode) prevails when the magnetic field is aligned parallel to the direction of current flow and the other mode of radiation (B∥ mode) prevails when the magnetic field is aligned transverse to the direction of current flow. The B⊥ mode of radiation is investigated in considerable detail and it is determined that a reproducible anisotropic radiation pattern of twofold periodicity is obtained when a transverse magnetic field is rotated in the azimuthal plane around the InSb specimen. A computer analysis using a theory which incorporates an acoustoelectric interaction, involving off‐axis acoustic waves, shows good agreement with the experimental azimuthal radiation patterns of the present experiment and other related experiments. It is concluded that an acoustoelectric‐type interaction is involved in both the B⊥ and B∥ modes of low‐field noise radiation.
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