Fox-1B, AO-91 or AMSAT OSCAR 91 [3] is a United States amateur radio satellite.

It is a 1U Cubesat, was built by the AMSAT-NA and carries a single-channel transponder for FM radio.

Fox-1B is the second amateur radio satellite of the Fox series of AMSAT North America.

To facilitate a satellite launch as part of NASA's Educational Launch of Nanosatellites (ELaNa) program, the satellite carries a student experiment conducted by Vanderbilt University's Institute for Space and Defense Electronics.

The RadFx experiment at this institute hosts four payloads for the study of radiation effects on commercially available electronic components.