Measurement of RF Propagation into Concrete Structures over the Frequency Range 100 MHZ to 3 GHz

Abstract

A concrete structure with few ports of entry for RF was illuminated over the frequency range of 100 MHz to 3 Ghz to characterize the propagation of RF signals into the interior region of the structure. The construction of the building tested is standard for a blast hardened facility. Outside walls of the structure are two-feet thick concrete with horizontal and vertical rebar separated by only eight inches. And the flat roof/ceiling is three-feet thick concrete with a two-inch steel plate base. With only one door located at the rear of the building and one set of windows across the front of the building, RF penetration through windows, doors and walls can be isolated and characterized separately. Illumination of the concrete structure is provided by using two log-periodic antennas driven from two wideband linear amplifiers for the frequency range 100 MHz to 1 Ghz. Above 1 Ghz two traveling-wave tube amplifiers were used to reach 3 Ghz. Swept frequency measurements of the RF attenuation into the building were performed by using automatic network analyzers with the amplifier/antenna configurations. Logarithmic spacing of the frequency samples was usedin the frequency sweep process. In all cases the antennas were oriented to produce a vertically polarized electric field with a horizontally polarized magnetic field. The RF penetration is characterized by transfer functions that are ratios of the penetrant field to the incident field. In order to remove the antenna-to-measurement point distance variable, both the penetrant field and the incident field were recorded at the same distance from the antenna. The incident field is measured by directing the antenna toward an open region and measuring the horizontal magnetic field component at the surface of a large metal plate resting on the ground. Considering radiation conditions obtain, the incident electric field is computed from the magnetic field measurement. Measured data for the penetrant field are compared with theoretical predictions to verify the results. Interior measurements indicate that the rebar attenuation of the electric field was not significant above 120 MHz while the rebar attenuation of the magnetic field was not important above 200 MHz. Near the window there is little attenuation at all frequencies. However, there is significant attenuation, more than 50 dB, through the concrete roof/ceiling for all frequencies.