阵列型微波黑体的发射率分析

Emissivity study of the array shaped blackbody in the microwave band

在微波波段,用于标定温度和辐射功率的发射率接近于1的标准发射率器件,即微波频段的黑体,结构形式一般为表面涂覆吸波材料的金属锥体阵列.这种黑体器件常用于为微波辐射计提供参考亮温,要求具有高发射率和均匀的温度分布.对此类黑体器件的发射率评估主要基于基尔霍夫热平衡定律,即通过评估反射率来确定发射率.已报道的研究集中在黑体发射率随频率的变化趋势,较少针对其随方向和极化状态的变化趋势.本文针对此类周期型排布的黑体,提出基于Floquet模式分析的反射率评估方法,相比已报道的基于后向散射的评估方法,具有更大的适用范围.基于这种方法,对某黑体的发射率随频率、角度和极化状态的变化规律进行了计算分析.分析结果表明:此黑体发射率在X到K_a波段内随频率提高而增大;在发射率较低的低频处,垂直极化与水平极化的发射率随俯仰角的变化趋势不同,并且存在垂直极化发射率随俯仰角增大而明显降低的现象.这些规律均与其物理上低频段内涂层对电磁波的衰减特性相符合.

Different from that in the optical band, the blackbody in the microwave band is constructed in a coated cone array structure. The blackbody of this type can be used in calibrating microwave radiometers with standard brightness radiations, and needs to have a uniform surface thermal distribution and high emissivity. The emissivity study of such a blackbody can be performed based on the Kirchhoff＇s law of thermal equilibrium, in a reflection determination routine. The emissivity characteristics varying with frequency have been intensively studied, but their variations with direction and polarization have not received much attentions. Starting from the Floquet mode analysis, a reflection evaluation scheme for the blackbody is presented, which is more robust than that based on the back-ward RCS determination. Based on the presented scheme, the trends of emissivity varying with frequency, direction, polarization are studied, for a microwave blackbody design. Results show that the emissivity rises as the frequency rises in a range from X band to K, band; and in the low frequency band, the trend of the vertical polarization emissivity varying with elevation angle is different from that of the horizontal polarization emissivity, and there exists an obvious phenomenon that the vertical polarization emissivity declines with the increase of elevation angle. These phenomena are related to the electromagnetic absorption characteristics of the coating layer.