so ive been messing around with a little 40m QRP rig i built from scratch, mostly following a design i found online with a few mods. got it putting out about 4 watts which is fine but the thing runs pretty warm even at that power level and im starting to wonder if my final stage efficiency is worse than it should be. the IRF510 i used gets noticeably warm after like 10-15 minutes of operating even with the small heatsink i put on there.

for reference im running it off a 13.8v supply and the current draw is around 800mA when transmitting which feels high to me for 4 watts out. that works out to like 36% efficiency which... i dont know if thats normal for this kind of design or if somethings off with my matching network. the lowpass filter and output network were wound by hand so those are probably not perfect.

has anyone else built something similar and can tell me if those numbers sound reasonable or if im chasing a real problem here. happy to share the schematic if anyone wants to take a look at it.

36% isnt great but its not shocking for a class E or class C final that isnt perfectly tuned. honestly the IRF510 is a popular choice for homebrew QRP rigs but it can be a bit finicky with the gate drive and you can lose a lot to heat if the switching isnt clean. what does your gate drive waveform look like if you have a scope handy? that'd tell you a lot more than the current numbers alone.

also the hand-wound toroids - how many turns and what core did you use for the output transformer? if the impedance matching is off even a little bit you'll dump more power as heat than you're radiating. i built a 20m rig a few years back with similar symptoms and it turned out one of my toroids was wound on the wrong mix core, had grabbed a type 2 instead of type 43 out of my bin. ran hot as heck until i swapped it.

yeah that efficiency figure does sound a bit low to me but i wouldnt panic yet. when i built my first QRP rig i was convinced something was wrong for weeks and it turned out the output was actually fine, i was just measuring things badly with a cheap dummy load that wasnt actually 50 ohms. worth double checking your power output measurement method if you havent already.

also for portable use and just getting on the air honestly 4 watts is plenty - ive made some really nice contacts running 3 watts into a simple wire antenna. if it works and youre making contacts the efficiency can be a project for later. but do keep an eye on that IRF510 temp if youre running it portable in summer, those things can get unhappy fast without enough airflow.