Refractive index of concrete. Knowledge of the dielectric properties of construction material is important because the reflection and transmission characteristics of buildings and rooms are governed by these properties. The complex refractive index of a plain concrete plate mixed from Portland cement, gravel, sand, and water was measured at 57.5 GHz for use in designing and testing millimeter-wave communication systems.68 The measured refractive index of the concrete 14 months after concreting was 2.55 – 0.084. (a) Using the measured values for concrete, calculate and sketch the magnitude of the reflection coefficient at the air-concrete interface at 57 .5 GHz as a function of the incidence angle varying between 0° and 90° for both perpendicular and parallel polarization cases. Assume the concrete to be semi-infinite in extent. (b) For a 5-cm thick concrete wall having air on both sides, calculate the magnitude of the normal incidence reflection coefficient at 57.5 GHz and compare it with the result of part (a). (c) Repeat part (b) for a thickness of 10 cm.