mikem:
The RF22 library was written for the RFM22B devices and AFAICS, the tx_ant and rx_ant pins dont need to be connected for the RFM22B. May be different for the RFM22 devices, but RF22 library does not support them.
Ah that explains it - haven't seen RF22B silicon myself.
For the RFM22 all that's needed to support the pins is
spiWrite (RF22_REG_0B_GPIO_CONFIGURATION0, 0xD2) ; // TX state
spiWrite (RF22_REG_0C_GPIO_CONFIGURATION1, 0xD5) ; // RX state
And then connect GPIO0 pin to TX_ANT and GPIO1 to RX_ANT as in the RFM22 datasheet.
The RF22B's lower power levels mean its matching network doesn't need to be isolated from the RX input during TX as it can take the power levels without frying. For the RF22 this isn't safe at high powers (17dBm is 4.5V p-p into 50 ohms) hence the antenna switch chip on the RFM22 module.