so i've been wanting to build something from scratch for a while now and last weekend i finally sat down and put together a crystal set based on a design i found in an old handbook from like 1978. nothing fancy, just a coil wound on a toilet paper tube, a variable cap i pulled off an old am radio, a 1N34A germanium diode and some high impedance headphones i grabbed at a hamfest for two bucks.

got it picking up a couple of the stronger AM broadcast stations no problem but i'm wondering if there's anything i can do to improve selectivity without going crazy with a more complicated design. the coil is wound with 26 gauge wire, about 60 turns, and i've got the tap set maybe a third of the way up but i'm not 100% sure i have the antenna coupling right. tried a long wire out the window maybe 30 feet or so and that helped signal strength but now everything bleeds into everything else and i cant really separate the stations at all.

anybody built one of these recently or have suggestions for the coil geometry or coupling? also wondering if i should bother adding a preselector stage or if thats overkill for what this is

yeah the long wire is probably killing your selectivity, thats a classic issue. the antenna coupling loop is really the key thing here — if you're just tapping directly onto the main coil you're loading it down and Q goes right out the window. what you want is a separate smaller coupling coil, loosely coupled, so you can bring in the antenna without hammering the Q of your main resonator. even just 5 or 6 turns of wire near the bottom end of the main coil and then you move it closer or farther to adjust coupling. makes a huge difference.

also 60 turns on a cardboard tube is probably fine but if you can get some space between turns you'll get better Q, close wound works but spread winding or even basket weave if you're ambitious will help. the 1N34A is the right call for a detector, some guys try to use silicon and wonder why it doesnt work well without forward bias.

i built one last year basically as a joke and ended up spending like three weekends on it trying to get it to reject the local 50kw blowtorch station lol. never fully solved it but the separate coupling coil thing the other guy mentioned does help. also try shielding the variable cap if you havent, mine was picking up hum from just my hand being near it.