How does an antenna work with harmonics?

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if you fed a rectangular waves to a radio antenna, would it still produce radio waves? does it have to be a sine wave?
 

Answers and Replies

  • #2
FactChecker
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No, it does not have to be a sine wave. In fact, a rectangular wave can be thought of as a specific sum of sine (and cosine) waves of various frequencies. That is the basis of the Fourier series. The antenna will respond to each sine wave as though it was not part of a rectangular wave, then all the responses to the sine waves can be summed to get the total antenna response to the rectangular wave.

Shown below is a sequence of better approximations of a square wave using more terms in the Fourier series. Each line is plotted a little higher so that you can see them separately. Clearly, the more terms there are, the closer it gets to a square wave. An antenna will respond to each sine wave as though it is alone. Then the individual responses can be summed to get the total response.
1575655092095.png
 
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  • #3
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if you fed a rectangular waves to a radio antenna, would it still produce radio waves? does it have to be a sine wave?
Of course, it would produce radio waves but true broad-spectrum mess. Square wave can be simply turned into decent sine wave by inserting a loading coil between output of generator and an antenna.
The principle is same as in this video. This would make antenna more or less electrically short though.
 
  • #4
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Of course, it would produce radio waves but true broad-spectrum mess. Square wave can be simply turned into decent sine wave by inserting a loading coil between output of generator and an antenna.
The principle is same as in this video. This would make antenna more or less electrically short though.
A single square wave contains several sine waves of different frequencies and amplitudes. It would be possible to tune to any one of those and tune out the others (although not completely). The amount of power lost by tuning out the other frequencies depends on the response of the filter to the other frequencies.
 
  • #5
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A single square wave contains several sine waves of different frequencies and amplitudes. It would be possible to tune to any one of those and tune out the others (although not completely). The amount of power lost by tuning out the other frequencies depends on the response of the filter to the other frequencies.
It also depends on the frequency of square wave. For example if that frequncy is 3x higher than quarter wave resonant frequency system with loading coil, than the response of the system to the 3rd harmonic is particularly strong
 
  • #6
sophiecentaur
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Most radio antennae are not designed (it's very hard!!) to operate over even a single octave of frequency range. Moreover, Transmitting amplifiers seldom handle a huge range of frequencies. About an Octave is about as much as you will ever get. A UHF Broadcast Travelling Wave Tube (a typical 'wide band' amplifier) has a wide enough bandwidth to handle a number of separate UHF channels but not over an octave frequency range. Even matching a transmitter to an antenna for a number of different channels is difficult. Most systems use a small Fractional Bandwidth.

So the question in the OP doesn't make a lot of sense in the practical world. However, RF antennae are usually capable of handling a Modulated RF signal which can carry a good looking square wave where the sharpness is limited by the Bandwidth allocated to the channel. An analogue TV transmitter can send 'good' black / white / black/white patterns that will satisfy the requirements of highish quality TV. The Sideband structure of the transmitted signal will consist of a fundamental and a number of (enough) harmonics within the channel width.

Perhaps that was what the OP really meant.
 

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