Meaning
Composite dielectric constants represent the averaged electrical characteristics experienced by a signal as it travels between a conductor and a substrate. Microstrip effective permittivity accounts for the fact that some of the electromagnetic field exists in the air above the trace and some in the material below. This value is always lower than the bulk permittivity of the substrate itself.
The definition applies specifically to open transmission line structures on a circuit board.
Signal Propagation
Velocity and wavelength on the board depend on this calculated value. When designing a circuit, the microstrip effective permittivity is used to determine the physical width of the traces.
Material Property
Substrate thickness and the dielectric constant of the laminate are the primary factors that influence the result. A microstrip effective permittivity calculation must also consider the thickness of the copper and the presence of any solder mask. Because air has a permittivity of one, narrow traces tend to have a lower effective value than wide traces.
This relationship is essential for matching the impedance of the antenna feed lines.
Design Constraint
High-frequency circuits require a stable material to prevent the microstrip effective permittivity from changing with temperature or humidity. Any drift in this value will shift the resonant frequency of the antenna and the phase of the signals.
