so ive been working on this direct conversion receiver for 40m, mostly following the EMRFD design but with a few changes because i didnt have all the parts on hand. got it breadboarded last weekend and fired it up and honestly it works better than i expected in some ways and worse in others.

the problem is the thing is pulling in broadcast stations like crazy, there's this massive AM station somewhere nearby i think around 1.2 MHz and it's just bleeding into everything. audio is loud and distorted and it basically covers the whole band when conditions are right. i've got a low pass filter on the front end but clearly it's not doing enough. the filter is a 5 element chebyshev thing i wound on T50-2 cores, calculated for 40m but i wonder if my winding is off or the core material is wrong for this.

also getting a lot of hum, like 60hz hum that varies with the signal strength weirdly enough. not sure if that's the power supply or if i'm picking up the AM demodulation products or what. running it off a wall wart right now, probably should put a proper filtered supply on it but i wanted to see if it worked first.

anyone dealt with this kind of broadcast breakthrough before? feels like a shielding issue too because moving my hand near the board changes things noticeably.

yeah the broadcast thing is super common with DC receivers, they're just inherently wide open at the front end before the filter does its thing. a few things jump out at me reading your description. first, T50-2 is fine for that frequency range but make sure your winding is tight and consistent, any sloppiness in the coil geometry will shift the cutoff and kill your stopband rejection. also a 5 element LPF alone might not be enough if the AM station is that close and that strong, you might want to add a bandpass section tuned to 40m rather than just relying on the lowpass to do all the work.

the hand capacitance thing is almost certainly shielding, breadboard DC receivers are basically antennas for RF interference. even just putting the RF front end section in a small tin or wrapping it in copper tape can make a huge difference. i built a similar thing a couple years back and it was basically unusable until i got it in a proper enclosure. the hum varying with signal is interesting, might be intermod between the 60hz pickup and the detected AM, which would be a really weird artifact. definitely get off the wall wart though, that's probably making at least part of it worse.

had almost the exact same issue with a regen i built, turns out my filter toroids were wound so loosely the coupling between turns was doing weird stuff. i ended up just buying a pre-wound filter kit because i got frustrated but probably not the answer you want lol. the shielding thing is real though, even a altoids tin over the mixer section helped mine a lot. good luck with it, these things are annoyingly finicky but when they finally work its pretty satisfying