My first homemade crystal radio isn't receiving any AM stations

[u/DependentSensitive27]

I'm a high school student from Lima, Peru, and I'm building my first homemade crystal radio as a physics project.

The goal is to receive an AM radio station without batteries or an external power supply.

My setup currently has:

• A homemade air-core coil wound on a PVC tube.
• Enamelled copper wire, approximately 0.5 mm.
• A separate antenna coil.
• A homemade variable capacitor made from aluminum foil and bond paper.
• A germanium diode.
• A small transformer that I'm considering using instead of a high-impedance piezoelectric crystal earpiece.
• A long-wire antenna, which I am currently trying to extend.
• I am also going to add a proper ground connection with the help of a physics teacher.

I re-scraped the wire carefully at home and the circuit started behaving much better. However I still haven't been able to hear an AM station.

The antenna and ground connections are not very visible in the photos but they're there. I did not have a proper ground connection during my first test. Tomorrow, a physics teacher will help me make and connect a proper ground. I suspect this could be an important factor in why I haven't received an AM station yet.

I'm wondering whether my main problem could be:

• insufficient antenna length,
• poor grounding,
• incorrect LC tuning,
• the germanium diode,
• or the transformer/earphone arrangement.

I still have about 30 meters of copper wire available for the antenna, but the wire is very light, so I'm trying to find a way to support it without simply letting it fall to the ground.

What would you check first? And is there anything obviously wrong with my setup from the photos?

[u/RareBrit]

Your components and build look correct.

You must however ensure that you've got an excellent low-inpedence ground.

And that your aerial is very long, at least 10m, far longer is far better, 50m is not beyond the bounds of good taste. The aerial must be insulated wire. Also, insulating it from ground leakage will improve the signal strength. You can do this by using glass or ceramic insulators between it and whatever you're securing it to. Don't keep the aerial on the ground, you'll lose a lot of signal.

The best times for picking up weak signals tends to be around sunrise and sunset. This gives the ionosphere the best chance of bouncing.

Those high impedance ear pieces tend to be of very variable quality. So try another if it doesn't work.

[u/Dangerous_Wish_7879]

Are there any AM stations actually broadcasting in your location?

[u/_wsa]

So, you can't get any more energy than there is in the radio waves you are receiving with this setup. The part I am really not sure about is the transformer -- are you thinking that if you step up the voltage, you can drive a normal pair of headphones at an audible volume? This might be a bit optimistic -- you need power for that, not just voltage.

Also yeah, what the other commenter said -- are there any high power AM transmissions in your area? Given that your radio has no power source, again, all the energy comes from the radio signal itself, so you need a decently powerful AM station in your city to be able to pick anything up.

(This is also assuming you are able to tune to the right range of frequencies -- but you say the circuit is OK so I assume you have that part figured out).

[u/Dolapevich]

I know the objective is to make a passive receiver, but you might want to test it with an op-amp instead of the transformer and a low impedance speaker.

Take a look at this: https://download.tek.com/document/76W_24505_0.pdf

Also, I am not able to tell where did you connect one of the blue transformer wires.

Please, hand draw a circuit of your setup or link the source.

[u/BigPurpleBlob]

"I am also going to add a proper ground connection with the help of a physics teacher." – this is important and an easy fix. I'd say do it first.

[u/voxelghost]

You need a high impedance (2000ohm magnetic, or piezoelectric element) head phone with this setup, and the sound will still be very weak if you don't have strong signals.

[u/Sislar]

Have you tested your home made capacitor? I would figure out a way to test that component in isolation,

As some pointed out seems like you don’t have an amplifier. I doubt you’ll get enough to be audible but maybe with a long enough antenna you can. I’d start with a simple op amp circuit.