As most of you know, although I'm a very active participant in AAC's fora (because I love this place), I'm just a half-baked noobie when it comes to analog electronics. So I'm going to ask a few questions that may seem laughable to those versed in the elemental stuff of electronics and electromagnetism, but that are pretty deep stuff to me. I'd rather ask a dumb question and look stupid (and get the answers I want) than stay quiet and stay stupid.

In the next paragraphs, I'm going to make several statements explaining what I think I understand, and will also make several direct questions. I'll be very grateful for any corrections and clarifications related to what I'm about to say. Remember, my statements will actually be questions with the purpose of evaluating how much I really know about the subject.

Let's start with the way AM behaves, since I think it will be the easiest for me to understand at first.

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• The modulation of a signal is related to the continuous change of one of its parameters, so as to encode information within it, that will later be extracted and demodulated (decoded) by a receiver.
• In the case of AM, the frequency remains constant, and the amplitude is changed to encode that information. So a constant tone of x-Hertz in AM would look like this:

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Some of the biggest mysteries for me regarding radio AM ares these:

• Radio waves are particular zone within the electromagnetic spectrum that lies below a frequency of about 100 MHz
• Since radio waves are nothing more than photons of a certain frequency, and since the energy of a photon is directly related to its frequency, how is it possible to modulate its amplitude? In other words, what property of light is amplitude? It's certainly not color, from what I understand. Does AM work by changing the amount of photons being emitted by the antenna, and therefore the intensity of the signal?
• A radio emitter works by exciting the electrons in the atoms that form an antenna. Those electrons are raised to a higher orbit by energy being pumped into the antenna, and they later release photons when they return to their lower (and original) orbit. Those photons form what we call radio waves.
• A receiving antenna captures those photons, in such a way that they excite the electrons of the atoms of metal that form said antenna, and that change in their potential is later amplified and decoded by the receiver.

How many stupid things, wrong assumptions, and incomplete ideas have I said so far?
There are many other questions that I'd like to ask on this subject, but lets start with this few ones first.

 

Some of the biggest mysteries for me regarding radio AM ares these:

• Radio waves are particular zone within the electromagnetic spectrum that lies below a frequency of about 100 MHz
• Since radio waves are nothing more than photons of a certain frequency, and since the energy of a photon is directly related to its frequency, how is it possible to modulate its amplitude? In other words, what property of light is amplitude? It's certainly not color, from what I understand. Does AM work by changing the amount of photons being emitted by the antenna, and therefore the intensity of the signal?
• A radio emitter works by exciting the electrons in the atoms that form an antenna. Those electrons are raised to a higher orbit by energy being pumped into the antenna, and they later release photons when they return to their lower (and original) orbit. Those photons form what we call radio waves.
• A receiving antenna captures those photons, in such a way that they excite the electrons of the atoms of metal that form said antenna, and that change in their potential is later amplified and decoded by the receiver.

1) Radio waves range from sub-hertz frequencies to a practical limit of 1 terahertz - way beyond your 100 MHz.

2) Not photons, but electrons are what are causing the EM field and thus the radio wave.

3) Still not photons. Current going into an antenna creates an EM field which radiates into the surrounding medium. This is the radio wave.

4) Again, not photons. An EM field radiating into an antenna creates a current in the antenna, which is then amplified and decoded.

A radio wave is neither photons, nor electrons, but is instead a field created by the movement of electrons. A simplified analogy of this is a simple magnet. Put a piece of paper over the magnet and sprinkle iron filings and the filings will align themselves with the magnetic field. There are no fundamental particles that are the magnetic field, but the field is the RESULT of the magnet. A radio EM wave is the result of a moving electron (or many, many electrons) and has two components, an E-plane component and an H-plane component. The E-plane component has properties like an electric field and the H-plane component has properties like a magnetic field. They both exist together and one cannot exist without the other (they are orthogonal to each other).

 

A radio wave is neither photons, nor electrons, but is instead a field created by the movement of electrons

Fair enough... where do those electrons come from? Do they already exist in the vacuum and permeate the entire universe? Do they travel from one antenna to the other?
Don't take me wrong. I'm not questioning your credentials. I'm just trying to understand. And thanks for answering, btw.

 

The electrons are in the transmitting and receiving antenna. An electrical current in the transmitting antenna causes electrons to accelerate as they move back and forth. When an electron is accelerated, it emits electromagnetic radiation. Similarly, electromagnetic energy impinging on an electron can accelerate it back and forth causing an electrical current that can be detected.

Electromagnetic waves are dual natured -- they can either be described as being wave-like or being particle-like. In different situations treating them one way makes both intuitive and computational sense while treating them the other way leads to all sorts of problems. Which way is best depends on a number of things, but usually you treat them as waves until you are talking about situations in which interactions with individual particles is important.

But to answer one of your questions, in the photon theory a given photon's energy is related to its frequency, so the intensity of the light beam altogether is determined by how many photons (the photon flux) are being emitted.

 

Thank you, Bhan. I knew I could count on you for this sort of question.

