r limit for radiated emissions standards) and a minimum shielding of 20dB the SE(db) = 20 log d (mm) F(MH2)) maximum hole size you can allow is 1.6cm. If individual perfor- ations are spaced closed together (hole spacing < (23.9) + 20 log 1 + In reduction in shielding over a single hole is approximately propor- E-FIELD TRANSITION TO PLANE WAVE H-FIELO FOR r = 1 METER (48 MHz) H-FIELD COPPER: Hy = 1 1111111 10 kHz 100 KHZ 1 MHz 10 MHz 100 MHz 1 GHz 10 GHz 1 barrier thickness FREQUENCY E-FIELD TRANSITION TO PLANE WAVE FOR r = 1 METER (48 MHz) H.FIELD H-FIELD IRON: He==200 u,= . 017 10 kHz 100 kHz 1 MHz 10 MHz 100 MHz 1 GHz 10 GHz FREQUENCY t barrier thickness Figure 23.25 Composite reflection and absorption losses for copper and iron (Source: Tecknit) 23/20 Safety and EMC tional to the square root of the number of holes. Thus a mesh of 100 Where screwed or riveted fastenings are used they are spaced 4mm holes would have a shielding effectiveness 20dB worse than a as closely as possible. A good rule is no farther apart than 2/20, single 4mm hole. Two similar apertures spaced greater than a half where a is the wavelength at the highest frequency of interest. wavelength apart do not suffer any significant extra shielding reduc- Keeping machining tolerances as tight as possible within cost tion. constraints. Where this still does not give a flat enough mating surface (which is likely to be most cases) or an environmental 23.13.3 Seams seal is needed, or where good seam conductivity without fasteners is required, such as at a hinged panel, conductive gaskets may be used. These are available either as a wire mesh An electromagnetic shield is normally made from several panels or spiral over rubber, or as an elastomer loaded with a conduc- joined together at seams. Unfortunately, when two sheets are joined tive material such as silver flakes. Selection of the right gasket the electrical conductivity across the joint is imperfect. This may be depends on environmental factors and the required conductiv- because of distortion