Modulated EMF waves emited from antenna

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Driverj30t9

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Sorry if this isn't the best place to post this. Maybe electronics in the tavern would be but I can't post there yet.

My question is though, when a trasmitter/transceiver modulates a carrier wave with a persons voice, how does the modulated signal of your voice and the carrier stay together leaving your transmitter/transciever? Do they stay together through a magnetic field attraction or something? Like when they go through the coax or from your antenna through a mediun such as air why don't they seperate?

It's two signals combined correct? I know music and voice contain lots of different frequencies I think it's like 3KHz to 20 KHz, so all those frequencies represent the modulated signal put onto the carrier wave? Just don't understand it enough yet.

I always pictured an EMF wave leaving the antenna as just one frequency or signal.
 

Murphy625

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Sorry if this isn't the best place to post this. Maybe electronics in the tavern would be but I can't post there yet.

My question is though, when a trasmitter/transceiver modulates a carrier wave with a persons voice, how does the modulated signal of your voice and the carrier stay together leaving your transmitter/transciever? Do they stay together through a magnetic field attraction or something? Like when they go through the coax or from your antenna through a mediun such as air why don't they seperate?

It's two signals combined correct? I know music and voice contain lots of different frequencies I think it's like 3KHz to 20 KHz, so all those frequencies represent the modulated signal put onto the carrier wave? Just don't understand it enough yet.

I always pictured an EMF wave leaving the antenna as just one frequency or signal.

The modulated signal and the carrier wave are the same thing. When the person's voice stops talking, only the carrier wave is left. When the person starts talking, the carrier wave becomes the modulated signal.
They are NOT two different waves once they leave the transmitter.

Imagine being in a pond and splashing. The water in the pond is the carrier, the waves from you splashing around is the modulation. You could not separate the waves traveling across the pond's surface because they are the pond. Your splashing has simply changed the pond.
 

k7ng

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Murphy625, that is as good an analogy as I ever heard. And not verbose. Congrats.
 
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Hey Driver :)
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A good question, and I can understand your mystery in understanding 'modulation.' I am going to assume you are referring to AM - amplitude modulation- though, of course there are many 'modulating' techniques to send intelligence by radio frequency.
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I found your question particularly poignant since I was asking it years ago as a young teenager-
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"Just how does an audio frequency get carried by a radio signal ?"
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I was taking my first licensing exam (UK,) and was fore warned I would probably be asked this very question. (my exam was in front of government radio 'inspectors' -it was part written, part verbal !)
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And sure enuff, I was ask'd .....my response, a well rehearsed one, was --
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"Amplitude modulation is a super imposed audio frequency upon a radio frequency carrier."
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A pregnant pause, followed by - "Very good" :)
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I had no idea what I had just said, but all-in-all I aced the exam. It wasn't until years later in university that I came to any understanding of what AM was. If you want the physics of it we can get into stuff like Fourier Analysis, and such, but that will take us into some complex math.
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Here's a site that explains this far better than what I can confuse you with :)
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https://www.tutorialspoint.com/analog_communication/analog_communication_amplitude_modulation.htm
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If you want to go no further with this subject, then, if asked as I was lo those many years ago- simply respond--
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"Amplitude modulation is a super imposed audio ............" a guaranteed Government appoved answer (Smiles :) )
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Lauri :)
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RFI-EMI-GUY

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Here is one, how do the E field and H field get glued back together after they reach the far field region?

Sent from my SM-T350 using Tapatalk
 

needairtime

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Ultimately, yes only one wave leaves the antenna. The 'carrier' is initially a pure, even-amplitude sine wave; when you modulate it, that sine wave is no longer pure - the peaks are no longer the same height (for AM).

FM (frequency modulation) I suppose is a little tougher but not too bad to visualize and code/decode.

When we get to [suppressed carrier] SSB (single side band) ... then what? (Where's the carrier ? :))

(It still baffles me how to generate/demodulate SSB, yes I had to "parrot" that 90° out of phase balanced modulator addition and subtraction phooey for the exam, but in order for it to *really* stick in my head, I need to build the actual circuit and see it working - which I plan to do - ... same thing for digital mode quadrature modulation...)
 

