An Outdoor Bistatic Scattering Assessment Using Array Antennas

Abstract

Passive coherent location (PCL), which is also known as passive radar, is of in- creasing interest lately. In a passive radar system, the transmitting and receiving antennas are placed at difierent locations. Therefore, it detects targets' bistatic scattering instead of monos- tatic scattering in conventional radars. In this work, we conducted experiments that evaluate the bistatic radar cross section (RCS) from targets in the UHF band. The variation in the amount of bistatic scattering was examined and analyzed by changing the parameters which include the target's shape, orientation, distance to antenna, and antenna polarization. 1. INTRODUCTION In 1936, German deployed radio frequency radar at warship Graf Spee for the flrst time. Since then, the radar technology has dramatically evolved and played an important role on the battle- fleld. Among the many difierent types of radar, the air surveillance radar is of crucial importance. However, the conventional air surveillance radar, which continuously emits strong radiation in the order of hundreds of kilowatts or even megawatts, is likely to expose its location. Therefore, the concept of passive radar has been brought up and implemented. Unlike tradition active radars, passive radars do not require high power transmitters and come with receivers only. Passive radar receivers continuously monitor the background radio signals, such as commercial TV broadcasting, mobile telecommunication or satellite signals. The presence of air targets may perturb the magni- tudes, phases and even frequencies of background signals. A passive radar may therefore be able to detect the perturbation and determine if an air target is present. In the passive radar operation scenario, the transmitting and receiving antennas are not placed at the same position. Unlike traditional radars, which detect the monostatic back scattering from the target, passive radars exploit the bistatic RCS property of targets (1{4). In this paper, we designed a series of experiments that measures the bistatic RCS values of elevated targets in the UHF band. The experiment set-up includes two antennas, one is for transmission, and the other is for reception. Because the signal magnitude may be very weak due to propagation, an RF transmitter of 10 watts at the UHF band was assembled to conduct measurements. The components used in the transmitter include two stages of amplifler, attenuators, a power supply, a circulator and a termination. The mock-up targets that scatter impinging radio frequency waves are placed in front of directive antennas. A network analyzer was employed to provide radio source and measure the properties of scattered signals. In order to minimize scatterings from background clutters experiments were conducted on a roof top. The design and performance improving measures of the antennas used in experiments are pre- sented in Section 2. The measurement range set up as well as measurement results and analysis are provided in Section 3. A conclusion and future work topics are drawn in the end. 2. DIPOLE ARRAY ANTENNA FOR BISTATIC RCS MEASUREMENTS