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In a basic course of Electromagnetic Field, we arrived to introduce antennas. We studied the behaviour of the antenna by computing the electric field created when a sinusoidal input (the current flowing in a dipole) is applied to the dipole. I have a question: is this approach due to the fact that a generic signal (then, a generic current feeding the antenna) can be seen as a continuous sum of sinusoidal signals (Fourier Transform)? In this way, I should be able to apply superposition principle (at least in theory) for every sinusoidal component of the generic signal. And what about the electric field? I think it should be given (if my approach is correct) by the sum of an infinite number of plane waves, being each plane wave the effect of the corresponding sinusoidal component.

Can this principle (that is, to see a generic signal as the sum of sinusoidal signals, which should be always possible because phisical signals always have Fourier Transform) extended to other fields? For example, waveguides? As a consequence of my previous reasoning, when a video signal is applied to a coaxial cable, in the coaxial cable an infinite number of waves propagates: each wave should be the effect produced by a single sinusoidal component of the video signal.

Thank you!

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  • \$\begingroup\$ Antennas are usually driven by a single, tuned sinusoid, which is why we're talking about quarter wavelength, or similar. The cables have a definite delay given by the distributed inductance/capacitance, delay which is universal to any signal applied to the input. Mismatches in impedance carry reflections. I might be wrong, but you seem to be fixed on Fourier, or infinite sums (given your previous question). \$\endgroup\$
    – a concerned citizen
    May 23, 2018 at 10:44
  • \$\begingroup\$ I think the answer to the question as written is "yes"; you can decompose a real signal into subsignals and analyse them separately, then recombine. \$\endgroup\$
    – pjc50
    May 23, 2018 at 10:45
  • \$\begingroup\$ Yes, I'm fixed on Fourier... This is because in every course (electronics or EM fields) we study everything with sinusoidal input only, then my question: what about if we apply a generic signal as input? (Example: when I listen to radio I don't hear sinusoidal signals...) The only possibility (based only on my electronic engineer studies) is that every signal can be seen as a sum of sinusoidal signals, thus justifying a studio based only on sinusoidal inputs... You'll see another question later on, which will be the last one (I hope...) \$\endgroup\$
    – Stefanino
    May 23, 2018 at 10:58
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    \$\begingroup\$ You haven't paying full attention to what you've been told about the Fourier transform, if that is your question! You can decompose any sufficiently well-behaved signal (what you call "generic") into complex sinusoids with the Fourier transform, then do your analysis on these. There's nothing "generic" where this description doesn't actually work, unless you're leaving the realm of power-limited signals. \$\endgroup\$
    – Marcus Müller
    May 23, 2018 at 11:05

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The information is modulated on the carrier frequency. You should search for FFT of different kinds modulations . For example:

AM:

enter image description here

OFDM:

enter image description here

...and many others.

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