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The term RF "amplitude modulation" is a misnomer.

12/01/2021 10:45 AM

I don't have an in-depth knowledge of radio communication. I only know these basic.

We have music or speech as input to the microphone. This input is variation in air pressure making sound waves. The transmitter circuit has the ability to produce an electrical signal waveform with a time series of voltage/time (this electrical signal waveform is an "analogous waving", waving in voltage imitating the waving of the sound - thus the term "analog" signal). Now the radio station has a fixed radio frequency to broadcast, say f Hz. Assume we take a pure sinusoidal voltage waveform with frequency f. We superimpose the analog signal onto this base waveform producing a amplitude modulated sine waveform with frequency f.

The transmitter somehow is connected to some resistor - the antenna. This antenna will emit em wave with the same frequency f. Users with radio tuning in to this frequency f will get the broadcast of the radio station.

Now, we have the term "amplitude modulation" to describe this manner of radio transmission. It is assumed that the electric/magnetic fields E/M have undergone modulation in their amplitude. We assume the em waves of frequency f has the same "analogous" waveform as the electrical signal in the transmission antenna. The only difference is that now it is the E/M fields that are varying instead of voltage/current in the antenna. But this picture of amplitude modulation may just be an illusion, not reality.

We have the picture of our radio wave having an electric field E varying in space, but this E field is only from our Maxwell model of electromagnetic waves. But no one has ever measured such E fields nor how they vary in strength. It is all just from our electromagnetism theory - basically Maxwell's.

If we accept the photon theory of light, then the radio waves are just photon. So the "amplitude variation" is just variation in energy of the photon and E-photon = hν; h= Planck constant. As frequency f is fixed, what is modulated is the photon energy E-photon.

So I'll rather say "amplitude modulation" may be a misnomer. More rightly it should be termed "RF intensity modulation".

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#1

Re: The term RF "amplitude modulation" is a misnomer.

12/01/2021 11:57 AM

This is insulting to my profession. You state that you do not have an in-depth knowledge of radio communication yet you want to change the definition and terms used in radio communication. That is an insulting case of chutzpah you have there. Do you really think all of the textbooks and lectures should change terms because you refuse to learn?

First of all, while this modulation technique is predominantly used in radio communication amplitude modulation is a mathematical definition that can be applied to any time-varying attribute. (The link is a really nice and informative demonstration.)

Second, in Physics the intensity of magnetization has a completely separate definition from the propagation of electromagnetic fields.

These terms have special meanings. You should learn them if you're going to use them.

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#2

Re: The term RF "amplitude modulation" is a misnomer.

12/01/2021 3:12 PM

Posting in online forum is often "tentative". Should we post our views only after we are 100% sure of its correctness?

If it is so, then I will in future post here only if I have a 100% certainty in my views.

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Re: The term RF "amplitude modulation" is a misnomer.

12/01/2021 3:31 PM

Respecting others with knowledge is how I have gained my knowledge. Any conversation with the phrase "I know nothing but it should be..." is a conversation with at least one fool. I guess I'm the fool for providing any knowledge.

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#3

Re: The term RF "amplitude modulation" is a misnomer.

12/01/2021 3:30 PM

It just depends on how one wishes to view the situation. Mathematics gives one the opportunity to view a physical phenomena from different perspectives from the likes of Messieurs Laplace and Fourier, n'est-ce pas?

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#5

Re: The term RF "amplitude modulation" is a misnomer.

12/02/2021 7:23 AM

<...em waves of frequency f has the same "analogous" waveform as the electrical signal in the transmission antenna...>

For amplitude modulation what one actually gets in the frequency domain is two sidebands as well, at the sum of and the difference between the carrier wave f and the audio wave frequency.

For frequency modulation there are loads of other sidebands at further sums and differences, which is why more bandwidth is needed for these signals to be transmitted and received cleanly compared with amplitude modulation; there is no scope for frequency modulation on the "long" wave, because there's not enough room for it.

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

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