[ulnarkressty]

I find these guides a bit disappointing in that they always give the example of a wire loop generating an EM field, which one could sort of imagine given day to day experience with direct current and conductors. However this all breaks down when looking at regular monopole antennas - how can the conductor conduct when one of the ends is just hanging in the air? AC magic I suppose.

Similarly they are very short on details on what exactly is going on when the EM field is generated. I guess it consists of photons, but where exactly do they come from and how are they generated using (in some cases) only milliwatts of power?

[amadsen]

It doesn't really "generate" a field. The EM field is always there, permeating the universe. An acceleting electron will _disturb_ the EM field (depositing some energy and momentum into it), and this disturbance will propagate through the field at the speed of light (naturally, since at the right energy level such a disturbance is what we call light). At high energies the disturbance will be finely localized in space and behave like a particle, which we call a photon. It's fine to refer to it as such also at lower energies, but slightly misleading because at the very low energies that we talk about here ("radio") it is very spread out in space and behaves more like a wave (with a wavelength of ~meters). In the case of AC, electrons are moving "back and forth" over a short distance (somewhat simplified but useful picture) with the same effect. Think about moving your hand up and down through water - you will create a wave.

[programmer_dude]

>The EM field is always there

Did you mean to say "the E field is always there"?

Loved this explanation otherwise.

[computerfriend]

It's one field, which can be split into E and B components if you like.

[georgeg23]

The B field is just the E field's relativistic effects to an observer not in the same motion frame.

https://wikipedia.org/wiki/Classical_electromagnetism_and_sp...

[H8crilA]

Both are always there if the circuit is running.

Also, both E and M fields are required to transport any power using electricity. This is also true for DC current. See https://en.wikipedia.org/wiki/Poynting_vector - the formula multiplies E by B, therefore if any of the two is zero no power (or information) transmission can occur. This is true over the air, as well as over a wire.

[rusk]

>The EM field is always there

Hmm sounds dangerously close to aether theory. I thought we had moved past this … that EM waves exist in their own right without need for a medium. Unlike sound say.

[gabia]

Aether refers to some historical ideas (which evolved over time), and though the word isn’t in use anymore the current model of fields has similarities (ofc in many ways very different). Have just been reading some of the history of this (Einstein plays a big role), it’s very interesting, thank you! :) You might be interested in this discussion https://news.ycombinator.com/item?id=27942970

[zmgsabst]

Wilczek points out that field theories are aether theories — as is relativity.

“Aether just means this one particular historical model!” is branding more than reality; and hides the fact that modern theories also describe an everywhere substrate of which particles are localized excitations and gravity is localized warping.

[rusk]

Way over my head unfortunately

[runlaszlorun]

Air is aether.

[p_j_w]

Air is not required for an E field to exist or propagate.

[jacquesm]

EM fields propagate just fine through vacuum, they are mediated by photons.

[jcims]

>AC magic I suppose.

Yep. Think of a long pipe open on one end and capped on the other. If you seal the end with your mouth and blow, it will quickly pressurize. But if you seal a loudspeaker to it and sweep through the frequencies you will find that the loudness of the speaker varies proportionally to how close the tone is to a resonant frequency of the tube.

The elections in a monopole antenna have a sort of elastic relationship to one another where the influence propagates at the speed of light rather than the speed of sound. They are also able to move quite freely within the conductor like gas molecules in the tube. So if you have an antenna that’s two meters long and play a ‘tone’ with the equivalent of the loudspeaker at 150 million hz, the tube of electrons resonates.

Just like the atmosphere can couple resonant cavities of the same frequency, antennas are coupled by the electromagnetic field to resonate at the same frequency. This provides a certain ‘gain’ of the energy at the end of the ‘tube’ relative to other frequencies that aren’t resonant, which is what feeds our ears or amplifiers with something differentiable from the rest of the noise.

Now, imagine you have your ear glued to another tube just like the loudspeaker tube and your other ear is plugged. You’re going to hear the world around you, but it’s mostly going to be at harmonics of that resonant frequency. So when your buddy asks ‘can you hear me?’ a you get this really ringy ‘mwaa mwoo mwee mwee?’. Now imagine he turns on the speaker tube on the other side of the room? It’s going to brightly stand out among all of the other things you hear, ooooooooooooooooo.

