Building Attenuation and the Impact on Product Susceptibility

Abstract

Electronic equipments can malfunction in the presence of high electromagnetic ambients such as near airport or weather radars or on upper floors of buildings near FM/TV antennas located on other nearby building roofs. Steel-reinforced concrete commercial office buildings offer natural shielding even in offices with large windows located around the periphery. Typical attenuation at A-M broadcast frequencies is 50 dB. At FM/VHF TV and lower UHF frequencies, the attenuation is about 4 dB and 8 dB respectively. When the product location is in the center of a floor of a building having approximately 25,000 sq ft, rather than at the periphery, this shielding may approximate 60 dB at A-M, 15 dB at FM/TV, and 30 dB at lower UHF frequencies. Thus, certain equipments, such as computers should be located in the core of a building if natural shielding protection is desired. Hostile electromagnetic ambients to susceptible equipments may exist in many locations. For example, field strengths up to 30 V/m at AM, FM, and VHF TV broadcast may exist within 100m of transmitter antennas. Airport or weather radars may develop field strengths up to 200 V/m at distances out to 1 km. These values correspond to levels in the open. However, for equipments intended to be operated indoors, the field strength becomes substantially less. Most equipments which may fall in the open can perform when located inside steel reinforced (girder construction) buildings, even though concrete and, glass are nearly transparent to radio frequencies. Based on NBS and Bell Lab reports and measurements performed by the authors, mathematical models of attenuation offered by steel-reinforced buildings are presented. Substantial attenuation in the A-M broadcast band is almost entirely due to the steel girders located a few hundredths of a wavelength on center. At FM and VHF TV frequencies, where the girder separation is of the order of a wavelength, attenuation is insignificant. At UHF and SHF frequencies appreciable attenuation exists from scattering and absorption. The attenuation is always greater in the center core of the building; the associated standard deviation is also greater. Based on the buildings attenuation and field strengths mentioned above, the mean attenuation effect of typical office buildings just inside the outer wall suggests that A-M broadcast signals are below 0.2 V/m. FM and TV emissions may reach levels of about 20 V/m and radar emissions may approximate 100 V/m. On the other hand, in the central office core of such buildings, all field strengths are below 10 V/m, where relatively few radiated susceptibility failures would result.