Limitations of methods for determining dielectric properties by the use of a standing-wave indicator

Conventional waveguide methods for determining the properties of dielectric materials are examined. The principal question considered is how to obtain maximum accuracy without the use of precision equipment or complicated experimental procedures. It is shown that the well-known open-circuit/short-circuit method inevitably produces a result which is the difference between two nearly equal measured quantities and is therefore unsatisfactory. It is argued that maximum accuracy is obtained if the specimen length is an odd number of quarter wavelengths, the specimen being terminated by a short-circuit. Under this condition a measurement of v.s.w.r. only is required, and minimum demands are made on the quality of the standing-wave indicator. It is shown that the advantages of this method derive from the fact that, in this arrangement, the energy stored in the specimen is a maximum.