I'd like to estimate the power density from a 13.56 MHz power source in a near-field WPT (wireless power transfer) scenario. Using spectrum analyzer with a receiving antenna will give the E-field measurements, while for near-field WPT the magnetic coupled resonance is the used method (non-radiation). So, I suppose I cannot use spectrum analyzer to measure what I want. I'm wondering if there is any measurement method to measure the power (in dBm) emitted from a source to be harvested by a loop antenna in near-field concept. Any advice would be highly appreciated.
If you are looking for the power to be harvested in the near-field using a loop antenna then it is not emitted power density you want to measure. Coupling to the near field will probably be dominated by inductive fields. These do not carry power unless there is an antenna there to pick up power, in which case that antenna subtly alters the near field so that the inductive field carries power to the antenna.
The way to measure the power that can be coupled is to use your loop antenna and a spectrum analyser. If your power source is simple, there may be simple equations describing the near field - see magnetic or electric dipole near fields - and the maximum coupling is simply calculated from the inductive magnetic field where the coil is and the area and number of turns of the coil.
If you are using a power amplifier as your source, the best way would be to relay on the PA (most PAs display the output power). If it is your own source circuit, you also can use an oscilloscope with a voltage probe and current probe for AC power measurement. For an accurate measurement, you may need to calibrate the probes very carefully though.
Thank you Prasad for your recommendation. Actually my source is a mobile phone. I'll check whether any app for determining the output power of a mobile phone. But according to Malcolm, what I should consider is the coupling coefficient, not the output power. I'll consider both.
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