A loop antenna crystal radio is one where the coil is a large loop and doubles as the antenna. It can be made portable because it has no long, horizontal antenna and no ground connection, though you can add either for better results.
The following is my loop antenna crystal radio in an extra large pizza box.
Below is the circuit diagram.
For this circuit, the diode has to be a germanium diode, in my case 1N34. The crystal earpiece was made by gluing a plastic tube to the opening in a piezo buzzer bought from Radio Shack. See here for details abouce crystal earphones/earpieces. There are 14 turns to the coil. They're looped around pieces of cardboard at each corner (see photo below.)
How to tune it
To tune the radio you rotate it, pointing the face of the box/coil at a radio station. If you don't hear anything then adjust the tuning capacitor in small steps until you do. If you still don't hear anything then rotate the radio again and adjust the capacitor again. Repeat until you hear something.
If you're still having trouble, try putting one hand on one of the aluminum foil capacitor plates/cylinders. It will act as a ground and improve reception.
It could be that you're receiving something but it's too quiet to hear. Try attaching an amplifier like I do here with my homemade amplifier to amplify the sound.
Video or how to make and use the portable loop antenna crystal radio
The following video shows step-by-step how to make this loop antenna crystal radio and then how to tune it to find a radio station.