As I was mentioning in my 1000.1000 Hardware selection, I have opted for the cool RFM69HW radio module. Weirdly enough, in quite a few sources (big distributor and ebay) the higher power HW module is cheaper. So there ie no reason not to get the higher power module, given quantity discounts. But I want it to operate at lower power most of times. The datasheet does not show any differences at lower power, so I had no reason not to go for the higher power module. It even says so on the features list on the front page, I can turn the power down to -18dBm.
What happens?
Unaware of the full details, frustration emerged when I started mixing in both types of modules. The Radiohead implementation I am using (1.71) makes sure no single power value will work for both module types: over 13 dBm and a W module will not work, under 14 dBm and a HW module will not work. Digging through the code shows no problems, it does not really work as expected, but it should. The datasheet makes things more clear which power amplifier to enable based on the level and the library implements it properly. We can see there is some overlapping in the ranges as well.
A nice graph shows the power levels, from which we can see that the power ranges in the middle are achievable with different configurations.
So doing some digging around the datasheet of the IC, same one present on both modules, we learn that the IC contains 2 types of amplifiers. PA0 is the lower power one and is always directly connected to the LNA input and both to the antenna. For higher power, one has to use additionally the PA1 and PA2 and optionally a BOOST setting to squeeze out the last couple of db. So how are these connected on the actual module. The RF69 datasheet shows a blurry picture of the module schematic for up to 13dBm output power, on page 76/77. We see that PA1 and PA2 never reach the antenna, just PA0.
Compared to that, the high power RFM69HW uses a switch which is driven by RX/TX pin, after some module reverse engineering. This means that only PA1 and PA2 reach the antenna, and never PA0, during transmission.
Read more: RFM69 output power