The problem is efficiency. Conversion factor aside, a lot of power is lost when we send it over the air. We would need a huge power source to transmit over any meaningfully large distance. Transmitting data requires power on both receiver and transmitter side btw.

Saw a research some time back using laser to transmit power that can keep a specially designed lightweight hobbyist airplane continuously in the air, as long as the beam is not interrupted. The plane just flew in circles a couple of feet in the air.

1. Energy doesn't necessarily needs a medium to be transferred. E.g. light and radiowaves can travels through vacuum and vacuum-like condition in the space.

2. Like I mentioned earlier, the problem is efficiency. Say you use 1W to send out a radiowave, how much of that 1W is received by the intended receiver? 0.1W? 0.01W? or 0.001W?? Why is radiowave not a good idea? Because it is transmitted in broadcast method i.e. the energy will be spread out over a large area where most will be lost.

3. Radiowave is a form of energy. Converting this back into electricity is simple and done everyday by RF receivers. There's no such thing as carry electric on radiowave.


You have it right about electricity but got it wrong about power transfer.
1. Power transfer doesn't means we need to use electrical current. Using electricity for wireless power transfer would in fact be very stupid because we already know electricity doesn't conduct well at all over the air.

2. Electricity is a form of energy, similar to sound and radiowaves. Whether we used it to transfer data or power, that's totally dependent on the application. E.g. the 240v power cord connected to the TV, that's purely for power transfer only. Or the way we capture sunlight on solar panels to charge up a battery, in which case power is being transferred using light waves over the air.

3. Also, there's nothing that says we cannot extract power from wireless data. In fact, that's exactly what touch'n'go transceiver and many other access card does. Power transfer of this type is only over very short range and for very low power application only.

1mW continuous power can charge a battery albeit over a long time. Boost charger circuits and capacitors are the key to utilizing low voltage/low current power sources. E.g. garden solar powered light usually have a rechargeable battery for storing the power absorbed during the day, and then releasing it during night time. It only powers a small led though.
This won't work for high power equipment e.g. laptop. Nobody is going to buy it if it takes 10 hours to charge up before we can start using it, and only to use it for 5 hours max.


That's one good example of power transfer.
Definition of Power is rate at which work is performed or energy is converted.
Electrical power has a source, then some wires and an electrical load at the end.
Similarly I can have a coal fired steam generator, some insulated pipes to carry the steam and a mechanical load at the end.


So it seems we've discovered 2 aspects to TS question,
1. Just grab power wirelessly from whatever there is out there. E.g. using wifi signals, or one can even harness power from those many power transmission lines that now and then go nears a residential area. Or solar/wind/etc power.
2. The generation and transmission aspect. E.g. if a salesman were to try and sell a generic consumer wireless laptop charger, I would weigh for e.g. how fast can it charge up, how much my electrical bill would be and the effect on my health.

The iron core as Xerone pointed out is to increase it's transfer efficiency but is not a must. We can have a primary winding that coils over a secondary winding with nothing but air in between and it'll still work. The transformer example simply illustrate there's no wired connection between the primary and secondary windings, thus proving power can indeed be transferred "wirelessly".

Again power transmission doesn't necessarily needs electrical current. I've provided an example where power transfer can be done without using nor involving electricity at all in my previous post. So what's wrong with my statement?

The definition of power is rate of energy conversion (E/t), or rate of work done(W/t). wiki: Power (Physics)
P = VI is for Electrical Power, that ain't wrong.
But there's also Mechanical power: P = ( F * d ) / t, where F is force and d is displacement. Force can be simply cause by steam or wind pushing a turbine, and displacement can simply be a bucket filled with water being lifted from a well.
Says who current and voltage must be involved in power?
There's also human power, horse power, solar power, wind power, etc... With so many types of energy in this world, how can ALL energy conversion i.e. power involved current and voltage?


So you neither rebut nor accept my statement this time.
Look if you read back my first 2 posts in this topic, I've already said efficiency is the main problem with wireless power transfer. The only reason we went around a big bush is because you disagree electrical current is not necessary for power transfer.
I'm not questioning your electrical knowledge, but you said my statement is wrong so the onus is on you to provide proof to back yourself up. We've already provide 2 examples where current is not involved in at least one of the power transfer path.
In fact, this whole side-tracked discussions appears to stem from your insistence that power must involved electricity.


That's exactly how passive RF sensors (access card like touch'n'go cards). Certain radio frequencies transmit relatively better over the air e.g. 27Mhz, 455Mhz, 900Mhz. These passive access card draws power from RF carriers waves generated by the transceiver. Range is very short but it's another example of no wires power transfer.

Err.. for simplicity sake, I'd assume the 1mW is net received power after transmission and conversion losses. And 1mW is also an arbitrary number for the sake of argument.
My argument that 1mW is sufficient to charge a battery is generally based on the design of solar powered night light.
Assuming the night light LED uses 3mA at 1.2V and needs to run for 8hours per night. That means simple total power drain is just below 29mW. Hence 3mW of continuous charging power for 12 hours per day during daytime will be sufficient for it's night time operation. It's a simple calculations but suggest plausibility. You can see why I think 1mW can charge a battery, just depends on what is the load and application.


First higher frequency doesn't means greater current. Simple example, our 3G network uses 2100Mhz, our 240v AC runs at 50Hz. I'm not an expert in this area though, so that's just the simple idea.

Next consider this. Let's say you are transmitting data using some sort of RF signal from one transmitter to another receiver. Now what's to stop me from say making another receiver that also reads the same RF signal, but only converts the electrical signal I received to powers say an LED?
Both electrical signal and electrical power are made of voltage and current. Whether it's a signal or power is our interpretation and application. Of course in order to create the signal, we modulate data into the signal waveform (commonly called carrier). And usually signal lines doesn't carries much power due to well, it's not designed to transfer power after all.

No I mentioned 3mW can be use to charge a battery for very low power applications. I did not say it must comes from wifi. The question for me was merely, can 3mW or 1mW charge a battery?
I interpret 3mW or 1mW as available net power. You interpret is as before conversion losses. So it's a simple misunderstanding. That in no way means I do not understand regulator efficiency and battery self discharging.

And it's only a small pool of light yeah but I'm only pointing out a product available on the market, not arguing about the pros and cons of it. Also the example is meant to point out such low amount of power is only suitable for very low power application whereby stressing the key issue of wireless power transfer --> transmission losses, which in my mind has always been the bigger picture.

I'm a firmware programmer, which in itself is a field that covers programming and a little on electronics. I'm not strong in analog electronics though so I'm curious about the amplifier you mentioned. Since the purpose here is merely to collect power and not the signal data, is there really a need for amplifier and mixer? I'm assuming the right antenna will convert the carrier waveform into similar electrical waveform. Since it's an oscillating waveform, then theoretically we can use the voltage doubler or boost circuitry concept to boost up to the right amount of voltage and current we want right?
And if amplifier requires power to function and is required in order to extract energy from RF signals, it would be a chicken and egg story and doesn't explains why passive RF tags work.

So passive tags work and there's really no need for amplifier and mixer in order to just extract power from RF signals, glad we cleared that one.
And yes a 1W RF transmitter probably aren't practical for wireless power transfer except for very low power application and of very short distance.
So can we move along now?