It seems that at least I had the photon flux concept about right (at least in my head, that is).
The dual nature of electromagnetic waves is something that I've also understood (or at least accepted) since high school too... on a theoretical level, that is. And the way an antenna works is more or less clear to me... electrons have to be in motion (mainly oscillating) in both transmitting and receiving antennas, with one inducing motion, while the other accepting it, for things to work.

My most basic question pertains to the nature of electromagnetic radiation itself. How is it related to photons? And if it's not transmitted through photons, then why every time scientists talk about the electromagnetic spectrum do they include visible light in it?
Is the answer as simple as "they're actually waves, until photons are needed to explain their behavior"? ... I know there are infrared and ultraviolet photons out there... and also x-ray and gamma ... so it follows that radio-frequency photons exist too, right?

 

Thank you, Bhan. I knew I could count on you for this sort of question.

So why do you always insist on thanking someone named, "Bhan"?

My most basic question pertains to the nature of electromagnetic radiation itself. How is it related to photons? And if it's not transmitted through photons, then why every time scientists talk about the electromagnetic spectrum do they include visible light in it?
Is the answer as simple as "they're actually waves, until photons are needed to explain their behavior"? ... I know there are infrared and ultraviolet photons out there... and also x-ray and gamma ... so it follows that radio-frequency photons exist too, right?

The Wikipedia article, https://en.wikipedia.org/wiki/Photon , might shed some light on it for you. You probably don't need to go much past the first paragraph. The key sentence is probably:

"The photon's wave and quanta qualities are two observable aspects of a single phenomenon, and cannot be described by any mechanical model."

Humans like to think in terms of mechanical models we can visualize, but electromagnetic phenomena just don't work that way. We can approximate it one way sometimes and get results that are good enough. We can approximate it the other way sometimes and get results that are good enough. But, in general, we just have to trust the math.

 

So why do you always insist on thanking someone named, "Bhan"?

Oooopsss... sorry about that, William... I suffer from typo-dyslexia sometimes

"The photon's wave and quanta qualities are two observable aspects of a single phenomenon, and cannot be described by any mechanical model."

Yeah... I tend to visualize radio waves as if they were analogous to sound waves... now I know I have to stop doing that if I want to make progress.

...in general, we just have to trust the math.

I feared it would come to that... so I'll just accept it, and move on. I'll give the link you posted a quick read, anyway. Thanks.

Last question (for the night) how can I visualize the amplitude of a radio wave? It's certainly not its energy level, nor its flux/density, is it? Or do I just have to trust the math on that one too?

 

Forget about photons at radio frequencies as they bring nothing but additional complexities to an already complex subject.

An analogy would talking about how the individual grains of wheat in an acre of wheat are affecting the wave like movements of entire fields of wheat in a tornado. A typical radio photon might have energy in the 10^-10 eV range as opposed to a gamma-ray photon at 10^9 eV range. It's easy to see that a huge number of RF photons would be needed to effect electrons and it would be almost impossible to build a single photon detector at RF.

How big is a RF photon?
https://briankoberlein.com/2015/04/14/thats-about-the-size-of-it/

 

Forget about photons at radio frequencies as they bring nothing but additional complexities to an already complex subject.

An analogy would talking about how the individual grains of wheat in an acre of wheat are affecting the wave like movements of entire fields of wheat in a tornado. A typical radio photon might have energy in the 10^-10 eV range as opposed to a gamma-ray photon at 10^9 eV range. It's easy to see that a huge number of RF photons would be needed to affect electrons and it would be almost impossible to build a single photon detector at RF.

How big is a RF photon?
https://briankoberlein.com/2015/04/14/thats-about-the-size-of-it/

View attachment 117791

Great article, thanks!

 

how can I visualize the amplitude of a radio wave? It's certainly not its energy level, nor its flux/density, is it?

The amplitude of a radio wave is simply its total power as determined by the number of photons per second and the energy of the photon in the wave.

 

Last question (for the night) how can I visualize the amplitude of a radio wave? It's certainly not its energy level, nor its flux/density, is it? Or do I just have to trust the math on that one too?

If we look at the propagation of a free space radio wave we can use the basis of the impedance of free space (limiting factor of the rate of change in electric/magnetic field) to determine the energy density and flux in that region of space by the measurement (or calculation) of the electric field within that space(area). The direction of propagation of an electromagnetic plane wave is in the direction of E(r,t)×B(r,t) so we know that energy flux points in that direction. The energy flux is usually known as the Poynting vector in EM but we can use similar types of mechanical wave equation expressions to find the energy in EM waves as they travel through space as long as they are compatible with EM wave physics.

 

Hello,

There is a series of videos about transmission lines and E.M. waves on learnersTV:
http://www.learnerstv.com/Free-Engineering-Video-lectures-ltv112-Page1.htm
and about oscillations and waves:
http://www.learnerstv.com/Free-Physics-Video-lectures-ltv082-Page1.htm

Bertus

 

I would try to help, but I'm obviously out-classed here.
I also have difficulty with this. "So, it's like the magnetic field around a metal magnet, but it wiggles, and the wiggling energy radiates. So a magnetic field decreases to zero in a few inches or a few meters. So why do radio waves travel, like, forever?"

