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I'm relatively new to antennas. In most of the textbooks, it is said that we want to minimize standing waves in the transmission lines by matching the characteristic impedance of the line with the antenna's impedance.

My confusion is regarding the following two questions:

  1. If the antenna and the line are perfectly matched there are no standing waves in the line?
  2. If so, what happens to these traveling waves once they reach the end of the dipole antennas. Most pictures show that there are actually standing waves on two sides of the dipole. It is clearly seen here that there are standing waves:

Dipole antenna standing waves animation

Image source: "Animation showing the sinusoidal standing waves of voltage, V (red) and current, I (blue) on a half-wave dipole driven by an AC voltage at its resonant frequency" from Dipole characteristics on Wikipedia, Dipole antenna

Does it mean that standing waves occur in line as well?

PS. An excerpt from C. Balanis book on antennas third edition page 18, which clearly says that there are standing waves in the dipole antenna.

enter image description here

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  1. If the antenna and the line are perfectly matched there are no standing waves in the line?

Correct. Standing waves occur as a result of reflections; if a reflected wave meets an incident wave on a transmission line they can interact to cause constructive and destructive interferance at certain points along the length of the transmission line, this is what causes "standing waves". Having a perfectly matched network means having no reflections and hence no standing waves.

  1. If so, what happens to these traveling waves once they reach the end of the dipole antennas.

Nothing special happens when they reach the end. The entire length of the antenna is responcible for coupling the wave into the air (or free space), which it does by oscillating at the frequency it is designed to transmit (think of it as vibrating when you shake it at a certain frequency).

The job of an antenna is to convert the impedance seen by the EM wave, from the 50ohm or 75ohm characteristic impedance of the transmission line, to the 377ohm impedance of free space. The better the antenna is, the less of the wave that reaches it will be reflected back into the cable, and the more will propagate through free space.

Most pictures show that there are actually standing waves on two sides of the dipole.

Correct. Although the antenna does not reflect any of the EM wave back unto the transmission line and cause standing waves in the transmission line, it does resonate at the frequency that it is designed to transmit at. Resonance however is not to be confused with standing waves.

I would advise you to read up on resonant circuits, I think it can help you understand what happens to the wave inside the antenna (it is roughly equivalent to a parallel RLC circuit)

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  • \$\begingroup\$ Thank you for your answer! Am I right that standing waves in antenna occur because antenna's and free space's impedances are not matched? Why don't we use 377Ohm antennas then? I'm also confused about your answer about the last part, where you say that it is correct that standing waves occur in the antenna, but then you say "does not .. cause standing waves in the transmission line". Standing waves occur in the antenna, even though there are no standing waves in the line? \$\endgroup\$ – TheDorkSide Feb 3 at 14:27
  • \$\begingroup\$ "Am I right that standing waves in antenna occur because antenna's and free space's impedances are not matched?". No, they occur because the antenna resonates. "Why don't we use 377Ohm antennas then?" We do, in a sense, all antennas are designed to couple to free space with an impedance of 377ohm, and to couple to the cable with an impedance of 50 or 75 ohm typ. "Standing waves occur in the antenna, even though there are no standing waves in the line?" Yes, again; read up on resonant circuits! you really need to understand them to be able to understand this. \$\endgroup\$ – Vinzent Feb 3 at 14:32
  • \$\begingroup\$ "Having a perfectly matched network means having no reflections and hence no standing waves". I read about resonant circuit, and it makes sense. However, I'm still confused about standing waves in the dipole. What could you say about this explanation: youtube.com/watch?v=RF5r64fmFhU. Here he shows that standing waves in the dipole occur as a result of reflection from the open circuit of the feed, where the dipole is just a bended part of the line. And most of the books I read give similar explanation. Can you recommend any textbook about resonant circuit explanation of antennas? \$\endgroup\$ – TheDorkSide Feb 16 at 6:19
  • \$\begingroup\$ The video you linked to is long and you didn't include a time stamp, I don't have the patience to watch through it all.. The simplest answer I can give is that while many people try to give intuitive explanations of how antennas work it is really not an intuitive subject because EM waves don't behave like anything we are used to (that we can observe) in the visible/tanglible world around us, you DO absolutely need to start with the math in order to really understand what goes on in antennas, and then when you fell at home in the equations you can form your own intuitive understanding. \$\endgroup\$ – Vinzent Feb 16 at 11:19
  • \$\begingroup\$ ..That sayed you keep referring to "standing waves" in the antenna, stop doing that, there are no standing waves in the antenna!, the antenna R-E-S-O-N-A-T-E-S., resonance is a completely different concept from standing waves. Marko put it well in his answer where he explains that standing waves have E and M fields that are in phase (power is being propagated from one end to the other) while when you have resonance, E and M waves are 90 deg out of phase, ie. the power is stationary, not propageting from one place to another, but stored in the energy of the resonating antenna. \$\endgroup\$ – Vinzent Feb 16 at 11:23
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I would like just to add a comment on previous answer:

The TEM wave has its E and H field completely in phase. In the matched transmission line, also the voltage and current are in phase (no standing wave), hence the standing wave means that voltage and current (E and H field) are 90 degrees out of phase.

Before becoming a TEM wave it is "born" in the vicinity of the antenna, which itself exhibts this strange phenomena: the EM wave has E and H field out of phase - near field, and then it transforms to TEM with E and H in phase - far field.

So, it is really difficult to answer if the antenna has a standing wave or not, as it is somewhere in between of transformation from electric signal to EM wave.

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