RFI-EMI-GUY

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Ultimately, yes only one wave leaves the antenna. The 'carrier' is initially a pure, even-amplitude sine wave; when you modulate it, that sine wave is no longer pure - the peaks are no longer the same height (for AM).

FM (frequency modulation) I suppose is a little tougher but not too bad to visualize and code/decode.

When we get to [suppressed carrier] SSB (single side band) ... then what? (Where's the carrier ? :))

(It still baffles me how to generate/demodulate SSB, yes I had to "parrot" that 90° out of phase balanced modulator addition and subtraction phooey for the exam, but in order for it to *really* stick in my head, I need to build the actual circuit and see it working - which I plan to do - ... same thing for digital mode quadrature modulation...)

What amazes me is how DSP can down convert, filter the channel BW, demodulate and also modulate using mathematics. When you think about it it makes sense, but that it all happens on a slab of silicon is pretty cool.
 

needairtime

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That might actually be a option, I wonder if I could chop off the whole back end of an existing receiver and sample the final IF output into a DSP (provided the frequency is low enough so that samples don't lose data) and recover audio that way... all in trying to hack a plain AM CB into an amateur HF receiver (or heck, even into a SSB CB receiver, this is not violating FCC rules if I don't transmit?)

Unfortunately to synthesize SSB as a frontend for transmission is a whole different issue and FCC gets involved ...
 
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".........Here is one, how do the E field and H field get glued back together after they reach the far field region ? " (RFI wrote :) )
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Hmmm... I'll take a stab at it, -- resorting to the fields found in a microwave waveguide. I *think* this might come close to an answer...... but don't hold me to it....(smiles :) )
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Okay, when we say "E-plane" we are not defining a singular plane, such as the X=0 plane. There are infinite E-planes, just as there are infinite H and transverse planes. When referring to an "E-plane probe" such as might be used in a coax transition, chances are it is located in the E-plane that is close to midpoint between the narrow walls of the wave guide.
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I'll use the example found in a rectangular waveguide--
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The TE10 (transverse electric) mode is the "normal" mode in which energy propagates in such guides.
In this case, none of the electric field lines cross the transverse plane. In TE10, magnetic field lines are circular in the H-plane-- encapsulating the electric field crests.
Or another way to look at this is that the electric field is normal to the broad wall while the magnetic field line is normal to the short wall... and yet another view is that the E field lines will always remain within the E-plane, while magnetic field lines circulate in the H-plane.
The maximum positive and negative electric field crests of the wave travel down the center of the waveguide along the broad dimension -- the electric field decreases to zero along the waveguide's narrow side walls.
It's a time domain issue- both fields appearing at the terminus together.
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My head hurt.......... :)
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Lauri :)
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(did I get off topic ?..., that did deal with antenna theory..... smiles)
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Driverj30t9

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Murphy625, that is as good an analogy as I ever heard. And not verbose. Congrats.

Agreed.

Hey Driver :)

Hi

A good question, and I can understand your mystery in understanding 'modulation.' I am going to assume you are referring to AM - amplitude modulation- though, of course there are many 'modulating' techniques to send intelligence by radio frequency

That's good. I was thinking more in terms of AM but also FM and PM, or SSB.

I found your question particularly poignant since I was asking it years ago as a young teenager-

and I'm just learning now, oh well better late then never.

I was taking my first licensing exam (UK,) and was fore warned I would probably be asked this very question. (my exam was in front of government radio 'inspectors' -it was part written, part verbal !)

Things have gotten better in regards to the tests that's for sure. Verbal seems unneccessary. So you could live in America and have a call sign from the UK? That could confuse some people. Might get all excited thinking they got some DX.


Amplitude modulation is a super imposed audio frequency upon a radio frequency carrier."

That's a good way to put it. I still don't understand how the imprinting is actually done. I will read up more on it. Also I see now I should have been saying baseband signal, low pass signal, un-modulated signal or message signal since you don't get a modulated signal until the baseband is added to the carrier.

