EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Beamin on October 17, 2022, 11:54:29 pm
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I have been watching daves debunking wireless energy woo woo and made me realize I dont quite understand how if both an antenna uses AC to radiate EM radiation, but so does an inductive heater. Also how come you dont get bleeding ears from a MRI machine if thats 10k watt FM modulated?
Wasn't teslas wireless energy based on this? And why it wouldnt work now a days because it would totally inyerfere with all RF gadgets. Radio: coined by its discoverers as "a useless laboratory curiosity"
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An antenna is designed to create radiation (in other words, far away from the antenna you can still 'see' the impact of the currents flowing in the antenna), but an induction heater is not designed to create radiation, but rather couple energy into another material where eddy currents create the heating1. An inductive heater can be thought of as a transformer, with the secondary winding being a near short, so big currents and thus huge heating.
A transformer is (generally) much smaller than the wavelength of the AC signal, and as a result, in far field, the current cancels itself out and you don't get radiation.
An antenna is (generally) on the scale of the wavelength of the AC signal, and as a result, in far field, the current cannot cancel itself out and you get radiation.
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1 I guess you could say an antenna generally also aims to couple energy into another element, but the operating principle is still different (far-field vs near-field).
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Induction heating
H field near field from work coil, 5 kHz...13.5 MHz eddy current in work heats, eg metals,
E field within capacitive electrodes, dielectric loss in work, eg plywood
See RCA, General Electric old application notes and books
Antenna...Tesla....Colorado Springs 1880s..1900 used 20..50 kHz at megavolt levels, 20..50 kW input to light a few 1/2w lamps nearby.
Since radio waves are radiated in all directions, the spherical angle intercepted and distance from TX..RX cause very high losses and very low efficiency.
See lod.org
Any text on electromagnetic theory explains the difference between electromagnetic waves and E, H, and near field vs far field.
Jon
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Radio waves are electromagnetic radiation, meaning you need both the electric and magnetic field acting together.
If you make one of those fields the other gets created from it, however it has to be at least in the ballpark of the wavelength for that to happen to any reasonable degree, hence why an antenna size is dictated by wavelength. Induction cookers are too small to be an antenna at the low frequencies they operate at.
As for why a MRI machine doesn't cook you like a chicken in a microwave is because you need to actually interact with the radio waves for them to do anything. Microwave ovens are specifically designed to get as much interaction between the radio waves and food as possible. They use 2.4GHz waves because that wavelength is particularly well absorbed by water (that pretty much all food contains) and they have a metal cavity that is specifically tuned to resonate at that frequency, that way the 90% of radio waves that didn't get absorbed in the food come bouncing back for another go, and another and another...etc.
Resonance is the magic sauce that can force energy across in scenarios where it doesn't really want to go across that willingly. It is the working principle behind teslas wireless energy transfer and is the exact same idea that is used to wirelessly charge mobile phones today. It is just two coupled LC circuitry exchanging energy.
But none of this solves the fundamental problem of wireless energy transfer... distance. Any form of energy, radiation, field etc... loves to spread out into the available space. So when your wireless energy transmitter and receiver are close together (such as wireless phone chargers) you can capture most of it before it has a chance to spread out so you recover most of the energy. However when you spread them apart to any reasonable distance you can't capture so much f the energy anymore, so most of it ends up radiated out to the environment while your energy receiver gets only a small fraction of it.
This massive loss of energy is fine when you are using radio waves for communication since you can transmit with 1kW and then build a receiver so sensitive that it can detect 1 nanoWatt. That way it works even tho 99.99999% of energy is lost. However once you get to wireless power transfer loosing 99% of your energy makes it useless. You can't afford to be constantly transmitting 1000W of power just so that your phone can pick up 10W of it.
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I have been watching daves debunking wireless energy woo woo and made me realize I dont quite understand how if both an antenna uses AC to radiate EM radiation, but so does an inductive heater.
No. Antenna and inductive heater are based on different principles.
