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Homework Help: Parabolas and radio waves

  1. May 25, 2013 #1
    1. The problem statement, all variables and given/known data

    Parabolic radio telescope. Dish is fixed and across its centre a cable is suspended carrying a beam on which equipment is mounted to pick up radio waves.

    Basically it's just a parabola on the cartesian plane.
    How would you adjust the parabola ( focus, directrix, vertex etc ) so that the maximum intensity radio waves may be intercepted.

    and any other ways such as size of dish or position of beam etc.


    2. Relevant equations
    N/A more of a worded problem

    3. The attempt at a solution
    I can find information about how the size of the dish would maximize the wave intensity etc but nothing specific which is what im looking for.

    Where should the focus be on the cartesian plane? e.g How far up the y axis? How far across the x axis?
    Same question for the directrix as they relate to each other ^^

    Should the vertex be at the origin of the cartesian plane? Or shifted to another point? which?

    Last edited: May 26, 2013
  2. jcsd
  3. May 26, 2013 #2

    Simon Bridge

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    Not enough information.
    i.e. where and what sort of radio source is it?

    in general - how do you use a parabolic dish to receive radio waves?
    Where would you point the dish relative to the direction of the radio source?
    Where would you put the radio-receiver in terms of the geometry of the dish?

    I cannot tell what question you are asking in "attempt 2".
  4. May 26, 2013 #3
    Edited original post
  5. May 26, 2013 #4

    Simon Bridge

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    Cool: that is clearer ... questions still the same.

    i.e. the equipment to pick up radio-waves ... where should you put that to get best reception?
  6. May 26, 2013 #5
    The only information i was able to find was about what types of parabolic antennas there are. So far the only thing i can come up with is to adjust the focal point so it is directly opposite the bottom of the dish (Move the focus further up the y axis). This would increase the latus rectum and therefore will be at it's maximum diameter. This larger diameter would then be applied to Area and Volume rules ( for the dish ) which would increase the reception of incoming radio waves.

    I'm still wondering about the vertex though... (i seriously can't find any more)
    Should the vertex be at the origin or shifted somewhere else (h,k)??? Stuff like this.
    Last edited: May 26, 2013
  7. May 26, 2013 #6
    There are many parabolic radio antennas all over the world. The biggest that I know of is at the Arecibo Observatory in Puerto Rico. I am pretty sure the fixed dish is parabolic although spherical reflectors will work also. For best reception the actual radio receiver should be placed at the focus of the parabola. If the dish is fixed, then the pointing can be changed a little by moving the location of the "sky room" Go look at the Arecibo Observatory website. Theoritically, all of the RF energy that hits the dish will be reflected into the receiving antenna located at the focus. Most parabolic antennas operate in the microwave region, i.e. 2 - 32 GHz. The TV satelite antenna you see all over the place are a section of a true parabolic antenna, not the entire surface of revolution. The receiving element is or should be at the focus. All of the antennas that trackl the Deep Space satelites are operated by the Jet Propulsion Laboratory. You can go to their web site and search for the DSN, meaning the Deep Space Network, or merely google DSS-14 and see what you get.
  8. May 26, 2013 #7

    Simon Bridge

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    Only where the source is approximately plane waves.
    But that is pretty much how people set up the dish IRL - the radio-wave receiving stuff goes at the focus of the dish.

    @rqyytrf: forget about looking stuff up for a bit - just think of the dish as a big concave mirror ... you want light (the radio waves) to concentrate on the apparatus at the end of the boom ... what do you do? You know this one already.

    You can adjust the shape of the mirror (affects focal length and directrix), and you can adjust which way the dish points by turning it.
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