so i finally finished my first scratch built QRP rig, been working on it for about three months on and off. its a 40m CW transceiver based loosely on the bitx design but i kind of went off script a bit with the final amp stage. running about 4 watts out which im pretty happy with considering i had no idea what i was doing when i started lol

took it out to the park last weekend with my end fed half wave and worked a guy in Ohio from Massachusetts which felt incredible for 4 watts. logged maybe 6 contacts in two hours which i know isnt amazing but for a first portable outing with homebrew gear i was thrilled

anyway my question is about efficiency. the rig draws more current than i expected on transmit, like around 800mA from a 3 cell lipo. that seems high to me for 4 watts out but i genuinely dont know if thats normal or if my final stage is doing something wrong. the transistor gets warm but not scary hot. anyone have a sense of what class of operation i should be aiming for and whether this math even makes sense

800mA at roughly 11-12v lipo voltage means youre putting in somewhere around 9-10 watts and getting 4 out, so your efficiency is running maybe 40-45%. thats actually pretty typical for a class AB final, not gonna lie. class E or class D switching amps can get you up in the 80-90% range but theyre a lot more fussy to build and tune, especially on 40m. for a first build id say youre doing fine. the transistor getting warm is normal as long as youre not cooking it, whats the part number? some of the old favorites like the IRF510 really want a heatsink even at low power or they get grumpy over a long CW session.

Ohio from Mass on 4 watts is a solid contact. once you get comfortable with the rig try some of the QRP nets, sparc or the 40m QRP watering hole around 7030, lots of patient operators there who are happy to dig out a weak signal.

congrats on the build, that's awesome. im working on something similar but keep getting distracted by other projects. the efficiency question is something i wondered about too when i was reading up on QRP finals. from what i understand class C is more efficient but apparently its not really usable for SSB, only CW and even then there are tradeoffs? i might be wrong on that, someone correct me if so. anyway your Ohio contact sounds great, i havent even gotten my rig off the bench yet so you're way ahead of me