I had no idea what I had just said, but all-in-all I aced the exam. It wasn't until years later in university that I came to any understanding of what AM was. If you want the physics of it we can get into stuff like Fourier Analysis, and such, but that will take us into some complex math.

I"ll get to that in time.

Never heard of that site before but looks like a good one. Thanks for the link.

If you want to go no further with this subject, then, if asked as I was lo those many years ago- simply respond--


Amplitude modulation is a super imposed audio ............" a guaranteed Government appoved answer (Smiles :) )

Will do.

Here is one, how do the E field and H field get glued back together after they reach the far field region

Don't know much about those. So what she said. Thought maybe you were making a joke. E field and H-field are the same as E-plane and H-plane then? H plane on a graph is veiwing an antenna from above with azmithual directions and e-plane is from like a ground point and there showing the radiation pattern, but that's the extent of my knowledge.

I see that signals can't be bonded together, didn't really think they could but I was trying to make sense of things.

Ultimately, yes only one wave leaves the antenna. The 'carrier' is initially a pure, even-amplitude sine wave; when you modulate it, that sine wave is no longer pure - the peaks are no longer the same height (for AM)

That's what had me confused because I read somewhere that the carrier wave remains unchanged but it doesn't once it's modulated. I think they meant the carrier wave is unchanging until it gets modulated.

FM (frequency modulation) I suppose is a little tougher but not too bad to visualize and code/decode.

Definetly a little bit yeah, but the waves just get shorter or longer based on the amplitude of the low pass signal.
 

Hans13

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That's a good way to put it. I still don't understand how the imprinting is actually done. I will read up more on it. Also I see now I should have been saying baseband signal, low pass signal, un-modulated signal or message signal since you don't get a modulated signal until the baseband is added to the carrier.

Give this video a go. Perhaps the first minute might help with AM modulation. At 2:37 is helpful as well because it shows demodulation of the AM signal. FM is later in the video.

"How AM and FM Works" (TF11-3482 - Frequency Modulation Part I: Basic Principles, 1964)

http://youtu.be/D65KXwfDs3s
 
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RFI-EMI-GUY

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I wasn't thinking in terms of waveguide with respect to E and H field but the same applies.

Where I get mixed up is with a dipole in free space. There is all this conjecture about making signal measurements in the far field as opposed to the near field yet no agreement on where that transition takes place and how it happens. Thus my joke about glue.

Sent from my SM-T350 using Tapatalk
 
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Smiles :)
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I knew I was not on point with my attempt ..... about half way thru it I realized this wasn't going to answer anything.
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Wave guide theory does get involved, tho..... :)
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Lauri :)
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majoco

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There are many ways of describing an amplitude modulated RF carrier once it has left the antenna - Fourier analysis, rotating vectors and so on. But one of the easiest ways of illustrating how it is produced is to consider a simple tube amplifier and modulation transformer in the plate circuit - an audio signal is applied to the primary of the step-up transformer, B+ or HT+ is connected to the plate of the tube through the secondary winding and for example consider the input audio as low frequency audio sine wave.. An RF signal is applied to the grid of the tube and the resultant current comes out of the plate. However one of the tube parameters is "gain versus plate voltage". As the plate voltage is varying from the applied audio signal via the secondary winding, the 'gain' of the tube varies, thus on the positive peaks of the audio sine wave the tube gain is high and conversely when the audio sine wave is at a negative peak the gain is low and so the amplitude of the RF carrier varies in sympathy. The now modulated RF carrier is picked off from the plate of the tube via a capacitor to stop the HT+ voltage and gets passed on to the antenna via a Pi-coupler or something to match impedances.
 

zz0468

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I knew I was not on point with my attempt ..... about half way thru it I realized this wasn't going to answer anything.
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Wave guide theory does get involved, tho..... :)

It used to be enough to tell guys that AM goes up and down, and FM goes back and forth. *sigh*

There's a guy with a website, his name is C. Nyack, who used to have some wonderful interactive applets that you could see an oscilloscope representation of all sorts of mathematical functions, Fourier transforms, various modulation schemes including AM, etc. It would be perfect for this discussion, but alas, he's converting the java applets to javascript-html-css format, and most of them are down.

