Wireless power is usually based on a narrow bandwidth (high Q) resonance at 6.78 MHz. The matching network tuning is very important, as is the oscillator frequency accuracy. However, reasonable efficiency should be achieved for +-0.05 MHz frequency offset, and modern crystal oscillators (e.g. from Murata) are very accurate at low frequency. More important is the detuning effect when the transmitter and receiver coils couple inductively, changing the resonant frequency. This can be solved by dynamic tuning.

 Than you for your answer, Tim.

Is this about near-field wireless power transfer? If yes, I think I haven't make my question clear enough.

What I consider is wireless power transfer with far-field radiative properties of electromagnetic (EM) wave, where radiation from current cellular networks or WLAN can be harvested by low-power nodes. e.g. sensors or even cellphones.    Traditional information transmission may suffer from transmit oscillator jitters, e.g. carrier offset. How is the effect of carrier offset in this situation ? Thank you.

We have been doing several projects on this in the past. Yes, if you go with resonance based methods (although it is more efficient), it will be affected by carrier offsets. One method we were experimenting was to track the carrier and adapt the resonator to capture maximum power. The other method is to use multiband resonator circuits and caputure the energy. best of luck

Thanks very much, Dr. Wijayakulasooriya.  That is very interesting.  For resonance based methods,  are there relating materials or published papers on both adaptive resonator and multiband resonator circuits?   Thank you.

For a long distance (RF/microwave) energy transfer, one may want to consider 2 factors:

1) You are unlikely to use coils, which are usually better suited for nearfield, and would more likely go for a more traditional antenna. An antenna which is to receive the electromagnetic waves will need to have appropriate impedance matching, but also good gain and appropriate orientation will define how much you can receive "from the air". Typically, it is easier to have high gain with narrow-band antennas.

In case if you want to received "from everywhere" and need wide bandwidth, the gain becomes more challenging to achieve (somewhat similarly to the gain-bandwidth product limit in amplifiers). 

The processes should follow normal antenna selection or design steps.

2) The microwave part and conversion to DC. This is well introduced in, for example,

C.R. Valenta, G.D. Durgin, Harwesting Wireless Power, IEEE Microwave Magazine, Vol. 15, No 4, Jun 2014.

Thank you, Albert.   In 1), you mentioned the  gain-bandwidth balance in the wideband antenna design. Are there any reference about this. Thank you again.

Dear Vasile. Thank you for sharing.  I agree with your idea of the possibility of ' harvesting more == increasing the electrosmog more'.  Maybe more regulations are needed.  

You also mentioned that the jitter does not count  when harvesting energy from a GSM node based on rectifying antenna and local storage. Is that because the jitter only changes the phase of the energy signal, but the amplitude remains the same? Thank you.