mikem:
According to Arduino Forum and to my tests, the tx_ant and rx_ant pins arent needed. I leave them floating and it seems to work fine.
Ah, I'm wondering if this is a difference between the DIP and SMT versions of the RFM22. The SMT version I have definitely have an antenna switch chip (6 pin near ANT) that must be driven correctly. However I suspect all the RFM22 modules have this as the chips RX side can't survive the full output power from the TX side (17dBm) without a switch according to the documentation. By contrast the lower powered RFM23 doesn't have one.
If the pins are left floating its unlikely you'd get any appreciable power off the device (limiting the range greatly) - there's also a risk of generating harmonics that splatter over other bands, which is not a good thing. There is also the risk of burning out the PA at full power since the VSWR will be badly off.