Measurement of small dielectric loss at microwave frequencies

The dielectric loss of liquids or solids, which may be of the order of 100 ¿rad, can be measured at frequencies of 10¿35 GHz by a bridge-like method. Liquid is admitted to a waveguide in one arm of the bridge, and its loss is determined from the change in attenuation with the change in length of the column of liquid; a solid specimen (which need not fill the guide) is measured by immersion in a matching liquid. The uncertainty in the measurement can be of the order of 10 ¿rad, and arises mainly from uncertainty about the wall loss of the waveguide.