Abstarct
The absorbing material studies presented in this Report are related to an ongoing project at the HUT Radio Laboratory for realising a submillimetre wave compact antenna test range (CATR) based on a hologram. Absorbing materials are needed in all indoor antenna test ranges, and an accurate knowledge of their properties is required for effective placement inside the range
Measurements of both specular and non-specular scattering from several submillimetre wave absorber materials and low-cost carpet materials are presented. The carpets were included in the tests because some earlier publications noted that they can be used in non-critical areas to reduce material cost. The frequency range in specular scattering is 200–600 GHz, and 300–400 GHz in non-specular scattering measurements. The constructed bistatic test bench allows automatic testing of the full continuous angular range of 0o–90o
The measurement results show large differences in performance between materials. The best specular performance of –50 dB was measured for the pyramidal TK THz RAM. However, the same material scatters considerable amounts of power into nonspecular angles. The scattered power from wedged-type TERASORB-500 and FIRAM-500 materials is concentrated close to the specular direction, and very little power is found at other angles if the wedges are properly oriented. Low-cost carpet materials have better than –15 dB reflectivities in most angles, and are very useful in the non-critical areas of the antenna range. The results presented in this Report can be used to optimise the absorber placement inside the antenna range, concerning both maximum performance and lowest cost
Contents
1. INTRODUCTION
2. DESIGN OF RADAR ABSORBING MATERIALS (RAM
2.1. INTRODUCTION
2.2 LOSS MECHANISMS
2.3 SINGLE-LAYER ABSORBERS
2.3.1 The Salisbury screen and the Dällenbach layer
2.4 MULTILAYER AND GEOMETRICAL TRANSITION ABSORBERS
2.4.1 The Jaumann absorber
2.4.2 Graded dielectric absorbers
2.4.3 Geometrical transition absorbers
2.5 OTHER TYPES OF RAM
3 SCATTERING THEORY
3.1 DEFINITION OF THE RADAR CROSS SECTION (RCS)
3.2 RAYLEIGH, RESONANT, AND OPTICS SCATTERING REGIONS
3.3 SIMULATION METHODS FOR SOLVING SCATTERING PROBLEMS
4 SPECULAR REFLECTIVITY MEASUREMENTS AT 200–600 GHZ
4.1 OVERVIEW OF THE TESTED MATERIAL SAMPLES
4.2 CONSTRUCTION OF THE TEST BENCH
4.3 MEASUREMENT RESULTS
4.3.1 Measured reflectivities for the materials
4.3.2 Effect of non-idealities
4.3.2.1 Direct coupling between the horn antennas
4.3.2.2 Dynamic range limitations at 500 and 600 GHz
4.3.2.3 Pointing errors
5 NON-SPECULAR REFLECTIVITY MEASUREMENTS AT 300–400 GHZ
5.1 CONSTRUCTION OF THE TEST BENCH
5.2 MEASUREMENT RESULTS
5.2.1 Measured reflectivities for the materials
5.2.2 Effect of non-idealities
5.2.2.1 Direct coupling between the horn antennas
5.2.2.2 Pointing errors
5.2.2.3 Effect of the receiver’s distance on the results
6 SUMMARY AND CONCLUSIONS