How is the " radiation signal" in the air converts to the voltage or current signal along the antenna? ...
Assume an antenna stretched along the electric field of the signal. For simplicity, consider the length to be lambda/4. In that case, when the peak of the electric field (consider a sine wave) is at one end, the other end is at a zero field.
Electric field is associated with a potential; thus the two ends of the antenna will see a potential difference.
Can this antenna extract much energy from the EM field? The answer is: it depends.
The LC circuit attached to the antenna has a characteristic frequency of resonance. At that frequency, the circuit can extract maximum energy from the EM radiation field.
The signal strength must be more than the noise present in the antenna. The noise figure is frequency dependent and the antenna can be tuned by changing LC values to different frequencies.
Remember that all diodes have a forward drop and a reverse leakage; they will not see signal below a certain level (strength).
For energy harvesting, you will need a large antenna that covers a large part of the electric field. Basically each antenna corresponds to a voltage source but above the ambient noise level.