I can imagine the energy being radiated off a dipole, and I've measured microvolts per meter on a receiving antenna, but how that energy gets to the moon and past doesn't seem like the field around a magnet to me! I don't know if the impedance of free space changes in Earth atmosphere, and I don't know how radio waves travel bazillions of miles in a vacuum. That's why I stay out of the RF Forum. I'm incompetent in this field.

 

I would try to help, but I'm obviously out-classed here....

You, are outclassed here? ... I must say, this has been a humbling experience. I just finished reading the link posted by Bahn (I had to double-check and re-type the name, since I misspelled it the first time... again ) and I don't think I fully understood more than 10% of it.

It seems that, at least for this subject, I'm going to have to be patient and learn and re-learn the basics... but I fear it will be something like trying to visualize the 4th dimension, or imaginary numbers... they're concepts that one just has to accept and have no real-world analogy.

crutschow's answer is the one that I most easily understood. So, if:

The amplitude of a radio wave is simply its total power as determined by the number of photons per second and the energy of the photon in the wave.

Then it means that in AM (as in all radio transmission techniques, I'm guessing) the amplitude of a wave diminishes in a way that is proportional to the square of the distance to its source?

 

The most successful scientific model we have is quantum field theory. In QFT, everything is a wave, so if you really want to get fundamental don't think of photons, think of field perturbations (waves). But I don't think QFT is much help when you're trying to develop an intuitive understanding of radio transmission.

Though we don't need to zoom in to the level of QFT to understand how radio waves propagate, we do need a field theory: electromagnetism. In this model, there are two orthogonal (perpendicular) fields, one electric and the other magnetic. These fields are a property of space itself, and a change in one field causes a change in the other -- if we apply a time-varying voltage and get the electric field moving, the magnetic field will move in kind, and vice-versa. This is how EM waves propagate.

To put a simple visual to it, imagine an electron sitting still at the end of an antenna. The electric field associated with it extends out past the antenna (to infinity, actually), but since the electron isn't moving, the field is still. Now we apply a pulse voltage to the antenna causing the electron to move. Like dropping a pebble in a pond, this perturbs the electric field, causing it to rise and fall "in place". But any change in the electric field causes a change in the magnetic field. Remember how the fields are perpendicular? This means that a time-varying electric field can be associated with a spatially-varying magnetic field (and vice-versa). So, in response to the electric field, the magnetic field also rises and falls in place, but a little bit further out. And since the magnetic field is changing, this causes the electric field to start changing (again, a little bit farther out), which causes the magnetic field to start changing ... and so on.

This can all be made precise with Maxwell's equations, but for an intuitive understanding, I think it suffices to visualize radio transmission in terms of ripples in a pond.

 

These fields are a property of space itself

That is yet another big mystery that I'd like to explore later on. That is the nature of fields themselves.

for an intuitive understanding, I think it suffices to visualize radio transmission in terms of ripples in a pond

That has worked for me, up to a point. And I'm trying to get past that... but maybe I'm just complicating stuff that (at least for me) could be kept simple.

 

These fields are a property of space itself,

A vacuum has magnetic field properties? (Blinking like a frog in a hail storm.)
It's such a stretch for me to imagine ripples in a vacuum!

 

... but I fear it will be something like trying to visualize the 4th dimension, or imaginary numbers... they're concepts that one just has to accept and have no real-world analogy.

Incidentally, there's really no reason to be spooked by complex numbers. As a set of numbers, complex numbers are equivalent to 2-dimensional real numbers (like the x and y coordinates on a graph). We can even toss out the j = sqrt(-1) business: any complex number in the form x+jy can be written in the form (x, y). As vector spaces, C and R^2 are isomorphic to each other (i.e., they're essentially the same thing). While it's true that if we include multiplication (and so treat C as a ring or field), there are some differences between C and R^2, but these are purely mechanical and not philosophical differences.

Besides the incredibly unfortunate name imaginary, there's nothing weirder or more unnatural about sqrt(-1) than, say, -1 or even 0. Like negative numbers and zero, sqrt(-1) was invented to be able to solve more types of equations, and beyond its privilege of being the algebraic closure of the real numbers, it's as ordinary as any other number. In most applications where complex numbers are useful (like electronics), we use them strictly for their 2-dimensional quality, which lets us encode more information in one number. In other words, if you can visualize and find real-world analogy for 2-dimensional graphs, then you can visualize and find real-world analogy for complex numbers.

 

A vacuum has magnetic field properties? (Blinking like a frog in a hail storm.)
It's such a stretch for me to imagine ripples in a vacuum!

Yes, and this unfortunately is where QFT is required.

 

A vacuum has magnetic field properties? (Blinking like a frog in a hail storm.)
It's such a stretch for me to imagine ripples in a vacuum!

They said space, not vacuum.
Why is this important?
It is important because WE LIKE to think that space is vacuum. And this mode of thinking is wrong. It is not vacuum. There are things floating there, hydrogen atoms, other stuff. They are not really close, but they are there.