Dielectrics at High Frequencies

Dielectrics are one of the three main categories of electrical materials, the other two being conductors and semiconductors. Common dielectrics used in electronic systems are ceramics, glasses, gemstones, and polymers. Dielectrics, or insulators, are described as being very poor electrical conductors, and unlike conductors, allow for electrical fields to pass through them. However, the electric fields present within a dielectric are different from the electric field in freespace and are subject to the dielectric material properties. These properties are commonly described as the complex permittivity, or the dielectric constant and the loss tangent electrical properties of a dielectric. When a dielectric is exposed to an electric field, the electric polarizabilities of the scatters embedded within the dielectric induce dipole moments. The extent of the bulk displacement vector of a dielectric is a function of the strength of the electric field, the permittivity of free space, and the polarization density, which simplifies to the complex permittivity of the material and the strength of the electric field.

For RF applications, it is generally desirable to either have a dielectric constant precisely controlled for a given use case, or to have the lowest dielectric constant possible (air has a dielectric constant of ~1) to minimize the impact of dielectric loading on a nearby circuit. In the case of loss tangent, which is how much energy is lost from an electric field traveling through a dielectric, it is generally desirable to have the lowest loss tangent possible, unless the goal is to absorb electromagnetic energy.

In the case of dielectrics near conductors, such as with transmission lines, the dielectric fundamentally influences the impedance of the transmission lines by impacting the effective permittivity of the line. This is why RF circuit board materials often have lower complex permittivity and much more precisely controlled physical dimensions than circuit board materials not suited to RF applications. Some dielectrics’ complex permittivity characteristics change as a function of frequency, usually worse at higher frequencies. Otherwise, the reactive impedance of a circuit is a function of frequency, and in the case of capacitors, decreases as a function of frequency. This frequency dependent impedance phenomenon is also why certain circuit board materials are often advertised for a maximum frequency for use or are not recommended for high frequency applications.

The electrical characteristics of many dielectrics are also a function of the temperature, humidity, and even pressure in the environment. At some temperature, pressure, and electric field strength, virtually all dielectrics break down, which results in maximum operating ratings. The operating dynamics of dielectrics are sometimes the limiting factors in the ratings for transmission lines and other RF components and devices, especially if the dielectrics are polymer-based. For these reasons, many Hi-Rel Rf applications use ceramic, glass, and gemstone (mineral) dielectrics which tend to exhibit higher operating temperatures than polymer-based dielectrics.