High Temperature Millimeter Wave Dielectric Characterization Of Radome Materials

AbstractExperimental methods have been developed to determine the millimeter wave dielectricproperties of materials at temperatures up to 1600 °C. Accuracies of the order of ±1.5% fordielectric constant measurements and ±5% with a detection limit of ±0.0001 for loss tangentmeasurement over the entire temperature range have been demonstrated.IntroductionThe inherent advantages of millimeter wave radar systems in terms of all- weather capabil-ity when compared to infrared and optical sensors, and reduced weight and size when comparedto conventional microwave radars, have led to increased emphasis on the development ofmillimeter wave seekers for a broad spectrum of applications. Realizing these advantagesrequires determination of the millimeter wave properties of dielectrics to facilitatematerials development for passive components which can satisfy specific systems requirementsin terms of dielectric, thermal, and mechanical properties.In particular, a key requirement for millimeter wave radars in high speed missiles isthe successful development of suitable radome materials capable of withstanding extremeaerodynamic heating and thermonuclear blast temperatures, while maintaining low transmissionloss and mechanical and chemical integrity. Radome temperatures resulting from aerodynamicheating in general depend on the shape and local air flow patterns which control the heattransfer at the air - radome interface, as well as on the radiative and convective heatlosses. Depending on the particular radome design and its relative location on the missile,typical radome temperatures in excess of 1200 °C are expected for missile speeds in therange of Mach 6, while temperatures in the range of 1500 °C are expected at Mach 8. Thedevelopment of suitable materials for these applications clearly requires an ability toaccurately determine the millimeter wave transmission properties over the expectedtemperature range.In order to meet this current need, experimental methods have been recently developedat Rockwell International Science Center for determining the high temperature millimeterwave dielectric properties of materials. Temperature capabilities up to 1200 °C have beendemonstrated for a free -space transmission and reflection method using plate samples, andup to 1600 °C for a cavity perturbation method using small rod shaped samples. The resultsobtained with the two methods at 35 GHz are consistent to within the experimental accuracyof the measurements, and have absolute accuracies of the order of ±1.5% for the dielectricconstant determination and ±596 with a detection limit of ±0.0001 for the loss tangentdetermination over the entire temperature range.Measurement methodThe measurement principles for these two methods are summarized in the followingdiscussion.Free -space transmission and reflection methodFor this method, a plate of the sample material of uniform thickness is placed betweena set of high directivity transmitter and receiver horns. A schematic of the typical exper-imental arrangement is shown in Figure 1. The observables are the power transmission andreflection coefficients and /or their relative phase changes referenced to the incidentpower. In principle, measurements of any two of these four parameters uniquely determinethe dielectric constant and the loss tangent of the sample. In order to minimize diffrac-tion effects, the planar dimensions of the sample must be made large compared to the free -space wavelength of the incident electromagnetic radiation and typical dimensions of theorder of 10a or greater are generally required for accurate measurements. At 35 GHz,samples with uniform thickness of the order of 3.5 in. to 4.0 in. on each side are necessaryto meet this criterion. In addition, for very low loss materials, it is desirable to haveas large a sample thickness as practical so that the attenuation within the sample can bemade sufficiently high to be measurable to the required degree of accuracy.