The use of `ghost modes¿ to determine dielectric constant and loss tangent

There are two principal problems in cavity methods of determining dielectric constant and loss tangent. One is to ensure that the specimen is in a high field region, so that the Q-factor of the system differs by a small amount from that of the test specimen. The second is to account properly for power absorption in all parts of the system. A most convenient technique is to excite modes of oscillation in the specimen that do not correspond to normal propagating modes in the region outside, i.e. in the containing waveguide. As a result, fields outside the specimen are strongly evanescent and contain little stored energy, hence the title `ghost mode¿. Calculations have been made of all possible ghost-mode resonances for a dielectric specimen in the form of a disc, placed in a waveguide of circular section. It has been found possible to excite resonances of different types, which do not require changes to be made in the geometry of the system. The distribution of stored energy and wall loss is significantly different, and accurate determinations of loss tangent, dielectric constant and circuit losses can be made. The method is particularly suited to materials of high dielectric constant and low loss, but is not restricted to this class of dielectric material.