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Antenna Output - at a high level, how can i visualize the beam?

  1. Jan 29, 2009 #1
    I'm having difficulty visualizing what the output of an antenna looks like. Would someone care to explain?

    This is where I am stuck. Imagine we have a point source that transmits energy. If at time t0 a pulse occurs which outputs a spherical wave, and some time later (say t1) another pulse occurs... From this I can see that we may define frequency, wavelength (say the wave travels at speed of light), etc...

    Is this spherical wave made up of a bundle of cosine functions that propagate orthogonal to the wavefront? That is, do these cosine functions transmit outwards from the point source?

    What does this diagram represent?
    [​IMG]

    Is that the intensity of the cosine function given some spatial variable? So when we check the intensity in the "main" direction of the beam we may have, A(t)cos(O(t)) but if we angle off from the "main" direction we may have B(t)cos(O(t)) where A > B?


    Any help, or references to this would be highly appreciated.
     
  2. jcsd
  3. Jan 29, 2009 #2

    berkeman

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    Staff: Mentor

    I googled antenna pattern, and got lots of great hits. Here's the hit list:

    http://www.google.com/search?sourceid=navclient&ie=UTF-8&rlz=1T4GGLL_enUS301US302&q=antenna+pattern

    The diagram you show is for a multi-element antenna array, which has its main beam to the right. You could get this pattern with a multi-element Yagi antenna, for example, where the dipole elements are vertical, and you are looking down on the antenna and radiation pattern from above.

    Think of a 2-element vetical dipole array, where you have two dipole antennas spaced a quarter wave apart, and fed with waveforms that are a quarter wave out of phase. In one direction the vertically polarized (E-field) waveforms will add in phase, and in the other direction they will cancel. This gives you a front-to-back directionality asymmetry in the radiation (and receiving) gain pattern. I believe that with the 2-element array I just described, that you only get one main lobe, and one zero-gain direction.

    With more than 2 elements, or a 2-element array with wider spacing or different phase feeds, you will get multiple cancellation directions, and multiple lobes where the signal gain is bigger or smaller. The graph above shows the gain as a function of azimuth (again, assuming you are looking down on a vertical antenna array).

    Hope that helps. Post more specific questions after you get a chance to read through the google hit list.
     
  4. Jan 30, 2009 #3
    Thank you Berkeman. That google search really cleared some aspects up. However, I am still confused. I'll try to frame my example.

    Let's say we have a parabolic dish that transmits a beam to another parabolic dish. Let's say that it is has no side lobes, and that the radiation pattern is a rectangle function (i.e. Intensity=A for -3deg < theta < 3deg).

    What is this beam made up of?

    I know at far distances, things can be modeled as plane waves. So is a beam made up of a summation of cosine functions as I've tried to depict below.
    (beam edge)---------------------------------------------------
    ===== a1cos(O1(t))============ >>>
    ===== a2cos(O2(t))============ >>>
    .
    .
    .
    ===== aNcos(ON(t))============ >>>
    (beam edge)---------------------------------------------------
     
  5. Jan 30, 2009 #4

    berkeman

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    Staff: Mentor

    I'm less familiar with dish anttennas. If you had a vertical dipole transmitting to another vertical dipole, you just have the one frequency (say, unmodulated carrier).

    I would think in the dish case, you would only have the one frequency as well, and one phase if the carrier is unmodulated. Sorry that I'm not understanding your question.
     
  6. Feb 1, 2009 #5

    Ouabache

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    Homework Helper

    It sounds like you are trying to picture what the signals look like as they radiate from the antenna. What berkeman has explained does hold for a complex waveform being transmitted over a carrier wave. As you've noted, the information signals may be thought of as the summation of cosine functions (or sine functions if you like). Viewed on an oscilloscope, they won't resemble a cosine function. They appear as a complex wave.
    Here is an example of a complex wave made up of pure sinusoids
    [​IMG]
    (ref -- www.acoustics.salford.ac.uk)

    Before they leave your parabolic dish, they are modulated (mixed) with single carrier wave at an RF frequency. So the information signals leaving the antenna are not lined up, side by side, as you've indicated. Since information signals are relatively low in frequency, they do not radiate from an antenna. For example in an AM or SSB transmission, once the information signal is modulated with an RF carrier, the carrier frequency dominates the shape and the signals all appear the same, leaving your dish.
     
    Last edited: Feb 1, 2009
  7. Feb 1, 2009 #6

    Sorry for the stupid question. I think I am just "seeing" things wrong in my head.

    Maybe this analogy will help, when light hits a piece of film it "captures" the various energy levels at different spatial coordinates on the film:
    [​IMG].

    Imagine that we split the piece of film up into pixels, each of those pixels had different frequencies hitting them, thus the varying intensities.

    Is it the same with an antenna?


    Here's an example

    If we had a dish:

    1. it transmits a pulse of energy
    _____>
    (
    (
    ( _____>

    2. the pulse moves forward
    _______>
    (
    (
    ( _______>

    3. the pulse reflects off a target and comes back

    ( <_______
    (
    ( <_______

    4. the pulse is absorbed by the dish

    ( <---------
    (
    ( <---------

    5. now when the pulse is absorbed by the dish, is it similar to the film? Do we break the dish up into "pixels" and capture the frequencies and amplitude at each of these "pixels"?
     
  8. Feb 1, 2009 #7

    Ouabache

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    There are no discrete points on the dish receiving different pixels of spatial information. Instead the information is combined into a complex signal. If your pulse of energy contained useful information, such as the contour of a surface; the received signal is reflected off the dish and concentrated onto a collector element at its focal point. The signal then propagates from the collector, down the transmission line to the receiver circuit. It is inside this circuitry, that the surface contour information is extracted from the signal by demodulation.
     
    Last edited: Feb 1, 2009
  9. Feb 2, 2009 #8
    The lobes in your picture look a little anemic. Ideally, each lobe would be represented by some set of contours.

    In any case the electric and magnetic field carrier stengths down the center of a lobe should be

    [tex]E = (1/r^2) E_{0} cos \left( kr - \omega t) \right)[/tex]
    [tex]B = (1/r^2) B_{0} cos \left( kr - \omega t) \right)[/tex]

    [itex]\omega[/itex] is the frequency in radians per second
    k is the wave number, [itex]k= 2 \pi / \lambda [/itex]
    [itex]\lambda[/itex] is the wavelength
    r is the radial distance from the antenna; r is large compared to the wavelegth
    [itex]\omega / k = c[/itex], the speed of light.

    The carrier is modulated, as Ouabache was saying.
     
    Last edited: Feb 2, 2009
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