Performance evaluation is an important aspect in the study of microwave-absorbing material coatings. The reflectivity of the incident wave is usually taken as the performance indicator. There have been various methods to directly or indirectly measure the reflectivity, but existing methods are mostly cumbersome and require a strict testing environment. What is more, they cannot be applied to field measurement. In this paper, we propose a scheme to achieve field performance evaluation of microwave-absorbing materials, which adopts a small H-plane sectoral horn antenna as the testing probe and a small microwave reflectometer as the indicator.
When the size of the H-plane sectoral horn antenna is specially designed, the field distribution at the antenna aperture can be approximated as a plane wave similar to the far field of the microwave emitted by a radar unit. Therefore, the reflectivity can be obtained by a near-field measurement. We conducted experiments on a kind of ferrite-based microwave-absorbing material at X band (8.2–12.4 GHz) to validate the scheme. The experimental results show that the reflectivity is in agreement with the reference data measured by the conventional method as a whole.
Otherwise, the incident wave is called a parallel polarized wave when the incident electric field vector is parallel to the incident plane. For a microwave incident on the absorbing-material coating at an angle, it can be decomposed into vertical and parallel polarization components. Taking the vertically polarized wave as example, the propagation of the incident wave in the coating is shown in Figure 1.
The other flange at the mouth is used to reduce the induced current on the outer surface and at the same time increase the contact area with the absorbing-material coating during on-site measurement. The H-plane sectoral horn antenna is connected with a small microwave reflectometer via a coaxial-to-waveguide adapter to form the whole testing system as illustrated in Figure 3.
RESULTS AND DISCUSSION
After painting, the coatings were polished to keep the surface flat and clean. The whole painting process were controlled by the material manufacturer. The #1 absorbing material coating had a thickness of 0.5 mm, and the #2 absorbing material coating had a thickness of 1 mm. The H-plane sectoral horn antenna was tightly attached to the two absorbing-material coatings during the test, illustrated in Figure 4.
We proposed a scheme to achieve field performance evaluation of microwave-absorbing materials, which adopted a small H-plane sectoral horn antenna as the testing probe and a small microwave reflectometer as the indicator. When the size of the H-plane sectoral horn antenna is specially designed, the field distribution at the aperture can be approximated to a plane wave as the far field of the microwave emitted by a radar unit. Therefore, the reflectivity can be measured on-site.
The microwave reflectometer weighs less than 2.5 kg with dimensions of 260 mm × 180 mm × 70 mm, thus it is portable and suitable for field measurement. We conducted experiments on one kind of microwave-absorbing material to validate the scheme. Experimental results illustrated an agreement with the reference data as a whole. The maximum deviation was less than 10 percent of the reference data. By designing different H-plane sectoral horn antennas, the reflectivity at a wider range of frequency can be obtained.
Authors: Shaopeng Guan | Yongyu Wang | Daiping Jia