This weak-coupling approach can be used by industry for temperature-dependent dielectric measurements of high-value materials.
There is a need to perform accurate, high-temperature, complex dielectric constant measurements at microwave frequencies on materials, such as those on the surface of Venus (surface temperature 460 °C). One approach is to excite and detect a TE10n mode resonance in a waveguide cavity heated in a high-temperature furnace. The standard way is to use commercial high-temperature transition adapters attached to cavity end plates containing small iris holes that weakly couple microwave energy into and out of the cavity. These high-temperature transition adapters are not simple to make, and are rather large in size. The addition of the transition adapter units to the waveguide cavity leads to a long combined system that in many cases makes it difficult, if not impossible, to insert in conventional high-temperature furnaces.
In order to perform high-temperature dielectric constant measurements, one needs to employ high-temperature coaxial cables that transmit the microwave energy from room temperature to the high-temperature cavity. An option for replacing the high-temperature transition adapters is to use the center conductor of the high-temperature cables that acts like a dipole antenna to excite the cavity. This is accomplished by eliminating the transition adapter units and replacing the iris holes with the end of the coaxial cable. For the TE10n modes, microwave theory predicts that there should be no energy coupled into or out of the cavity when the coaxial cable center conductor is positioned at the center of the end wall (axis position of the cavity). However, it was shown that a weak-coupling condition could be attained by having only a slight non-uniform wire or positioning the wire slightly off axis. The amount of weak coupling could be controlled by adjusting the length of the coax center conductor inserted into the cavity. This study used a WR340 waveguide cavity excited in a TE10n mode. This approach can be used up to the highest temperature achievable by the coaxial cable and significantly reduces the size of the cavity system that needs to be inserted in a furnace to perform high-temperature dielectric constant measurements. It is also easier to construct than using a loop antenna at the end of the cable, which is another way to couple energy into and out of the cavity.
This work was done by Martin B. Barmatz and David E. Steinfeld of Caltech for NASA’s Jet Propulsion Laboratory. NPO-49262
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