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Optimization Criterion for a High Performance Absorber Design at VHF/UHF Frequency Band
Typical anechoic chambers for EMC, Far–Field, Near-Field antenna or RCS measurements comprise a metallic enclosure with internal surfaces covered by absorbing material, conventionally composed of pyramidal, wedge or similar shapes. It is common for antenna engineers to make the following assumptions regarding the performance of microwave anechoic absorber materials:
• Absorbing material reflectivity is monotonically improving if larger absorbers (e.g., larger pyramid heights) are used
• Reflectivity of larger absorbing materials is better at lower frequencies, allowing the extension of the operating frequency band of the absorber and chamber to a lower frequency.
• Absorbing material reflectivity is improved if a higher carbon loading is impregnated into the materials
In this paper it is demonstrated that these assumptions may not be applicable to the absorbing materials commonly used at VHF/UHF frequency bands. If the chamber operates down to VHF/UHF frequencies, then the height of foam impregnated absorber components may vary from 0.9m up to 2.4m or more. The reflectivity of these absorbing materials is critical at VHF/UHF, as the chamber cross-section may span only a few wavelengths at these frequencies, and the free space propagation loss of the waves reflected off the absorber is no longer an
enhancing factor to aid in the overall chamber reflectivity. Moreover, manufacturing of such absorbers is expensive and challenging for VHF/UHF frequencies. A variable carbon based loading as a function of absorber height may be implemented in the design to achieve superior performance at lower frequencies, and a simple criterion that can be readily implemented in the manufacturing process and which delivers optimum loading/reflectivity is important in the practical implementation of VHF/UHF chambers. Such criterion is formulated in the paper and its validity is confirmed by a series of absorber reflectivity measurements in the coaxial line.
