... Why is the OPLink Mini Ground not compatible with the Revolution with on-board radio? ...
Because the OPLink Mini Ground is bad, so is the matching OPLink Air. They're made with substandard parts and are off frequency. Since they're both made by the same people and off by the same amount they talk to each other but not OPLinks that are properly made and on frequency. I have 4 regular oplinks, a regular revo, and an oplink ground/air pair. The regular oplinks and revo all talk to each other (not at the same time of course
). The oplink ground and air only talk to each other and won't talk to any of the others.
In the capture below the top half is a normal, working oplink. The bottom is an oplink ground that won't talk to other oplinks. The glitch in the middle is me stopping the analyzer to switch oplinks. Notice the bad one is shifted higher in frequency. And it's RFM22b has the chip marking removed. Maybe they're using seconds or clones of the RFM22b modules.
In the second picture the left one is a real RFM22b module and works fine, the right is the oplink ground that won't talk to regular oplinks. On the right the chip marking has been removed. It's also a different chip package, the board markings are a little different, and the crystal a different make. The castellations along the edge of the board are a different design.