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

Resonant wavelength of a cavity resonator using different internal materials?

  1. Jan 2, 2012 #1
    Hello everyone,

    Recently I have been studying cavity resonators.

    I noticed that all cavity resonators have air in them, but I was wondering what the effects would be if something other than air was used? For the sake of simplicity lets say we put a dielectric material in it with a dielectric constant of 50.

    What effect would this have on the resonant wavelength of the cavity and on the cavity in general?

    What if the cavity was filled with something that was more conductive than air, what would this do?
  2. jcsd
  3. Jan 3, 2012 #2


    User Avatar
    Science Advisor
    Gold Member

    This is known as dielectric loading and is very common. What it does depends on the geometry etc, but in general it will shift the resonance frequency downwards.

    The immediate effect of filling a cavity with something conductive is to reduce the Q value.

    The most common way of measuring the dielectric constant of a material is to measure the resonance frequency of a cavity with and without in the materials in the gap, by measuring the frequency shift and the change in Q you can get the dielectric constant and the loss tangent,

    edit: It is of course the wavelength that shifted upwards, and the frequency downwards....
    Last edited: Jan 3, 2012
  4. Jan 3, 2012 #3
    The wavelength of a travelling wave in a dielectric filled waveguide will be reduced for a given frequency because its velocity will be lower. The wavelength of a resonant cavity (assuming the same mode) will stay the same. That is determined by the boundary conditions (i.e. the shape of the cavity). The frequency would need to be reduced to achieve resonance.
    Filling cavities at microwave frequencies is only practical at low power levels. The dielectric gets hot, which is why coax isn't used much above 1GHz.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook