so ive been messing around with the idea of building my own QRP rig for a while now and i finally just dove in. picked up a copy of Experimental Methods in RF Design and honestly its a bit overwhelming but im getting there. right now im trying to put together a simple 40m CW transceiver, nothing fancy, just something to prove to myself i can do it.

my main thing right now is the PA stage. im using a IRF510 MOSFET like i see in a lot of the EMRFD projects and the homebrew community stuff, but im getting way more heat than i expected even at what i think is around 3-4 watts out. is that normal? i dont have a dummy load right now im just using a light bulb thing i threw together which probably isnt ideal. also not sure if my LPF is really doing what its supposed to, i have a scope but no spectrum analyzer so i cant check harmonics properly.

the receiver side actually works pretty well, i used a NE602 mixer and a 4.00 MHz crystal filter i pulled from an old CB radio, it picks up signals fine. just the transmit side thats giving me grief. any pointers from folks whove been down this road would be great, or just reassurance that this is normal first-build chaos haha

the IRF510 running hot is pretty normal honestly, those things are a bit notorious for needing proper biasing or they just idle at way too high a current. what does your quiescent drain current look like? if youre not setting the gate voltage carefully you can end up burning off half your battery just sitting there. a lot of the older EMRFD-style designs have you trim a pot to set bias and its easy to leave it too high.

the light bulb dummy load should be fine for basic testing, people use them all the time, just know the impedance changes with temperature so your power readings are going to be a bit approximate. for harmonics without a spectrum analyzer you can do some rough checking with a second receiver if you have one, just tune around and see if you hear anything on 80m or 20m while youre transmitting. not perfect but gives you a clue. whats your LPF cutoff? if you wound the toroids yourself double check your turns count because thats where most people go wrong first time around.

oh man i went through basically the same thing with my first 40m build last year. the NE602 receiver section is really solid for a first rig, i had mine picking up signals the first night which felt amazing. transmit side was a nightmare though, took me like three weekends to get it actually putting out clean power.

one thing that helped me was borrowing a friend's SWR/power meter just to get a baseline, even a cheap one tells you more than the lightbulb situation. also are you running it off a regulated supply or just a battery? i had some weird instability when i was running off a wall wart that wasnt properly filtered. anyway keep at it, getting that first homebrew contact is genuinely one of the best feelings in the hobby