High power, high Q, variable frequency oscillator design

[Vasiliy]

Hi everybody.
We (me and a friend of mine) are going to play around with wireless energy transfer using two strongly coupled inductors. The coupling is achieved by designing the inductors having the same resonant frequencies, and as high Q as possible. The transmitter's diameter is ~0.5m while that of the receiver is only about 5cm. After performing some very basic calculations we concluded that we won't be able to get the efficiency of more than a few percent, therefore we will have to compensate for this with high input power on the transmitter.

The specifications for the oscillator on transmitter's end:
1) Variable frequency: 10-50MHz
2) High power operation: ~40W
3) Low output resistance
4) Input voltage: up to 48V

The questions:
1) Can all these specifications be met altogether? If not, which specifications should be relaxed?
2) What configuration of oscillator should be used?
3) Should we try to design high power oscillator or design a regular oscillator and use a power amplifier?

We suspect that this kind of design is uncommon, therefore, even if you don't have a definite answer, we will be glad to hear various considerations and dilemmas that you can think of in conjunction with this kind of design.
Our background: students for electrical engineering. We have designed some simple oscillators and pll's using SPICE, but never actually built anything.
Thank you

[Fraser]

With 40W in the HF bands you need to consider a transmitting licence and interference to others.

You can resonate a loop antenna at HF quite easily and the Q may be high but that makes loop tuning very critical indeed. The oscillator source may be a simple VCO module from Mini-Circuits. These are both cheap and easily used. The PA stage will not be anything special and may be taken from amateur radio information sources.

The energy efficiency of your power transfer will be very poor indeed, but you already know that. Also consider the effect of the strong RF field on the device being powered...RF will do nasty things to electronics at the levels of power you are considering.

All tests in RF energy transmission that I have been involved with, used VHF or UHF with very high gain Yagi antennas at TX and RX ends. Power transfer efficiency was chronic due to path losses. You also wouldn't wish to stand in the beam of the TX Yaggis if high powers were in use.  We ended up with a stack of eight 24 element UHF Yagis at TX and RX ends of the test....not exactly portable.

[wkb]

At least run your experiments based at an ISM frequency.  See http://en.wikipedia.org/wiki/ISM_band

When I was repairing (so with the PCB removed from the metal enclosure) my Metcal soldering station which runs at 13.56MHz a lot of electronics on my bench got quite confused.  Like for example the temperature control circuit of my Weller WECP soldering station.

SO..  thread with caution I'd say.  It could very well be illegal, like Aurora already noted, to transmit this kind of power without licence.

[Ajahn Lambda]

These guys have it right.

There are so very many "gotchas" in this, I don't really know where to begin.  I suppose with distance, because that will be your greatest enemy:  get it as low as absolutely possible, and observe the inverse square law.  This is probably where your 'efficiency' is being 'lost'.  The second challenge is coupling, but I'll let you find out about that on your own.  Entire books and courses exist to cover all of the EMC aspects at which you're going to be looking.

More power isn't always better (bah, I'm a gear head and I'm saying that?!?    ), particularly in the frequencies you've suggested,  but mostly for a reason which Aurora and wkb have already pointed out:  it is more than likely illegal.  Anything more than a handful of watts, maybe less, and certain people will be knocking on your door.  Hopefully before anything harmful has occurred.  The only exceptions are ISM bands (which even then have certain rules), and/or if you're doing your experiments in a specially-designed EMC laboratory which will suppress RFI.  On the surface, what you're proposing is essentially a VHF broadcast transmitter.  There are so many radio communications on this range of frequencies that you'd likely cause major problems for anyone within the broadcast area - several dozen miles for a 40W transmitter and proper aerial.

When I was an EE student, several projects similar to this one were undertaken as group lab projects or even corporate-sponsored term projects.  Virtually none succeeded -- more than anything it was a learning experience, and perhaps that was the only product of such endeavors.  A result is a result, no such thing as failure, etc.

[Vasiliy]

The issue of interference and licensing should be checked. The main point is that for our application it may be possible to use some sort of Faraday's cage because the final product is aimed to be used under ground (in some sort of a cave). The main problem in this aspect is finding a place to perform the experiments, before the final installation. Thank you for pointing this out.

Aurora suggested to use a regular VCO, in series with power amplification stage at the output. Can you suggest what configuration will be best for our case? In our opinion, the main requirements from this stage are:
1) The transmitter is very high Q inductor, therefore the impedance matching is critical (do we get this right?). Therefore, the PA stage should have as low output impedance as possible.
2) It should have low total harmonic distortion in order to minimize the losses.

Thank you for you replies

[wkb]

You might want to have a look at this one http://www.freepatentsonline.com/4626767.html

[Vasiliy]

Hi everybody.

We have made some research, and are ready to begin designing and building our system (not really ready, but we have to start some time).

As a preliminary practice, we would like to build a scaled version of the system: relatively small transmitter at lower power and frequency.
The specs are:
Freq - 1MHz
Output power - ~8W
Voltage supply - 12 V

The transmitting antenna is 5 Ohm and 50 mH.

For preliminary project we decided to use a function generator as an oscillator (therefore 1MHz) and a lab voltage supply.
The output stage will be class E power amp, based on RF power MOSFET. We realize that using this transistor is an overkill, but we prefer to minimize chances for every possible future problem related to amplifier's main transistor resistance/capacitance/linearity/etc.

Some of our concerns:
1) If we find out that function generator current capability isn't enough to drive the power MOS, what is the easiest way of overcoming this?
2) The laboratory we are working at has really good equipment, but, just to be sure - is there a possibility of damaging the voltage supply (it can go up to 35V, 3A) while feeding the power amp?
3) Is frequency selective network the same as impedance matching network? In class E, we are going to use large coupling capacitor to the output, and install a frequency selective network in order to achieve a resonance at required frequency (after the capacitor). If we test the circuit and find out that its resonance is at required 1MHz, does it mean that the impedances match? If no, what is the criterion for matching (it seems problematic to measure VSWR)?

Any additional tip/advice will be more than welcome.
Thanks

[amspire]

You are making a 8W transmitter that operates in the middle of the AM broadcast band. Is that legal?

[Vasiliy]

As long as you prevent the radiation from getting out of your lab, it's perfectly legal (or, at least, no one will know that you're criminal )