One of the biggest problems with crystal radios is their lack of portability. The long line antennas and earth ground requirement is always a problem. One good way around this issue is to use a loop antenna as shown below:
This extremely simple design works surprisingly well. I can routinely hear Radio Romania and China Radio International broadcast at 6Mhz from Cuba two thousand miles away.
Admittedly the selectivity could be better so I am going to try an antenna tuner to try to get better signal isolation.
If you want to hear the standard North American AM band just add more turns to the antenna to lower the reception frequency.