I've been noodling on the power amp stage for a few days. My first thought always goes to the IRF510 RF MOSFET as it's a classic for homebrew radios, has loads of example circuits, and is as cheep as chips. However, it can be a bit fiddly. The biggest problem is the high gate capacitance. 180 pF requires a lot of gate power just to keep it conducting, perhaps as high as 1 W continuous to get full amplification! That's basically a non-starter for a low power portable. Fortunately there are a lot of options out there if you're willing to pay a bit more. I've seen suggestions for 2SK3476 from Toshiba but it's unobtainium right now. However, there's another device from the same family which could be an even better match, the 2SK4037.

This device is designed for 470 MHz but these devices often have acceptable response down to DC. Typical output power is 36.5 dBm with a gain of 11.5 dBm. We'd like more gain but this should be acceptable for a first shot, we can always add another drive stage if we want to get up to a full 5W. This device doesn't list input capacitance but the similar 3476 has an input capacitance of 53 pF so we can assume the 4037 is in the same order of magnitude. At $2.49 in quantities of 10, it's not as cheap as the IRF510 but it's not going to break the bank either. I think I'll buy a couple and see how they work in the HF.

I'll put together a first take on an amplifier stage and post an update. Then it's time to order parts and start breadboarding the design to see if it works.