9 ms·

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

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?

Frummy 3y ago
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 3y ago
  No, the "photons" here are not created by that phenomenon but rather by accelerating electrons. https://en.wikipedia.org/wiki/Larmor_formula https://en.wikipedia.org/wiki/Larmor_formula
  - Frummy 3y ago
    Cool, thanks for letting me know.
max_ 3y ago
Its abit outdated.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 https://catalog.hathitrust.org/Record/001617948
amadsen 3y ago
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 3y ago
  >The EM field is always there Did you mean to say "the E field is always there"? Loved this explanation otherwise.
  - computerfriend 3y ago
    It's one field, which can be split into E and B components if you like.
    - georgeg23 3y ago
      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_special_relativity https://wikipedia.org/wiki/Classical_electromagnetism_and_sp...
  - H8crilA 3y ago
    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 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 3y ago
  >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 3y ago
    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 https://news.ycombinator.com/item?id=27942970
jcims 3y ago
>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.
amelius 3y ago
Circuit theory breaks down when it comes to antennas.
- rusk 3y ago
  Circuit theory breaks down at high frequencies and the appropriate generalisation of Kirchoffs is embodied in Maxwell’s equations.
  - jacquesm 3y ago
    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.
- H8crilA 3y ago
  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://en.wikipedia.org/wiki/Poynting_vector https://www.youtube.com/watch?v=bHIhgxav9LY https://www.youtube.com/watch?v=bHIhgxav9LY It is a very useful tool in practica.
  - colanderman 3y ago
    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.
  - jacquesm 3y ago
    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 https://www.omnicalculator.com/physics/skin-depth
ajsnigrutin 3y ago
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 3y ago
  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 3y ago
    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 3y ago
      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.
NovemberWhiskey 3y ago
>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.
myth2018 3y ago
> 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 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 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.
eternauta3k 3y ago
> 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.
jerry1979 3y ago
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 https://www.youtube.com/watch?v=-F7KYLO4Bkg

Frummy 3y ago

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 3y ago
No, the "photons" here are not created by that phenomenon but rather by accelerating electrons. https://en.wikipedia.org/wiki/Larmor_formula https://en.wikipedia.org/wiki/Larmor_formula
- Frummy 3y ago
  Cool, thanks for letting me know.

max_ 3y ago

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 https://catalog.hathitrust.org/Record/001617948

amadsen 3y ago

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 3y ago
>The EM field is always there Did you mean to say "the E field is always there"? Loved this explanation otherwise.
- computerfriend 3y ago
  It's one field, which can be split into E and B components if you like.
  - georgeg23 3y ago
    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_special_relativity https://wikipedia.org/wiki/Classical_electromagnetism_and_sp...
- H8crilA 3y ago
  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 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 3y ago
>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 3y ago
  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 https://news.ycombinator.com/item?id=27942970

jcims 3y ago

>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.

amelius 3y ago

Circuit theory breaks down when it comes to antennas.

rusk 3y ago
Circuit theory breaks down at high frequencies and the appropriate generalisation of Kirchoffs is embodied in Maxwell’s equations.
- jacquesm 3y ago
  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.
H8crilA 3y ago
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://en.wikipedia.org/wiki/Poynting_vector https://www.youtube.com/watch?v=bHIhgxav9LY https://www.youtube.com/watch?v=bHIhgxav9LY It is a very useful tool in practica.
- colanderman 3y ago
  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.
- jacquesm 3y ago
  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 https://www.omnicalculator.com/physics/skin-depth

ajsnigrutin 3y ago

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 3y ago
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 3y ago
  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 3y ago
    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.

NovemberWhiskey 3y ago

>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.

myth2018 3y ago

> 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 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 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.

eternauta3k 3y ago

> 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.

jerry1979 3y ago

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 https://www.youtube.com/watch?v=-F7KYLO4Bkg