Many different ones, some COTS, some home-made
Existing radios have been small and simple
They’ve also been slow (e.g. Libertad-1 at 300bps with AX-25) or power-hungry (MHX series receive idle current @ 115mA)
This is an unfinished / unresolved area of concern for the CubeSat community.
Bands / Frequencies
Primarily amateur due to licensing restrictions: 144MHz, 433MHz, 920MHz, 2.4GHz
Patch (GeneSat), x4 (Delfi-C3), stubby (MAST), tape measure (CPn), genetic algorithms (NASA ARC)
Omni required if no attitude control. With ADAC, can improve antenna gain