Ok, now, he hands you a roll of paper and says draw along the roll relative to how loud it is. Up is louder down is softer, and he goes back and starts messing with the volume knob:

oooooOOOOOOoooo....oooooOOOoo..ooOoOo

The thing you draw on that roll of paper looks like a sound wave.

And now you know how AM radio works.

[myth2018]

> how can the conductor conduct when one of the ends is just hanging in the air? AC magic I suppose

I struggled with these matters too. I wouldn't call it "AC magic", but "RF magic".

Notice that you need a different mental model to understand those circuits. In DC and low-freq AC, it's enough to think that voltages and currents manifest themselves instantaneously in a conductor. But that is a (useful) simplification. Roughly speaking, the energy supplied by the power source takes time to propagate over the wire [0].

Having that in mind, take a look at the animated gif in this page: https://en.wikipedia.org/wiki/Dipole_antenna. I believe it's gonna make more sense now.

This mental model also helps to understand why a magnetic loop isn't simply a piece of short-circuited wire -- for a DC or low freq AC circuit, it is indeed a short-circuit; for an RF circuit, it is not. [1]

[0] https://en.wikipedia.org/wiki/Velocity_factor

[1] this mental model is not enough to fully understand any antenna. Many designs also depend on other phenomena, like the electromagnetic interactions between its components, and interactions with its surroundings. But I think that, equipped with this mental model, you'll be able to research further in case you feel interested.

[ajsnigrutin]

Yep, AC is magic.

And not just antennas.... look at computers... at 3GHz (DDR4+ ram), the wavelength is ~10cm, so looking at a single sine wave, you have 1.8volts at the cpu, zero volts 2.5cm away from the cpu, -1.8v 5cm away, zero 7.5cm, and again 1.8 volts 10cm from the cpu... now look at the distance between the cpu and ram, the other frequencies that come with the square(-ish) waves, and all the math to make a basic RAM read/write work.

Just a normal wire (or cable - the simplest component in DC electronic circuits, just a line, that does nothing) changes everything... you send a (voltage) signal in, and the current has to flow at some rate, even before the "signal" (field) reaches the other side of the cable, to "see" if the other end is "open" or soldered together or if some sort of a resistor is soldered there.

[jacquesm]

That's not how current propagates. Current propagates - broken analogy warning - in the same way that marbles would propagate through a tube. You push a marble in on one end and assuming the tube is full another marble pops out the far end. So the flow rate of the electrons is vastly lower than the flow rate of the signal itself!

[ajsnigrutin]

Of course, but currents here are a result of a field that moves those electrons one way or another, and electrons move, before the field reaches the other end (where a resistor is.. or is not.. or a short circuit is), so in your analogy, marbles move down the tube, before they "realize" that the tube is closed on the other end.

[jacquesm]

Fair enough. It's an analogy anyway and it is a broken one in more ways than one (for instance: it doesn't deal with propagation of pulses very well because these can be reflected from the end of the tube, you'd have to model it as a tube full of marbles and springs to account for that) but it serves for some discussion purposes.

[max_]

Its abit outdated. But it think you may enjoy reading through Oliver Lodge's book, "Signalling Through Space Without Wires"

[0] https://catalog.hathitrust.org/Record/001617948

[NovemberWhiskey]

>regular monopole antennas

There is no real monopole antenna - there's always another "half", like a ground plane, a counterpoise system or even just the body of the person holding a handheld radio.

[Frummy]

Photons are created when an electron drops to a lower energy level. Like your voicebox and the sound you make when you speak, it doesnt come from somewhere but is created right then

[eternauta3k]

No, the "photons" here are not created by that phenomenon but rather by accelerating electrons. https://en.wikipedia.org/wiki/Larmor_formula

[Frummy]

Cool, thanks for letting me know.

[amelius]

Circuit theory breaks down when it comes to antennas.

[jacquesm]

It's at best a leaky abstraction but for some domains it is very useful. Antennas are 'magic' from the point of view of Ohms law and so on but once you move to the electromagnetic domain you will find equivalents for most of the elements in the original abstraction: impedance, reactance and so on.

[SAI_Peregrinus]

Yep, and even common nonlinear elements like diodes and transistors are non-Ohmic.

[jacquesm]

No, they are just non-linear: as the voltage goes up the current goes up in jumps because the resistance changes in a non-linear way, but it is still resistance and you can still express the system at any point of the curve using Ohm's laws and you can still compute the power lost in any part of that system using the simplest equations. A coil or a capacitor (or even a piece of wire, but there the effects are very small) are components that do not follow Ohm's laws, you need to add more parameters than just current, voltage and resistance to work out what's happening.

