Antenna theory it seems is always described assuming that the transmitted or received signal being talked about is a pure sine wave usually at the resonant frequency. Standing waves for example are always depicted as perfect sine waves.

So everyone gets the impression that the waveforms on our antennas and transmission lines are all perfect sine waves.

However, it seems to me that the signals on an antenna including standing waves are hardly ever a perfect sine wave and are usually not exactly at the resonant frequency. For example, an LSB voice signal does not look like a perfect sine wave at all.

Do antennas work differently for perfect sine waves than for the far more usual messy looking waveforms i see on my oscilloscope ?

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    $\begingroup$ > are hardly ever a sine wave. Why do you think that? If you look on an oscilloscope, you'll find they are very much sine waves. The modulation makes very little difference to the sine-ness of them. I guess you could call them 99.9% sinusoidal, which would be indistinguishable to the naked eye. $\endgroup$ – tomnexus Dec 4 '19 at 12:30
  • $\begingroup$ @tomnexus i changed the question a bit to clarify. $\endgroup$ – Andrew Dec 4 '19 at 21:39

No, because of the superposition principle. Every signal that isn't a sine wave is expressible as a sum of sine waves, and an antenna's response to the sum of two signals is the sum of its responses to the separate signals. This property is called "linearity" and it's true as long as you don't run excessively large amounts of power through the antenna (approaching power levels that would destroy it anyway).

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  • $\begingroup$ Superposition is the answer, but when I was at MIT the past I didn't buy was linearity. Everything I knew was non-linear. $\endgroup$ – cmm Dec 4 '19 at 18:57
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    $\begingroup$ Id suggest looking at the modulation frequency, say 2kHz, modulation an 80 M carrier at 3.5 MHz. That is an almost 2000::1 ratio. If you look close enough at the resultant signal, it is a sin wave. $\endgroup$ – cmm Dec 4 '19 at 18:59
  • $\begingroup$ @cmm well, everything is nonlinear, but antennas are linear enough until suddenly they're not. I also agree with your point, but I didn't want to complicate things by mentioning it as well :) $\endgroup$ – hobbs - KC2G Dec 4 '19 at 23:01
  • $\begingroup$ I am K1UZK. Pleased to meet you. It pissed me off and confused me in early EE courses because I knew that resistors were inductors, and wires had mutual capacitance and Inductamce, and the response from T $\endgroup$ – cmm Dec 5 '19 at 1:18
  • $\begingroup$ TAs was to just put my head down and ignore what I knew from experience was true. Finally got a professor, Dr. Robert Burns, who didn't tell me I was stupid. We talked in useful depth about models, linear regions, and the range over which each approximation applied. Finally, a teacher worthy of the institution. $\endgroup$ – cmm Dec 6 '19 at 11:43

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