Below are detailed tips and tricks for getting your crystal radio to work.
As well, here's a video that also goes through the crystal radio tips and tricks.
A crystal radio's antenna should be as long as possible. A car antenna won't work. I've managed to get a wire around 12 feet/4 meters long to work, but I live near a few radio stations. In general, the farther away your radio stations are, the longer the antenna has to be.
The antenna should be straight, though you'll likely have some near the crystal radio itself which isn't lined up with the rest of it.
Pointing it at a radio station gives best results for picking up that radio station, but that isn't always possible and so it isn't likely you'll be doing that.
The antenna can be any type or size of wire, but it should be electrically insulated from other metal or other electrically conductive things (e.g. water).
I've succeeded in putting multiple same-length wires in parallel and connected together only at the crystal radio. It seems to help as long as the wires are separated by at least around 4 inches/10cm.
A ground is a thing that is a good source or sink of electrons i.e. it gives electrons freely and also takes them if you want to give it electrons instead. Some good grounds are as follows.
Grounds - The Earth
The Earth is a good ground, but you have to make good electrical contact with it in order to use it. Ground that is dry is not a good electrical conductor and so it's hard to make good electrical contact with it. Wet or damp soil is better since water makes it a good conductor. A temporary way of improving the conductivity of the Earth ground is to pour water into it.
Connecting to the Earth ground can be done in a variety of ways:
Ground - Your body as an okay ground
Your body is also an okay ground and will often help if you have no other ground around. To use it as a ground you want to make contact with as much of your body as possible. In the photo below I'm using my portable crystal radio and you'll notice that my left hand is on the aluminum foil plate which makes up the outer cylinder of my capacitor. That plate of the capacitor is the one that would normally be connected to an Earth ground if one was available. I'm making as much skin contact as possible with it.
Go over every place where you've connected a wire to something or two wires together or two metal objects that should be touching and make sure they are in fact touching and are doing so firmly, not loosely.
Some of these places may not be obvious. For example, if you have a tuning coil as shown in the photo below then the tops of the tuning coil have had their enanmel insulation removed from them and the underside of the tuning bar should be making contact with one or maybe two turns of wire. It's easy to think that it is but in reality it might not be making good contact.
Also, whatever piece of metal is being used as the tuning bar may have a coating of some sort that acts as an electrical insulator, preventing electricity from flowing to/from it. Make sure to scrape any coating off before connecting anything to it, and that includes where it makes contact with the coil.
The doide you use must be a germanium diode and not a silicon diode. Germanium diodes are harder to find but I've bought them from local electronics stores and by ordering them online. There are many places to get them online.
Some examples of diodes that work are: 1N34, 1N34A, 1N60, 1N91, 1N119, 1N270, 1N277, 1N283. You can also look for what are called "equivalent" diodes to any of these.
Some online sources are:
For most crystal radio circuits you need to use crystal earpieces, also known as piezo earpieces
You can buy these online. Here are some sample sources:
You can also make some that work fairly well. This page shows you how to make your own crystal earpieces.
You can use ipod style earbuds as well but you'll have to modify the crystal radio a bit. I show you how to use earbuds here.