Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Creating shadow of an object by radio waves

  1. Jun 21, 2011 #1
    Hi guys,
    We all know that a shadow could be generated by radiating light waves to an object.
    Would it be possible to design a system with radio waves, an object made by a special material and a sensor plane in order to create a shadow of the object on the plane?
  2. jcsd
  3. Jun 21, 2011 #2


    User Avatar
    Science Advisor

    The difference between light waves and radio waves is mostly a matter of frequency and method of detection.

    The sharpness of the shadow depends very much on the wavelength of the radiation hitting the object and also on how opaque the object is to the radiation.
    Radio waves will pass easily through a brick wall and fairly easily through a human, but they are stopped by an area of metal.
    Long wavelengths would cast a more fuzzy shadow than short wavelengths due to diffraction effects around the edges of the object.

    Since we can't see radio waves, we would need some way of detecting their presence or absence in the shadow area. This is normally done with some type of antenna which has to be comparable in length to the wavelength of the radiation.
    This would certainly favour short wavelengths as radio antennas can be many meters long for longer wavelengths.

    So you would probably need to be using high microwave frequencies (ie above 10 GHz) and this could be achieved with a dish antenna that scanned across the object while transmitting.

    It isn't really possible, but those are some suggestions on how this might be achieved.
  4. Jun 22, 2011 #3
    Thank you very much for your very clear answer, vk6kro.
    It is really appreciated.
    Wish you all the best! :D
  5. Jun 22, 2011 #4


    User Avatar
    Science Advisor
    Gold Member

    Anyone here ever heard of a tower shadow? It is when an antenna is mounted to the side of a tower which causes a favored direction in sensativity.
    Last edited: Jun 22, 2011
  6. Jun 22, 2011 #5


    User Avatar
    Science Advisor
    Gold Member

    Or going under a bridge while listing to your autos FM radio.
  7. Jun 23, 2011 #6


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    This is exactly the same effect that you get with an H antenna. One element (the reflector) is fed and the other is made (appropriately) just a bit longer and at the correct spacing. It absorbs and re-radiates a signal from the driven element. The two resulting signals add in phase in tyhe wanted direction and cancel in the 'backward' direction. Adding shorter, 'director' elements, in front will have the effect of making the antenna more directive. (A Yagi antenna)

    In this case, there is no power loss but there are occasions where an absorbing structure (say a hill or building) is in the way, most of the incident power is just absorbed and dissipated resistively - there is no 'reflection'.
  8. Jun 23, 2011 #7
    Yes to both questions.

    To a large extent it depends on the scale you are interested in. Propagation plots reveal the shadows created by hills even at VHF frequencies. For shadows at smaller scales you must use higher frequencies.

    Manufacturers of omnidirectional antennas sometimes publish the antenna patterns of the antenna by itself and when mounted off the side of a tower. Again, the effect is greater at higher frequencies.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook