so i finally pulled the trigger on one of those QSK-40 kits ive been eyeing for like two years. got it all soldered up last weekend and honestly the build process was way more satisfying than i expected, took me about 4 hours spread over two evenings. its putting out right around 4 watts on 40m which i think is about right for the design.

anyway my question is about efficiency — im running it off a 3s lipo pack i had leftover from an RC car project and i keep reading that QRP rigs can be pretty picky about supply voltage. the kit manual says 12v nominal but ive seen guys run these down to like 9v without too much issue. does the output power drop significantly or is it more of a gradual thing? im trying to figure out how long i can run this thing from the battery before i lose a noticeable amount of signal.

also just curious what antennas people are using for portable ops with low power rigs. i have a random wire i throw up in trees when im out but ive been wondering if an end fed halfwave would be worth building. seems like every QRP guy i see on youtube has one but i dont know if thats just because theyre easy or if they actually perform better for this kind of thing.

yeah the voltage thing is pretty real with those kits. most of the simple single-band designs are running a class E or class C PA stage and the output drops pretty noticeably below about 10.5v in my experience. not a cliff edge exactly but you'll probably lose a watt or more by the time the battery sags to 9v under load. i'd keep an eye on voltage while transmitting, not just at rest, because lipo packs can dip more than you expect when you're drawing current for a key-down.

on the antenna question — end fed halfwaves are genuinely good for portable work, its not just youtube hype. the main practical thing is you only need one support point if you're doing a sloper which makes it way easier to throw up in the field. a random wire works fine too but you'll want a decent ATU and a good counterpoise situation or you end up with rf on your coax braid and all kinds of weirdness. for QRP id honestly say the antenna matters more than it does with higher power stuff because you dont have the extra watts to compensate for a mediocre setup.

nice work getting it built, i remember finishing my first kit and just staring at it for a while before i trusted myself to power it up lol. i'm still pretty new to QRP myself but ive been doing portable stuff with a little 5w rig for about a year now and the end fed halfwave thing is real, i made contacts into europe from a hill in vermont with 5 watts and a piece of wire in a tree so they definitely work. just make sure your 9:1 or 49:1 unun is decent quality if you build one, the cheap ferrite cores can be lossy and at QRP levels every db matters more than you'd think