Complex permittivity & Loss tangent

0. relative permittivity & susceptability

$ \varepsilon = \varepsilon_0 \varepsilon_r $ : absolute permittivity (often called simply permittivity)
ε_r = 1+χ_e : relative permittivity or dielectric constant
χ_e : electric susceptability


1. Complex permittivity of lossy media

$$ \varepsilon_c = \varepsilon_0 \varepsilon_r - j  \frac{\sigma}{\omega}\qquad (F/m) $$

It is customary to include the effects of both the damping and the ohmic losses in the imaginary part of a complex permittivity ε_c:

ε' : 전자파의 파장과 propagation과 관련된 항목
      medium안에 저장된 에너지와 관련된 항목
ε'' : 손실과 관련된 항목 (감쇄 손실 & 저항성 손실), 
       ε''가 음수이면, absorption loss을 의미
       ε''가 양수이면, gain을 의미

2. Loss tangent

a measure of the power loss in the medium

δ_c: loss angle

3. Good conductor & Good insulator

Good conductor : σ 》 ωε
Good insulator    : σ 《 ωε

A material may be a good conductor at low frequency but may have the properties of a lossy dielectric at very high frequency.