only nanowatts? With an antenna, could this low level of power be drawn from radio/TV signals? It is enough to power a crystal set radio receiver.

Nanowatts are -60 dBm or what you would expect to receive on your cellphone when you have decent but not great signal. Many radios operate down to the -110 dBm/MHz PSD or lower ranges for high performance stuff. So you could use hits those power levels with energy harvesting on radio waves but only if you were in the right location, on the right band, and it would possibly cause problems for the systems that are trying to use those signals for their intended purposes.