We present a design chart and a manufacturing process for mm-wave absorber consisting of two spacers (poly-carbonate) and two-resistive sheets (polyethylene terephthalate deposited with Indium Tin Oxide). The conventional design chart gives us necessary information to make a desirable absorber. Based on the design chart, a multi-layered type absorber was manufactured and it is concluded that a significant absorption level (-20dB) is attained at a wide-frequency range of 46-66GHz.

In this paper, we discuss an application of range Doppler imaging to measurement of reflected wave intensity from a moving object without using an anechoic chamber. The wave intensity reflected from a metal plate moving in the horizontal direction toward the antenna is typically 40-50 dB higher than that in the case without using the plate, and the estimated radar cross sections for a metal plate and sphere show good agreement with the theoretical value. The measurement of wave absorption by the present method suggests that frequency characteristics of the observed reflection loss are in close agreement with those of the calculated loss. These results show the reliability of the present experimental system and suggest that the method is applicable to wave reflection measurement not in an anechoic chamber but in an ordinary laboratory room.

The antennas for subsurface radar are usually covered with a conducting cavity to prevent the radiation field from affecting the electromagnetic environment and to protect the received field from external noises. Furthermore, radiowave absorber is attached to the interior wall of the cavity in order to suppress the multiple reflections in the cavity. In this paper, the characteristics of the two-dimensional cavity-backed antenna having the absorber and the over-all properties of this subsurface radar due to buried objects are numerically analyzed by the Finite-Difference Time-Domain method. It is shown that the pulse propagation in the ground is confined to the narrow region due to the cavity. It is also shown that the multiple reflections in the cavity are effectively suppressed by choosing the suitable absorber, and so that the distinctive pulse echo can be obtained.