Inductive heater uses near field inductive coupling in order to transfer energy. While antenna using far field plain wave in order to transfer energy. Antenna also has magnetic field and can be used as inductive heater if you use at very close distance, but antenna goal is to emit electromagnetic plain waves in the far field, while inductive heater goal is to avoid emission of electromagnetic plain wave in the far field. The difference between them is a near field structure which depends on geometry.
Antenna has magnetic flux in their near field as a side effect, it is unwanted but hard to avoid. Induction heater is designed to produce magnetic flux in the near field intentionally.
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Microwave ovens are specifically designed to get as much interaction between the radio waves and food as possible. They use 2.4GHz waves because that wavelength is particularly well absorbed by water
This is actually not true - there is nothing special about the behaviour of water at 2.4 GHz that is used for microwaves - the loss tangent of water even gets higher for lower frequencies down to a few MHz or even KHz - this is why submarines can't use radios. When you look at industrial microwave ovens they can also use lower frequencies (I think 915 MHz is another common frequency for industrial uses).
Don't quote me on the following, but I believe that 2.4 GHz is used in microwave ovens because it simply happened to be a nice size of the magnetron back when they were commercialized. The fact that the first microwave ovens for consumer use worked at 2.4 GHz is why we have an ISM band at 2.4 GHz - nobody wanted that spectrum anyways.
As to why MRIs don't fry your brain: They don't use continuous waves. They pulse (which is the source of the banging sound some MRI machines make) at quite low frequency. As a result, while the instantaneous field strength might be very high, the actual transferred power is very low (you are only rotating the molecules a few times a second or less, compared to the 2.4^9 that you find in microwave ovens).
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Wasn't teslas wireless energy based on this?
Energy is always wireless, because it cannot flow within wire. When you transfer some electric power through wires, the energy flows in the space between wires, usually in the wire insulator or in the air between wires. Even if you look at capacitor, energy is stored in the insulator near the capacitor plates. So, there is no sense to talking about "wireless energy", it's the same like talking about "wet water" :)
And why it wouldnt work now a days because it would totally inyerfere with all RF gadgets
Transferring energy as RF waves is not a good idea. Because it will affect all electronics, especially wireless communication devices, radio receivers and other things. In addition, it is not safe for your health to stay near high power transmitter. And after all it has low efficiency, because a lot of energy will be lost due to emission into environment and heating things around you.
The example of using RF wave to transfer energy is a high power laser beam. As you understand it is very dangerous to stay near such device, if that beam hits your body your body will get damage. Exactly the same thing happens when you try to transfer energy as RF wave.
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38 years in the high power side of induction heating and I only have one anecdote about far field interference from it.
It was told to me as it occurred before my time, and I have forgotten the details and frequencies.
Destructive interference started on a HF aviation system in eastern USA.
With some DF, the USA authorities contacted the Australian aviation authority, and after searching the source was isolated to an induction heater in Sydney.
It was a 50 kW Triode Colpitts, and the cause was not directly from the induction magnetic field, it was from a parasitic oscillation that had developed on the anode.
18,000 km, maybe a world record for QRM?
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Microwave ovens are specifically designed to get as much interaction between the radio waves and food as possible. They use 2.4GHz waves because that wavelength is particularly well absorbed by water
This is actually not true - there is nothing special about the behaviour of water at 2.4 GHz that is used for microwaves - the loss tangent of water even gets higher for lower frequencies down to a few MHz or even KHz - this is why submarines can't use radios. When you look at industrial microwave ovens they can also use lower frequencies (I think 915 MHz is another common frequency for industrial uses).
Don't quote me on the following, but I believe that 2.4 GHz is used in microwave ovens because it simply happened to be a nice size of the magnetron back when they were commercialized. The fact that the first microwave ovens for consumer use worked at 2.4 GHz is why we have an ISM band at 2.4 GHz - nobody wanted that spectrum anyways.
As to why MRIs don't fry your brain: They don't use continuous waves. They pulse (which is the source of the banging sound some MRI machines make) at quite low frequency. As a result, while the instantaneous field strength might be very high, the actual transferred power is very low (you are only rotating the molecules a few times a second or less, compared to the 2.4^9 that you find in microwave ovens).
