A dielectric is an insulating material that, when placed in an electric field, stores energy by polarising its molecules. The key RF properties of a dielectric are its relative permittivity (dielectric constant) and loss tangent.

Relative Permittivity (\(\varepsilon_r\))

The dielectric constant \(\varepsilon_r\) relates the permittivity of the material to that of free space: \(\varepsilon = \varepsilon_r\varepsilon_0\). It determines how much wavelengths are shortened in the material: \(\lambda = \lambda_0/\sqrt{\varepsilon_r}\).

Loss Tangent (\(\tan\delta\))

A real dielectric also dissipates energy. The loss tangent represents the ratio of the imaginary to real part of the complex permittivity:

Lower \(\tan\delta\) means lower dielectric loss. Attenuation in a transmission line increases with loss tangent and frequency.

Common PCB Substrate Materials

• FR4 — \(\varepsilon_r \approx 4.4\), \(\tan\delta \approx 0.02\). Standard PCB material, adequate to ~5 GHz.
• Rogers 4003C — \(\varepsilon_r = 3.55\), \(\tan\delta = 0.0027\). Low loss, stable with temperature, suitable to 77 GHz.
• PTFE (Teflon) — \(\varepsilon_r \approx 2.1\), \(\tan\delta < 0.001\). Excellent high-frequency performance, used in coaxial cable dielectrics.
• Alumina (Al₂O₃) — \(\varepsilon_r \approx 9.8\), \(\tan\delta \approx 0.001\). Used in MMIC substrates.

Frequency Dependence

Both \(\varepsilon_r\) and \(\tan\delta\) vary with frequency. For PCB design above 1 GHz, always use substrate data measured at the operating frequency rather than the low-frequency value printed on datasheets.