1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Dipole antennas

  1. Nov 26, 2015 #1
    Alright.

    I have also a few questions concerning dipol-antennas:

    first:

    It has often been said, that the dipol antenna, in order to receive a certain frequency, needs to have a length of λ/2.
    I dont see a clear reason for that. If one would not know that, one had to calculate the Capacity and Inductance of that piece of antenna (which certainly depend on the thickness of the antenna, too)
    And each of these quantities can only be approximated. So how does it come, that we can hit the resonance by just letting the length be λ/2?
    Is there a simple explanation?
     
    Last edited: Nov 26, 2015
  2. jcsd
  3. Nov 26, 2015 #2

    nsaspook

    User Avatar
    Science Advisor

    You don't need to be at exactly λ/2 to radiate but that's one point where we have a standing wave on the antenna so the fields reinforce each other to maximize EM radiation in some direction.
     
  4. Nov 26, 2015 #3
    First:
    Referring to the video:
    Generally, the antenna is a resonant circuit. So it should be single-mode system, and that single mode (ground state) is then oscillating at resonance frequency.
    Is that correct? The lecturer then continues with antennas, with length larger than λ/2. Such antennas can then be exited by the same frequency with which the λ/2-antenna was exited, but the standing waves look different (they have more node-points). Actually those longer antennas can be thought of as being a join of many λ/2-antennas. Am am correct so far?

    Ok, but why is it at that point? Or to put it differently:

    Why is it not possible to create the same current pattern of a λ/2-antenna inside a λ/3-antenna, for example?

    As I said earlier,my approach was: The resonance frequency of a piece of wire with length λ/2 is f=c/λ. And the resonance frequency should also be calculable by f = 1/ (2Pi Sqrt(LC)) . And one should then find, that λ = 2Pi/c * Sqrt(LC). Problem here is, that L and C are difficult to find. Therefore I was either searching for a way to find L and C, or for a more instructive explanation.

    EDIT: How do you quote, so that the name appears?
     
  5. Nov 26, 2015 #4

    nsaspook

    User Avatar
    Science Advisor

    We use resonant (where the elements have electrical min/max nodes at the ends) antennas because it usually makes the matching of the antenna to the transmission line and RF source much easier as a mainly resistive impedance for max power transfer. Non-resonant antenna elements when combined with the reactive tuning elements needed to null antenna reactance and match the resistive source impedance can work just as well to radiate and receive.

    It's hard to explain in words the physical relationship of how the fields move from the transmission line to the ends on the elements so I'll use another video first to see if you can see the connection. Electrical signals do travel down wires (as fields) at nearly the speed of light but the average electron velocity is very small so a rf frequency voltage is generated between the feed point and the end of the antenna from current flow.

     
    Last edited: Nov 26, 2015
  6. Nov 26, 2015 #5
    that video does not explain why an emitting antenna with length l radiates EM-waves with wavelength λ = 2l.

    EDIT:

    What happens, if I have a sending antenna in free space, and a receiving antenna somewhere in a substance with ε>1? Can I still receive with the same performance as if the receiver were in free space?
    Or do I have to adjust the length of the antenna? The wavelength of the EM-wave in the substance would decrease. But the frequency would remain the same. From my point of view, the antenna would receive with same performance, because only the frequency is relevant.
     
  7. Nov 26, 2015 #6

    nsaspook

    User Avatar
    Science Advisor

    The emitting antenna radiates EM-waves with a wavelength equal to the free-space wavelength of the rf source signal. The length and matching components of the antenna governs the coupling of the EM energy surrounding the antenna conductors to the free space impedance of about 300 ohms as waves.
     
  8. Nov 26, 2015 #7
    Therefore the receiving antenna in the substance would need to be adjusted, because the coupling would be different, because of the different impedance of the surrounding substance? And this is why the length of the antenna need to be adjusted to the new wavelength inside the medium?

    Ok, but does that answer why an emitting antenna with length l radiates EM-waves with wavelength λ = 2l? You have been saying, that the antenna is a kind of impedance-transformation. Well, if the antenna is alone, with no other components attached to it, then the impedance of the "non existent" components would be zero.
    But it is impossible to match the 300 ohms to the zero ohm. I am really confused...
     
    Last edited: Nov 26, 2015
  9. Nov 26, 2015 #8

    davenn

    User Avatar
    Science Advisor
    Gold Member

    because as explained in the video or here on Wiki ....
    https://en.wikipedia.org/wiki/Antenna_(radio)
    its isn't the way the current and voltage distribution is when at resonance. See the animation in the section above the bit I quoted
    It primarily has to do with the voltage distribution on the dipole. For a low feedpoint impedance at a given frequency the voltage at the
    feedpoint needs to be at a minimum. As the voltage at the feedpoint increases, so does the impedance. This results in an impedance mismatch between antenna and transmitter/transmission line resulting in significant losses because reflected power of the mismatched impedance at the feedpoint.


    Dave
     
  10. Nov 26, 2015 #9

    davenn

    User Avatar
    Science Advisor
    Gold Member

    hilite the bit of text you want to quote and a little blue tab appears with +quote/reply, click on reply

    I cant make sense of that ??

    The antenna emits EM AT the wavelength of the freq being generated by the transmitter
     
  11. Nov 26, 2015 #10
    why? Why cant the minimum stay at the feedpoint? The voltage on the antenna is a standing wave, so it is the superposition of two waves travelling in opposite direction.
    These voltages waves, how fast do they propagate in the antenna? lightspeed?


    EDIT:

    and another question is: If we use plane waves, where E and B are in phase, how does the antenna deal with that? Because for a resonantly driven antenna, E and B are 90° shifted? Is this a problem?

    and what about the antenna, which is inside a substance? Do I have to adjust the length of the antenna to the new lambda inside the substance? Sorry if I have overlooked the answer
     
    Last edited: Nov 26, 2015
  12. Nov 26, 2015 #11

    davenn

    User Avatar
    Science Advisor
    Gold Member

    I answered that in my post above post #8

    slightly less if it is bare wire or other conductor ( eg. aluminium tube) around 95 - 98% of c


    just a small correction it is the magnetic H field we work with not the B field

    yes we do. putting the antenna inside, say, a fibreglass radome, increases the antenna to air capacitance, increasing the capacitance
    decreases the resonant freq of the antenna. Therefore we have to cut the antenna slightly shorter to bring its resonant freq back up to
    where we want it to be


    Dave
     
  13. Nov 26, 2015 #12
    Ah ok, thank you.

    Maybe that question should be put differently: Why is it for the voltage wave not possible to keep his minimum at the feedpoint, while the phase velocity, i.e. driving frequency is being changed.
    Sorry that I dont see the answer in #8. But the fact, that the propagation speed of voltage/current wave is nearly c, isnt this the answer?
     
  14. Nov 27, 2015 #13

    tech99

    User Avatar
    Gold Member

    Any conductor will respond to a passing EM wave, it does not depend on length. However, a half wave dipole is one of those antennas which has a resistive source impedance, arising from its resonance, which is convenient and efficient for the design of the feeder and receiver.
    An EM wave travels along a conductor at nearly the speed of light because of the electrical properties of the conductor in vacuo. LIke a tuning fork supporting a sound wave, where inertia and springiness determine the wave velocity and hence the wave length, the permeability and permittivity of the vacuum determine the speed of the wave on a conductor, and it works out to be the speed of light. LIke a string which is half a wavelength long and supporting a standing sound wave, a conductor will do the same with an EM wave.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Dipole antennas
  1. Antenna Theory (Replies: 2)

  2. Antenna resistance (Replies: 4)

  3. Optical Antennas (Replies: 13)

Loading...