What is the difference between an oscillating magnetic field, and an EM wave?

[chippytea93]

In EEEVblogs latest video () he makes a point that RFID chips in credit cards are not RF antennas, they are transformers.
But aren't these things essentially the same? In a transformer, an oscillating electric field on the primary coil (AC power source) causes an oscillating magnetic field inside the iron core (or in the case of RFID - the air), and this in turn induces an oscillating electric field inside the secondary coil resulting in a voltage across it, and if a load is connected to the secondary coil a current can flow.
But isn't an oscillating magnetic field always accompanied by a oscillating electric field, therefore it is an EM wave? So in the core, or in the air, the oscillating magnetic field has an electric field component?
If it doesn't, why?
Why does an antenna produce an EM wave, but a transformer just produces a changing magnetic field?
A magnetic field propagates at a finite speed, and so a changing magnetic field would propagates through space with different values at different points in space and time, and is therefore a wave, so why is it not an EM wave when it propagates through the core of a transformer or the air as in the case of RFID?

[rfeecs]

I agree his wording was lacking.

There's a reason they call it NFC:  Near Field Communication:
https://en.wikipedia.org/wiki/Near_and_far_field
https://en.wikipedia.org/wiki/Near_field_communication

It is also possible to have a varying EM field that doesn't propagate.  A waveguide beyond cut-off is an example:
https://en.wikipedia.org/wiki/Evanescent_field

[retrolefty]

I agree his wording was lacking.

There's a reason they call it NFC:  Near Field Communication:
https://en.wikipedia.org/wiki/Near_and_far_field
https://en.wikipedia.org/wiki/Near_field_communication

It is also possible to have a varying EM field that doesn't propagate.  A waveguide beyond cut-off is an example:
https://en.wikipedia.org/wiki/Evanescent_field

 I agree also. This is simply RF transmission & reception despite his insistence "that this is not RF". What propagation mode is being used is what he probably meant to explain.

[IanB]

It depends what level of explanation Dave was aiming at. If you are intending to explain things in simple terms then there is no need to cloud the issue with details. Simply stated, the mode of operation of a transformer is different from the mode of operation of an transmitting/receiving antenna pair.

With a transformer there is direct magnetic coupling between two or more coils. With an antenna there is wave propagation through space.

If a child asks you why the sky is blue, do you give them a physics lecture?

[uncle_bob]

Hi

Antennas have both near and far fields. By convention, it's treated as a *propagating* EM wave in the far field. In a lot of cases the qualifier is left out because it is "obvious to the casual observer".

Bob

[rfeecs]

I think Dave's off-the-cuff explanation that it is best to look at this as a transformer is fine.

I'm just quibbling with the spur-of-the-moment wording:

"But the important thing to note here is this is not an RF system.  These are not antennas.  This is a transformer.  It works on magnetic fields instead of an RF field."

You can see how a small percentage of people could say "Huh?? But.."  leading to the questions that ended up posted here.

[Brumby]

It depends what level of explanation Dave was aiming at. If you are intending to explain things in simple terms then there is no need to cloud the issue with details. Simply stated, the mode of operation of a transformer is different from the mode of operation of an transmitting/receiving antenna pair.

With a transformer there is direct magnetic coupling between two or more coils. With an antenna there is wave propagation through space.

If a child asks you why the sky is blue, do you give them a physics lecture?

 

[damn_dirty_ape]

Like others have pointed out, a lot can get lost in translation when simplifying explanations. 

The overarching point is that near-field communication uses a sort of coupling effect which is distinct from traditional RF communications

[T3sl4co1l]

"RF" is not a fixed definition.

RF is a matter of perspective.

Typically, one calls a circuit "RF" when many reactances (and/or transmission line effects) must be controlled to get the desired response.  Often the circuit has attributes like narrow bandwidth, fixed operating frequency, constant (or a nominal range of) impedances on relevant nodes, things like that.

An "RF" circuit might be a 60Hz induction heater (in which case, the reactances are largely a matter of power factor correction for what's effectively a particularly inductive motor), or a ~1MHz AM radio (or you might argue an SDR for the same range, ~10MSps direct conversion, basically an oscilloscope, isn't RF), or a 200MHz wideband amplifier (or not), or a 80GHz monolithic distributed amplifier, or...

In this case, it's more correct to call it a poorly coupled transformer, because there is little to no evidence of E&M delay or transmission line effects: in the near field, the impedance is low, the magnetic field dominates, and the speed of light is fast enough not to care.

But one mustn't loose sight of these things.  It may act like a transformer, but that transformer has considerable self-inductance, which draws far more current than the signal.  Sensing that current directly will give you quite a poor SNR.  Canceling it with a capacitor instantly converts the circuit to an RF one, where there's a center frequency (Fo = 1 / (2*pi*sqrt(L*C))), bandwidth (BW = Fo * R / Zo, for series resonant), nominal impedance (Zo = sqrt(L/C)), and so on.

And as you can see, RF is a useful framework.  You can instantly recite simple formulas, and apply them to the circuit.  Transferring transmitter power to the coil, and recording the signal return, is easily accomplished with standard building blocks.

Tim

[Brumby]

If you really want to get pedantic, you could have an acoustic (physical displacement) wave operating at "RF".


Conventions often take on attributes that are not essentially linked to the fundamental concept.

[IconicPCB]

Does the fact that RF involve electro magnetic radiation come into reckoning.. after all it implies an orthogonal electric  and a magnetic field.

so without the electric field .. no RF radiation

[T3sl4co1l]

Does the fact that RF involve electro magnetic radiation come into reckoning.. after all it implies an orthogonal electric  and a magnetic field.

so without the electric field .. no RF radiation

This is essentially what "near field" is about: in the far field, all the, well, near fields have died out, and E and M are perpendicular and proportional (the proportionality being |E| / |H| = Zo ~= 377 ohms).  In the near field, E and H can be largely independent, differing greatly in magnitude and direction.

