The penetration depth is defined as the depth which the power is decayed to 0.368 (l/e) of its maximum value [13]. It is varied depending on the loss factor and the resonant frequency. Usually, the higher loss factor is, the lower the penetration depth will be. When the wavelength is increased, the penetration depth is increased as well. The relationship between wavelength and penetration depth is expressed as (6) where dp is penetration depth (cm) and c is speed of light (3x〖"10" 〗^"8" )