Microwave Materials Dielectric Measurements
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MICROWAVE MATERIALS DIELECTRIC MEASUREMENTS DON PURINTON AND LEON STIBOREK Texas Instruments Inc., MS 8019,
McKinney,
Texas
75069
ABSTRACT Texas Instruments Defense Systems and Electronics Group develops radars, missile systems, infra-red systems and electronic systems for the Defense Department. Materials having special microwave properties are needed for radar systems, missile systems that home on microwave emissions, microwave emitter locator systems, and jamming systems. The microwave materials appear in the form of antennas, lenses, radomes, windows, dielectric loading materials, potting materials, dielectric transmission lines, microwave absorbing materials, adhesives, sealants, coatings, and foams. Naturally, the complex permittivity and permeability of these materials need to be known to enhance their application and control their properties in manufacturing. Texas Instruments has a microwave materials test laboratory that includes permittivity and permeability measurements as well as physical and thermal properties. The samples come in many different forms, measurement at different frequencies are needed and the electrical properties cover a wide span (conductors to dielectrics and foams to ferrites) . For this reason, many different methods of dielectric and magnetic measurement are used. These various methods will be discussed below. DIELECTRIC MEASUREMENTS Measurement Choices. Measurement choice is determined by the combination of desired measurement frequencies, available sample characteristics, permeability of sample and measurement temperature range. Measurements for frequencies above 18 GHz are most easily made by the free space measurement of flat panels that are large compared to wavelength (W> |07 ). For frequencies between 8 and 18 GHz waveguide measurements are usually most convenient and for broad band measurements below 8 GHz (.05 to 8 GHz) coax line measurements may be practical. The physical characteristics and sample availability often determine the measurement choice. Elastomeric materials can be conveniently measured in coaxial lines, waveguide and resonant cavities because they will conform slightly to fit the waveguide or coax line without error generating air gaps. They will also lay flat on the bottom of a resonant cavity. Kapton is made only in thin film form so that the resonant cavity measurement is most practical. Anisotropic fiberglass composite laminates require measurement with respect to the propagation orientation during application. For radome laminates flat sheet free space measurement is often
Mat. Res. Soc. Symp. Proc. Vol. 189. 01991 Materials Research Society
490
convenient and waveguide and resonant cavity measurements are also used. For PCB laminate, stripline measurement techniques are used to match the propagation mode. Small samples of materials are often measured in the resonant cavity or a coax line because panel or waveguide samples require a sample size of material larger than is available. Permeability measurements require more effort than dielectric
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