Tricky stuff!

[H8crilA]

Circuit theory is demonstrably wrong. The energy doesn't flow inside of a wire, it flows around the wire.

https://en.wikipedia.org/wiki/Poynting_vector

https://www.youtube.com/watch?v=bHIhgxav9LY

It is a very useful tool in practica.

[Manuel_D]

Not really in the intuitive sense: https://www.youtube.com/watch?v=iph500cPK28

Someone actually went out and tested this: https://www.youtube.com/watch?v=2Vrhk5OjBP8

The electric field is very tightly constrained around the wire. The amount of energy that reaches the light through the gap is incredibly small, not nearly enough to turn on a real light bulb. The bulk of the energy does indeed have to take the path through the wire. I find that video usually leads people to an even less correct understanding than the marbles-in-a-tube analogy.

The Veritasum video can essentially be summarized, "fun fact: two parallel wires act like an antenna, and an extremely small amount of energy reaches the other end before the rest of the energy takes the long path through the wire.

[colanderman]

I'm not sure circuit theory states that energy flows within wires? Current, yes, but energy transfer is the product of current and voltage, and voltage of course exists only between wires.

[wbl]

The Poynting vector is the obvious choice but we cannot do the experiment to tell between the various possibilities one can get by adding more terms that obey the constraints.

[jacquesm]

Energy does flow 'inside' a wire at low frequencies. The skin effect gets higher and higher as the frequency goes up until you reach a point where it is all skin effect. But even that 'skin' isn't idealized it definitely has a thickness, about 30 u at 5 MHz and 6.5 at 100 Mhz.

There is a neat little calculator here:

https://www.omnicalculator.com/physics/skin-depth

[H8crilA]

No. DC energy also flows around the wire. And I mean around, entirely outside of the conductor, not in the skin or anything like that. In particular it's not the electrons that carry the energy, though they can receive it from the fields and depose inside the conductor (like in a light bulb).

Wires, or conductors in general, are so useful because they allow us to manipulate the EM fields and channel the energy very efficiently.

It's an very common misconception coming from circuit models.

(the movement of electrons, and thus the deposition of the energy via "resistance", may indeed be limited to just the surface of the conductor; this is what that calculator shows)

[jacquesm]

Complete nonsense, the current carrying capacity of a wire is directly proportional to the surface area of the cross section of the wire. If it were the skin it would be proportional to the circumference and it clearly is not. I have no idea where you came into this idea but it is just completely wrong. You can test it for yourself with $50 worth of gear.

[H8crilA]

You can invalidate the circuit theory with, well perhaps not $50 worth of hardware, but a bit more:

https://youtu.be/oI_X2cMHNe0

I'm not sure how do you plan to invalidate Maxwell's theory, which shows that energy flows entirely outside of wires, with $50. Or any other amount of equipment. It would be very much welcome in the physics community.

This misconception is even more widespread than "planes fly because of Bernoulli" because it's so incredibly useful when designing most circuits/PCBs. Though it is, fundamentally, a lie.

PS. Circuit theory is made to match reality for the case showed in the video via the concept of a transmission line.

[colanderman]

@H8crilA is talking about energy, you are describing current. Energy flow is the product of the electric and magnetic fields [1]; electric field within a conductor is zero, therefore energy flow within a conductor is zero.

[1] https://en.wikipedia.org/wiki/Poynting_vector

[rusk]

Circuit theory breaks down at high frequencies and the appropriate generalisation of Kirchoffs is embodied in Maxwell’s equations.

[jacquesm]

Indeed, it's a bit like Newton vs Einstein.

The one is a special case of the other, it doesn't make the former wrong but the latter gives better answers in situations where the special case constraints aren't satisfied.

[jerry1979]

Quick answer: The antenna is like a tube, and the energy stuff is like liquid that sloshes back and forth in the tube generating a kind of wiggle.

This Canadian military video should help: https://www.youtube.com/watch?v=-F7KYLO4Bkg

[eternauta3k]

> how can the conductor conduct when one of the ends is just hanging in the air?

At frequencies where the wire radiates, you can't model a wire as an open resistor. You need to model it as a bunch of Rs, Ls and Cs, and there you will see the current flow.