Medical MRI does not use radio waves: the body is inside a coil or other resonant structure that is pulsed with current at a radio frequency (typically 64 MHz for 1.5 T static magnetic field) to generate a pulsed oscillating magnetic field inside the patient to measure the nuclear magnetic resonance.
There is also a pulsed "gradient" field, superimposed on the "static" DC magnetic field, that varies the resonant frequency (proportional to the local magnetic field) in a controlled manner as a function of location to "encode" the signal as a function of position to do the imaging.
The reaction between this pulsed gradient field and the static magnetic field stresses the coil structure to produce the audible noise.
Regulatory safety limits include limiting the "SAR" (specific absorption rate in W/kg) of the RF field and limiting the induced electric field along the patient from the pulsed gradient field.
"Line", "audio", and "RF" are examples of frequency ranges, and do not necessarily imply radiation.
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I have been watching daves debunking wireless energy woo woo and made me realize I dont quite understand how if both an antenna uses AC to radiate EM radiation, but so does an inductive heater. Also how come you dont get bleeding ears from a MRI machine if thats 10k watt FM modulated?
Wasn't teslas wireless energy based on this? And why it wouldnt work now a days because it would totally inyerfere with all RF gadgets. Radio: coined by its discoverers as "a useless laboratory curiosity"
Hello,
I thought i already replied to this but the post is gone (again).
What i was saying was simply that any time you get a change in the field you get radiation, even if it is just a small amount. This is evident from Maxwell's fourth equation which is Ampere's Law with Maxwell's addition. It is usually given as a change in the electric field.
Since an electromagnetic field changes with current, anytime you get a change in current you get radiation. Even the 50Hz or 60Hz power line radiates energy because the current is always changing, and you can pick that up in audio or on a scope. It's not a huge amount but it is always there. The only thing that does not radiate anything is pure DC current. You get a static magnetic field that's it.
With an antenna, the field will radiate out at some set of angles depending on the construction of the antenna. There is nothing to block that usually or ideally.
With an induction cooker, the radiation is mostly a changing magnetic field and that induces a current in the nearby metal and that causes power heating. The metal absorbs a lot of the energy but not all so some could radiate outward.
Since an intense field can interact with almost anything, it is hard to project a beam of energy without making sure it does not hit anything unwanted. Thus the only way i could see the transfer of somewhat high amounts of energy wirelessly is with a very large number of directional transmitters that are spread out so that any individual beam can not hurt anything unwanted, but with a large number of correctly placed receivers the total energy received would be high. For example, a million 1 watt transmitters and a million receivers, the total power transmitted would be a million watts but it would be spread out over a large area so no one small area sees the entire amount of power.
I believe this method is used in the medical field to penetrate the body with an intense field without affecting the skin. The energy is concentrated only at the focal point inside of the body and not on the skin.
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The only thing that does not radiate anything is pure DC current. You get a static magnetic field that's it.
When someone moving past the wire with DC current, he can see changing magnetic and electromagnetic fields :)
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The only thing that does not radiate anything is pure DC current. You get a static magnetic field that's it.
When someone moving past the wire with DC current, he can see changing magnetic and electromagnetic fields :)
Hello,
Yes but then there is still a dE/dt which means the field relative to the observer is no longer static.
Compared to an AC current which is always changing. Maybe we could move in sync with the AC field and change that too. Physical movement adds a new dimension to the problem.
In the Maxwell equation it doesnt matter how the E field changes as long as it changes.
Obviously the generator effect is an example of a change in distance with a static field.
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Microwave ovens’ operation (https://en.wikipedia.org/wiki/Dielectric_heating) is described on Wikipedia, including early attempts at 10MHz.
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Energy is always wireless, because it cannot flow within wire.
If this were true, wires would not get hot when you pass electric current through them. The heat is generated inside the wire (especially with DC), and by energy balance that heat must have resulted from the conversion of a corresponding amount of electrical energy. If the heat source is inside the wire, the electrical energy must also be inside the wire.
