Right, I've been digging into this a little deeper. The Semtech sx1231 can output +13dB and has a minimum voltage limit of 1.8V. The sx1231h has a max output of +20dB (100mW), and a minimum voltage of 2.4V.
Now the RFM69 uses a sx1231h (as confirmed by the version number register which is different to the sx1231). However, the chip is marked RF69, and there is a datasheet for it:
www.orcam.eu/res/Datablad/rf69v12.pdfThis shows that for up to +17dB the voltage can be as low as 1.8V (using PA1 and PA2, normal power settings) . From +17dB to +20dB it uses PA1, PA2 with high power settings and needs 2.4V. So it appears the 2.4V spec is for the full output power using PA1, PA2 and high power settings.
The RFM69CW is restricted to +13dB from the RFIO output, and doesn't use PA1, PA2 or the high power settings, and according to the RF69 datasheet only needs 1.8V. Even though this is missing in the sx1231h datasheet the thing that makes it credible is that the sx1231 is also +13dB without the PA amplifiers and also has a minimum voltage of 1.8V.
Now I'm curious as to what the other 2.4V component is, the RFM69CW and RFM69W have only passives and crystal, unless you're saying it's an oscillator? All the reference designs use a crystal and both pins 4 and 5 of the sx1231h are connected to the device and the other two are ground, so I'm sure it's a crystal.
My conclusion is that the RFM69CW and RFM69W are OK down to 1.8V.
Mark.