You could drag a slider button and change modulating frequency, or modulation index, or whatever, and see an o-scope and spectrum analyzer view as the waveform changed. It very clearly shows what happens (or doesn't happen) to the carrier when it's AM modulated, and things like the Bessel nulls on an FM carrier when the modulation index is exactly 2.4... neat stuff.
 

Murphy625

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If I might make a suggestion, perhaps a $15 investment into an SDR dongle and the free associated software might also help.

The spectrograph allows one to see the different types of modulated wave forms as they are being actively used on the various ham bands.

You can see Narrow band FM and compare it to Wide band FM, upper and lower sidebands, AM broadcasts, and even digital modes.

Then, looking at the bottom spectrograph displays, you can see the audio waveforms as well and how they correlate with the modulated signals.

As they say, a picture is worth a thousand words..
 
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"It's two signals combined correct? I know music and voice contain lots of different frequencies I think it's like 3KHz to 20 KHz, so all those frequencies represent the modulated signal put onto the carrier wave? Just don't understand it enough yet. "

.....Driver wrote, and once again he mirror'd my mystery of Amplitude Modulation as as teenager. To be perfectly honest, I still revolve this in my mind, for AM is a signal that varies in amplitude- not in frequency. Young Lauri once tried (a variation of) this thought puzzle out on her elders --
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"Okay, my wise Yoda Masters (I was a Smart A** even in those days :) ) - " If I take an RF carrier and vary its amplitude, should I not get just a signal that varies in strength, but does not convey any audio qualities ?"
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"Like, if I whistle (an audio signal,) but only vary the strength, not the tone (frequency,) is that not an Amplitude variation ?.. Combine that with a hum (will call it a carrier) only the over-all strength of the whistle should vary in the amplitude. If I vary the whistle's tone, but not its amplitude with respect to the hum, there will be no variation in the amplitude, but the tone will be heard to vary in frequency, right ? Is this the same as AM radio modulation, and if so why is it different for radio but doesn't work for audio ?.... or does it ?"
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"Making our heads hurt, you do" was the response.
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Years late I grok'd it all, but Driver, I understand your question now. :)
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Answer it, you ask of me now, Hmmmmm ?
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Lauri :)
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_________________________________________________________
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PS---
Oh, and Driver, you ask'd about my UK license- I was living there and sitting the exam was no problem for a Yank... ")
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RFI-EMI-GUY

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(snip)
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"Okay, my wise Yoda Masters (I was a Smart A** even in those days :) ) - " If I take an RF carrier and vary its amplitude, should I not get just a signal that varies in strength, but does not convey any audio qualities ?"
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(snip)
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You are still conveying "information" in the variation of signal strength.
 

zz0468

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You are still conveying "information" in the variation of signal strength.

And the problem becomes one of physics and mathematics, which require that information being conveyed occupies bandwidth. So, the process of varying the amplitude at a specific information rate adds sidebands containing the information.

Even on-off keyed morse code occupies bandwidth greater than the single pure carrier frequency. Don't believe it? Watch it on a spectrum analyzer. The faster the code speed, the wider the occupied bandwidth. You can dampen the rise and fall times of the CW, and narrow the bandwidth, but then it limits the rate it can be keyed.

The "problem" (it's not really a problem) is that by altering any characteristic of a pure perfect sine wave, be it frequency, phase, or amplitude, for that instant, it's no longer a pure perfect sine wave. And if it's not a pure and perfect sine wave, it's got other frequency components in it, and that increases the bandwidth.

And to RFI-EMI-GUY, my comments aren't directed at you, I'm just using your post as a jumping off spot.
 
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zz0468

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If I might make a suggestion, perhaps a $15 investment into an SDR dongle and the free associated software might also help...

...As they say, a picture is worth a thousand words..

That's actually a very good idea.
 
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