vfone said:Assuming 10dB path loss and full transmitting power, you can get less than 200mW near the antenna.
According to Transmitting Power vs Time distribution, GSM (as any other cellular systems) is transmitting only a small period of time at full power.
vfone said:This is 200mW peak power. If you want to rectify this that means you have to look for average power, in GSM case divided by 8 (1 burst).
So at full power you get 25mW. You can use a simple zero bias diode detector for this, and for maximum efficiency have to match it with the antenna.