for copper and iron versus frequency. typical materials differs depending on whether the electric or mag- netic component of the field is considered. Shielding effectiveness 23.13.2 Apertures below 20dB is considered minimal, between 20dB and 80dB is average, and 80dB to 120dB is above average. Above 120dB is The curves in Figure 23.25 suggest that upwards of 200dB attenua- unachievable by cost effective measures. The perfect electric shield tion is easily achievable using reasonable thicknesses of common consists of a seamless box with no apertures made from a zero materials. In fact, the practical shielding effectiveness is limited by resistance material. This is known as a Faraday cage and does not necessary apertures and discontinuities in the shielding. exist. Any practical shield will depart from the ideal of infinite Apertures are needed for ventilation, for control access, and for attenuation because of two factors: viewing indicators. Electromagnetic leakage through an aperture in a thin barrier depends on its longest dimension (d) and the minimum 1. It is not made of perfectly conducting material. wavelength (a) of the frequency band to be shielded against, and to 2. It includes apertures and discontinuities. a lesser extent on its width (h). For wavelengths less than or equal to twice the longest aperture dimension there is effectively no Shielding effectiveness of a solid conductive barrier can be ex- shielding. The frequency at which this occurs is the 'cut off fre- pressed as the sum of reflection, absorption, and re-reflection losses quency' of the aperture. For lower frequencies (a > 2d) the shielding (Figure 23.24) as in Equation 23.7. effectiveness increases linearly at a rate of 20dB per decade (Figure Shielding 23/19 23.26) up to the maximum possible for the barrier material, given Comparing Figure 23.25 and Figure 23.26 shows that for all practical purposes shielding effectiveness is determined by the by Equation 23.9 for d >> thickness. apertures. For frequencies up to 1GHz (the present uppe