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If this were true, wires would not get hot when you pass electric current through them.
wires are get hot because electric field around wire pushes free electrons in the wire metal. Some pushed electron colliding with atoms and it leads to temperature grow. But these electrons speed is much-much slower than energy flow speed around wires...
For comparison, average electrons flow speed in the wire is about 0.1 mm per second.
While average energy flow speed around wire is about 200000000 meters per second.
If the heat source is inside the wire, the electrical energy must also be inside the wire.
Heat source is electron which colliding with atom. But energy is carried by electromagnetic field around wire. Electromagnetic field cannot penetrate into conductor, so the energy is carried primary outside wire.
You can simply test that wire circuit can carry electric energy much-much faster than average electrons flow speed in the wire.
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Electromagnetic field H [Amp/metre] does penetrate into the conductor pair of the transmission line.
Current density J {Amp/metre_sq] only exists because the conductors have electrical conductivity.
The simplest cases eg a pcb conductor transmission line having width and thickness spaced above a ground plane and constant properties can be solved manually.
That model can be reduced one dimension (y) with two boundary points Y1 and Y2 mirrored so only a half solution is needed and the ground plane is mirror.
The diffusion equation solves for J and H.
In one dimension, diffusion equation reduces to a 2nd order ordinary differential equation which has tabulated analytical forms of solutions.
The current density in the conductor has magnitude and phase and is of the form J/J_surface = cosh [ (1+j) y/delta ) / cosh [ (1+j) b/delta )
The magnetic field in the conductor has magnitude and phase and is of the form H/H_surface = sinh [ (1+j) y/delta ) / sinhh [ (1+j) b/delta )
where delta is the penetration depth "skin effect" and b is thickness of conductors
Usually all this is done numerically, with the user drawing the model and assigning boundary conditions.
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Electromagnetic field H [Amp/metre] does penetrate into the conductor pair of the transmission line.
Current density J {Amp/metre_sq] only exists because the conductors have electrical conductivity.
But electric current (electrons flow) cannot carry energy, energy is carried by electromagnetic field. Electrons movement is a result of interaction between electrons and electric field around wire. So, energy is carried by electric field...
If you remove electric field, electrons will be stopped and there will be no current... :)
It's unusual, but the same thing happens with water in the pipe. The water particles doesn't carry energy, the energy is carried by electromagnetic field between water particles...
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In an x-ray tube or electron linear accelerator, there is a flow of electrons from the cathode through an accelerating field to the anode.
The electrons hit the anode, and their kinetic energy is converted to heating the anode, except for a small percentage that produces useful x-ray radiation.
Yes, the flow of electrons carries energy from cathode to anode, and that energy equals the accelerating potential times the electron current.
If you shut off the heating current to the cathode, while maintaining the accelerating field, the current stops and there is no energy transfer to the anode.
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Electromagnetic field H [Amp/metre] does penetrate into the conductor pair of the transmission line.
Current density J {Amp/metre_sq] only exists because the conductors have electrical conductivity.
But electric current (electrons flow) cannot carry energy, energy is carried by electromagnetic field. Electrons movement is a result of interaction between electrons and electric field around wire. So, energy is carried by electric field...
If you remove electric field, electrons will be stopped and there will be no current... :)
It's unusual, but the same thing happens with water in the pipe. The water particles doesn't carry energy, the energy is carried by electromagnetic field between water particles...
Hi RadioL,
Post #15 was not my speculation, it is a solution using Ampere's Law and Faraday's Law .
Those laws are bound into the overall set of Maxwell's Equations.
The results are applied in all sorts of devices in electrical engineering, moreover can be demonstrated physically.
For you queries, the results show that dJ/dx is accompanied by dH/dx, and the effect is reversible.
So in a man-made transmission line ( be it busbars, pcb tracks, 450 Ohm twin or 50 Ohm coax, or waveguide,)
the H field exists in both of the conductors and the interposing "non" conductive medium.
The solutions in post 15 show the distributions in the conductors.
Generally for the overall thread, here is a reference:
http://www.dannex.se/theory/3.html (http://www.dannex.se/theory/3.html)
The "Near Field and Far Field" topic has some relevant graphs for delineation of evanescent and far field modes.