This is why I referred to impedance: there is such a concept in the near field, that the impedance of the fields can be something other than Zo.  High impedances correspond to capacitance and dominant E fields, low impedances to inductance and H fields.

It's not fair to say "no" radiation, but the farther the field impedance is off from Zo, the less radiating field is coupled.  This basically gives Chu's antenna tradeoff (see http://www.antennasys.com/antennasys-blog/2011/7/30/the-antenna-tradeoff-triangle.html ), because very mismatched fields are characteristic of compact resonators, while fields already near Zo are characteristic of large antennas that couple easily (low Q).

And of course you can always do worse, say by putting a resonator inside a metal shielding box.  It'll have resistance, but practically none of it will couple to outside radiating fields.

Tim

[Ammar]

You are mixing up E and H fields with propagating EM waves. If you really want to know what it is all about you can try "Fundamentals of electromagnetics with engineering applications" by Wentworth. Unless anyone else knows a better textbook.

[EEVblog]

It depends what level of explanation Dave was aiming at.

I explained this in another thread:

https://www.eevblog.com/forum/blog/eevblog-889-credit-card-rfid-theft-protection-tested/msg960305/#msg960305

Quote:

Quote from: retrolefty on June 12, 2016, 09:25:53 PM
So your intent was good but instead of properly explaining about the two fields comprising an RF signal, E and H,
you went the direction you did of stating that RF and magnetic fields are different things.

I didn't want to spend any time in this video explaining the details, apart from the fact that I thought it was important to offhandedly mention it's a transformer and not an antenna as a beginner might have thought it was.
Heck, you can spend several semesters at uni trying to understand near and far field theory etc.
To think I'd be able to explain something like that in a few minutes (or even 10 minutes) in a video not focused on that is just completely impractical, it would have confused beginners more than helped them I think.

Try designing a transformer using just basic antenna theory. Or vice-versa design an antenna using just basic transformer theory.
Near field and far field matters.

[EEVblog]

I think Dave's off-the-cuff explanation that it is best to look at this as a transformer is fine.
I'm just quibbling with the spur-of-the-moment wording:
"But the important thing to note here is this is not an RF system.  These are not antennas.  This is a transformer.  It works on magnetic fields instead of an RF field."
You can see how a small percentage of people could say "Huh?? But.."  leading to the questions that ended up posted here.

Sure, but I'm not concerned about the 1% who know and care about this stuff deeply enough to quibble over how I worded something off-the-cuff.
I was concerned about the 99% who might see the traces and think it's an "antenna", when in practice it's the magnetic field that is going to dominate and is better thought of and analysed as a (pretty lousy) transformer. I thought it was an important and interesting enough point to mention that.
I wasn't about to go back and re-shoot that to clarify for the 1% who already know it, it's not worth it. Nor is it worth it to try and fix such a complex explaination with a text overlay in editing.

You can bet your arse that if I had called it an antenna and said it "transmits RF" I would have had everyone going  instead of 1%.
And if I had delved into near and far field theory I would have lost 99% of the audience and wouldn't have even had the time to cover it correctly anyway, it would have been pointless.

[EEVblog]

Like others have pointed out, a lot can get lost in translation when simplifying explanations. 
The overarching point is that near-field communication uses a sort of coupling effect which is distinct from traditional RF communications

That's the point I was trying to get across, nothing more. And there would be a huge number of people who didn't know that hence why I thought I'd mention it.

[Chris Mr]

Dave does a fantastic job.

He is enthusiastic and manages to show the vast majority of people things they would otherwise never be aware of.  In doing so he makes a subject that let's face it, can be pretty dry, into something entertaining and interesting.

I have no idea how many people have taken up (or even just got interested in) electronics as a result of his efforts but it will be a lot.

To go into more detail than is necessary to explain something is relatively easy, it ends up taking hours, people's faces glaze over, turn off and go do something else.  To do what Dave does is really hard, to pick the essence and make it interesting.

Anyone who thinks they can do better - TRY IT 

[Buriedcode]

If a child asks you why the sky is blue, do you give them a physics lecture?

Sadly, I have.  But I gave my niece the option of the beautiful explanation or the cool one. Regardless of her answer I mentioned Rayleigh scattering because physics falls under both categories.

[EEVblog]

If a child asks you why the sky is blue, do you give them a physics lecture?

I would 
But in this case no one asked "why", and it's not the place in a video whose purpose was to test the effectiveness of a product. Horses for courses.
Ideally I should have left out all mention of how RFID worked, but it's my style to throw in little off-the-cuff tidbits like that.

[luxfx]

If a child asks you why the sky is blue, do you give them a physics lecture?

Yes... Guilty as charged

In regards to the question though -- this is the difference between an emission vs a dipole magnetic field, right? It might behave like an antenna, but that's only because it's inside the transmitters field?

[Brumby]

I've not ventured into the video blogging enterprise, but it doesn't take an Einstein to realise for an effective presentation, you have CONSTRAINTS!

Time is an important one - but even more so is the necessity to keep the material engaging from start to end.  I think this is where Dave does a reasonably good job.  This video being a prime example.

The distinction between electromagnetic radiation and transformer coupling was made to a degree that was adequate and appropriate to the presentation.  To have gone any further would have bored a lot of the audience sh*tless and buried the purpose of the video in a dry pile of excrement.

If people want to take things deeper, then there are many other avenues available.  This forum being one.


JMHO