Microwave measurements of dielectrics in free space

The measurement of dielectric properties in the microwave band is usually accomplished by placing the sample in a waveguide or resonant cavity. Free-space methods, while lacking the general versatility of waveguide techniques, exhibit certain advantageous features: the problem of maintaining a precision fit between the sample and the guide is avoided; certain structures, for example, “honeycomb” materials and artificial dielectrics, can not be measured by waveguide methods but can be measured in free space; and at millimeter wavelengths, waveguide procedures become more complicated while free space techniques retain their simplicity.