Permittivity is a property of a medium or a region of space. On this page, I'm going to give its meaning as it relates to antenna theory, with only a small explanation on the physics behind it. The permittivity, as we saw relates the electric flux density to the electric field. The permittivity is given in units of Farads/meter; since Farads relates to capacitance, a material with a higher permittivity can be thought of as storing more electrical energy.
The permittivity of a vaccum (often called free space) is given by:
The above is roughly the permittivity of air on earth. Suppose we have some other dielectric medium (glass, rubber, wax or other non-conducting material). The permittivity describes how an electric field is affected within a medium. The E-field tends to polarize the molecules within the material, which make up a net electric field that opposes the applied E-field. As a result, the total E-field is less than it would be in a vacuum. This effect is quantized in terms of the permittivity - which can be anisotropic (direction dependent) and frequency dependent.
As an antenna engineer, the permittivity affects the speed of propagation of a wave through a medium and also its wavelength. The permittivity of a medium is most often given as a relative permittivity:
The speed of wave propagation in a given medium is given as:
Hence, if a medium has , then the speed of wave propagation in the medium will be half as fast as in free space (1.5*10^8 m/s). As a result of the speed being slowed, the wavelength of a plane wave decreases in size as well (the frequency remains constant):
The above fact is often used in antenna-miniaturization: since resonant antennas are often a half-wavelength in size, if they are placed in a medium with a higher permittivity the required length decreases and hence the antenna will be smaller.
Permittivity alters the direction of travel of a wave incident upon a medium through snell's law. In addition, the reflection and transmission coefficients of a wave travelling from one medium to another is influenced by the permittivity.
See also permittivity from Maxwell's Equations for more information on the physics and properties of permittivity.
Maxwell's Equations (External Link)