Hello,
Fully agree with FvM. This is a double tuned circuit. When you use a single oscillator, it is likely that the oscillator will "jump" between two frequencies because of the double tuned circuit.
Regarding 3..10 MHz and 2m distance, you will very likely exceed EMC regulations and maybe radiation safety limits (ICNIRP guidelines).
I assume that the distance is large with respect to coil diameter. You are right, using resonance in the receiver coil greatly enhances the "capture area" of the coil, hence the overall efficiency.
Before spending lots of time experimenting, I would suggest you to first calculate the power transfer between the setup that you have in mind. When you are familiar with AC circuits and transformers this can be done. Maybe you want to do some actual Q factor measurements to calculate loss resistance for in your simulation.
Regarding frequency, when you stay on the low side, you can use regular switching mosfets. I made a full bridge power amplifier at around 700 kHz where I used a 10W oscillator to inductively drive the 4 mosfets. I have doubt whether this is easy to do at 3..10 MHz with standard SMPS mosfets.
IXIS RF and Microsemi (acquired APT) ) make special HF capable (expensive) mosfets. They have several application info that may give you some idea about the way to go. Forget bipolar devices, unless you have some bipolar RF power devices at hand.
Also radio amateurs (HAM) are using mosfets in their power amplifiers at the lower end of HF, based on easy to get standard SMPS mosfets. Probably you will not get >80% efficiency, but >100W output is possible. When you go for the highest efficiency, load mismatch is a problem.
My first thoughts go to a push-pull approach (with N-chan mosfets with source connected to ground, transformer coupling, Class C to E operation).
Added after 1 hours 55 minutes:
Hello,
Without the power amplifier you can do the analysis in combination with small signal measurements (except for the rectifier efficiency).
To quess the H field at certain distance you can use:
H = I*N*A/(2*pi*dist^3).
Only valid when distance >> diameter of coil
Product of I*N*A is called "magnetic moment [Am^2]
Vtx = 2*pi*f*L*I
Dist = distance between coils facing each other
A = surface area of coil (0.25*pi*diameter^2)
I = current through coil
N = number of turns.
f = frequency
Vtx = transmitter voltage across transmitting coil
u0 = permeability of vacuum
For the reception coil:
EMF[V] = N*A*uo*H*2*pi*f
When you use same size coils (so same inductance) for transmitting and receiving, the ratio EMF/Vtx equals the flux linkage factor that you can use in the model for the coupled inductors in a PSPICE program. Now add the series loss resistances based on the assessed quality factor of the coils and capacitors, and you can start a simulation.
For the RX coil, add a series capacitor that cancels the inductive reactance and add the load resistance. When load resistance equals series loss resistance of RX coil, your efficiency will never exceed 50%.
For the TX coil, drive with a sinusoidal current source, don't add a capacitor and "measure" the voltage across it. By doing this you can ignore the detuning effect of the mutual coupling. This means when you change the distance, the receive coil will not detune in simulation
The input power you can calculate from Vrms*Irms*cos(phi). When you do a time domain simulation, just multiply V(t) and I(t) and integrate the result (low pass filter will also work, output of LPF equals real input power).
When loss in TX coil turns out to be less then 50%, you can increase the load resistor at the receiver.
As there is 1/dist^3 in the H-field, efficiency decreases rapidly with increasing distance. You will notice this as increasing the distance means that you have to reduce the coupling coefficient in the inductor coupling.
Regarding losses in the coil, you have the skin effect issue, but also a proximity issue, so use the correct formulas. If in doubt, make an inductor and determine the quality factor (for example by using resonance with a known capacitor).