Click here to go to our page on isotropy and anisotropy
Click here to go to our page on Keffective
Click here to go to a page on measuring dielectric constant
Click here to go to our companion page on permeability
Click here to go to our basic concepts of microwaves page
Click here to go to our main microwave materials page
Click here to go to our list of dielectric constants of miscellaneous materials
Click here to go to our discussion of capacitors
Permittivity is a property which is associated with how much electrical charge a material can store in a given volume. The units of permittivity are Farads/meter. The Farad is named after Michael Faraday. Permittivity is what controls the value of a capacitor, beyond its physical dimensions.
Permittivity of free space
The permittivity in vacuum (free space) is denoted as 
0. Its value is 8.85418782E-12 Farads per meter (see our page on physical constants). No material has a permittivity lower than that of a vacuum.
Relative permittivity (the dielectric constant)
Materials other than vacuum have permittivity higher than 0, often they are referred to by their relative permittivity, denoted R:
material=0xR
In microwaves, we often refer to relative permittivity as the "dielectric constant". These terms are interchangeable, but "relative permittivity" sounds more scholarly if you want to pretend to be a nerd. Saying "relative dielectric constant" is like Yogi Berra saying "deja vu again". Quite often dielectric constant is referred to as "DK" or "Dk". It's not "DC" for obvious reasons, Grasshopper.
But lots of otherwise-smart people flub this microwave IQ test. Here is a screen capture from a well-known EDA software package you might recognize. The user has pulled up the information on his/her microstrip substrate parameters. Er is described as "relative dielectric constant". Come on, guys, fix that! This is a good reason for holding a design review on a software product....
Note: an alert professor points out the the relative permittivity is not always a constant for a given material, it can change with frequency, temperature, date of manufacture or supplier, or even direction of propagation. Pay attention to this valuable comment, because some day you will design a microstrip filter and find out the hard way that we usually don't know EXACTLY what the dielectric "constant" (which really isn't a constant) is. If you know the value within +/-1%, you are in better shape than the rest of us!
Most microwave materials have dielectric constant between 2.2 (PTFE) and 9.9 (alumina), because so many materials are engineered by mixing these two materials. If particle size is kept small compared to a wavelength, and the ratio of materials is well maintained from batch to batch, the mixture will be well behaved. See our pages on soft and hard substrate materials for data on a wide variety of materials. Here's a page on miscellaneous materials.
Permittivity can also be a function of direction, which leads us to the definitions of two classes of materials:
Isotropic material
A material in which the permittivity is NOT a function of direction. Most materials are isotropic, thankfully! More information on isotropy and anisotropy is provided on this page.
Anisotropic material
A material in which the permittivity IS a function of direction. An example is sapphire. The interesting thing about sapphire is it has the same atomic composition as pure alumina (Al2O3) but alumina is amorphous while sapphire is crystalline. Thanks to Luciano for pointing this out!
Effective permittivity
Note: Keffective is further discussed on this page.
In non-TEM transmission lines such as those realized in microstrip media, most of the electric fields are constrained within the substrate, but a fraction of the total energy exists within the air above the board. The effective permittivity (a.k.a. effective dielectric constant) takes this into account. The effective dielectric constant of a fifty-ohm transmission line on ten mil alumina is a number somewhere around 7, which is less than the dielectric constant of the substrate bulk material (9.8), but more than that of air (which is 1).
Another example of an effective dielectric constant is if you were to create a stripline circuit using two sheets of substrates with different dielectric constants. To a first order, the effective dielectric constant would be the average of the two materials' dielectric constants. A third example is coplanar waveguide transmission lines with air above the substrate. Here the effective dielectric constant is very nearly the average of the substrate dielectric constant and one (the relative dielectric constant of air). Thus the effective dielectric constant of CPW circuits on GaAs (ER=12.9) is approximately 6.5.
Measuring dielectric constant
This topic has been moved to a new page.
Complex permittivity and loss tangent
Permittivity is actually a complex number, so "epsilon" is made up of two parts:
(Thanks to Maarten for correcting another recurrence of us mixing up "permeability" with "permittivity" on this dyslexic web site...) Epsilon single-prime is the number we usually deal with, and causes no loss, and in most day-to-day engineering you don't see the prime notation. The imaginary epsilon double-prime is the culprit. Microwave engineers usually deal with the ratio between the two, which is called tangent delta, or tanD (say "tan-dee"), for short. If tanD is zero, there is no loss due to dielectric. For example, dry air has no dielectric loss.
Note that relative permittivity is the ratio of epsilon prime to epsilon zero:
The loss tangent creates a loss that is proportional to frequency. See our page on transmission line loss calculations for more information.
Loss tangent is often called "dissipation factor" and abbreviated "DF" or "Df" (as opposed to dielectric constant which is abbreviated "DK".