See post #3 about power.

Also if you use a loaded coil with short antenna, much of the RF power will be spent in the coil and mismatches.

With picowatt I think you can go a few mm away...

Unless you are going to amplify the signal. Maybe a MAR-1 or more in series can help.

Look here for data: http://www.minicircuits.com/pdfs/MAR-1+.pdf

Ioannis