Free-space permittivity measurements are useful for accurate measurements at high frequency (above 40 GHz), for elevated temperature measurements, or for composite samples that have structural variations on a relatively large scale compared with sample sizes required for other measurement techniques. For this method, a plate of the sample material is placed between a high-directivity transmitter and receiver horns. When samples with relatively large dimensions are used, collimated, lens-corrected conical horns may be used to generate a near plane-wave beam over the area of interest. When sample sizes are restricted to less than the required dimensions or when spatial resolution is desired to test for sample homogeneity, spot-focusing lens antennas may be used to focus the microwave signal into a small spot on the sample. A schematic of the measurement apparatus is shown in Figure 3-19. Samples are positioned at a given angle in the path of the incident beam, and the transmission and reflection coefficients are measured by two identical receiver horns suitably aligned with respect to the incident beam and the sample. The dielectric properties are then determined from the observed transmission and reflection coefficients (Ho, 1988). Alternatively, if an automatic network analyzer is used in these measurements, either the complex transmission (magnitude and phase) or reflection coefficient may be used to determine the complex permittivity of the material under test. Measurement of the transmission coefficient is often preferred, because it avoids some measurement difficulties involved in the determination of reference planes in the reflection coefficient measurements.