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Are all antennas traveling wave antennas?

  1. Mar 21, 2015 #1
    Hi, i've been reading a lot about traveling and standing waves, and i am right now reading about traveling wave antennas, to my understanding ALL antennas should be traveling wave antennas, but this is not the case of what i read online, and there is also not too much information about it.

    For instance, the wikipedia article states that the most common type of traveling wave antenna are the beverage and rhombic antenna.

    I can't find much information about this, care to explain to me why? Is it because the propagation pattern of the wave has to meet some standards in order to be in the traveling wave category? Or are all antennas(like the ones in the microwave dishes and all others used in transmission and reception) really traveling wave antennas as i believe? and so are all waves(talking about radio waves in this case) that propagate traveling waves? Does modulation play any role here?

    Thanks in advance
     
    Last edited: Mar 21, 2015
  2. jcsd
  3. Mar 21, 2015 #2
    Imagine the antenna wire is a transmission line driven from one end. As the electric wave passes each electron, it give it an acceleration, and this is what causes the radiation. When the wave reaches the end of the wire, it can be absorbed by a resistor or reflected. In the former case we have a traveling wave antenna - the wave travels only one way. But if the wire is open circuit at its end, the wave is reflected, and travels back along the wire the other way. As it does so, it again causes radiation as it passes each electron. The wire now has waves travelling in both directions, and the combined effect is to create a standing wave. The energy in the standing wave is stored energy, and if the transmitter is turned off, it will take a finite period of time for the energy to be lost. Most antennas are standing wave type, because there is no wasted energy in a terminating resistor.
     
  4. Mar 21, 2015 #3
    Hi, and thank you really much for your explanation, now i understand. But this makes me rise new questions. Before, in my mind, i thought all the antennas were traveling wave type because that's what they radiate, right? The radiation of all antennas is a traveling wave, since they radiate waves that travel through space. Am i right with this assumption? Is the radiation on a traveling wave antenna the same as on the standing wave antenna?

    I didn't know that what makes a traveling wave antenna is the way the wave propagates on the wire itself.

    Thank you again!
     
  5. Mar 21, 2015 #4
    The EM wave that is radiated from an antenna is a travelling wave, whether the antenna is a standing wave or travelling wave type. (By the way, it is possible to have standing EM waves, if, for example, there is a sheet of reflecting metal near the antenna or if you radiate the wave into a metal box).
     
  6. Mar 21, 2015 #5
    Alright, thank you!
    One last question, are both of them used for the same purposes? Or are there uses that one would work better at than the other would? If we're explicitly talking about them being travelingwave and standingwave
     
  7. Mar 21, 2015 #6

    davenn

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    @richjtf
    its a pity you didn't continue to quote the wiki article stating the difference in the travelling wave antenna and non-travelling wave antenna .....

    because therein lay the answer to your question :wink:

    cheers
    Dave
     
  8. Mar 21, 2015 #7
    ahaha, you got me. I read that, but i didn't quite get it, nor why does it do it. The answers tech99 gave me cleared every doubt out
     
  9. Mar 21, 2015 #8

    davenn

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    that was also answered :wink:

    personally for most of my radio work, I prefer resonant antennas .... there are ways to make them wideband
    for instance I have a log periodic yagi that is resonant from 2 to 12 GHz :) .... that's pretty spectacular bandwidth !!
     
    Last edited: Mar 22, 2015
  10. Mar 21, 2015 #9
    Thanks for pointing that out. I want to understand the topic widely, every time i get home from work college i sit down and read for as long as i can. I want to learn as much about antennas as i possibly can. Any good book or site where i can find nice information? I've learnt about and made half wave lenght dipoles only, so far..
     
  11. Mar 22, 2015 #10

    davenn

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    A VERY good source that is reasonably easy reading is the ARRL Antenna Handbook
     
  12. Mar 22, 2015 #11
    You
    You may have seen standing waves on a guitar string. The string is tuned to one frequency, so standing wave antennas tend to be best over a small range of frequencies. Often the antenna is about half a wavelength long for the frequency being used.
    A travelling wave antenna can work over a wide range of frequencies, but it must be very long so that most of the energy is radiated it reaches the terminating resistor.
    If there is space, a travelling wave antenna can be a simple way of making a directional antenna, as the waves tend to be radiated in the direction towards the resistor.
     
  13. Mar 22, 2015 #12
    That made a lot of sense to me, thank you. I don't really want to continue bothering you since you've helped me so much already, but i that made me rise a new question, and i don't think want to make a new post.

    In my mind i'm picturing both, traveling wave antenna, and the standing wave antenna. In my mind the traveling wave antenna charges change because of course, the electric current is AC. So that creates a changing electric field which creates a changing magnetic field and so on so it radiates.

    Then i picture a standing wave antenna, the current wave reflects at the end of the antenna creating a standing wave, which of course changes amplitude meaning charges change their polarity, i don't quite picture it the same way of the traveling antenna, but this is as well happens of course because of the AC source.

    Could i say that both type of antennas radiate the same way, because of the same reasons? I found myself talking to a friend of mine explained the new knowledge i had gained, and he couldn't understand why standingwave antennas radiate. I explained it to him the same way i just put it down, but he still didn't quite get it. It's easy to get confused because of the standingwave, name.
     
  14. Mar 23, 2015 #13
    There are two things at work with antennas. First, we drive it as a transmission line, so a wave travels along it. As this wave passes an electron in the wire, it gives it a brief acceleration. This makes it radiate. For a standing wave antenna, the mechanism is the same; there are transmission line waves in both directions, which we can consider separately. Each causes an electron to have a burst of acceleration and radiate.
     
  15. Mar 23, 2015 #14

    berkeman

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    Actually, it is a smoothly varying sinusoidal acceleration in both cases, no? :smile:
     
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