Reader antennas' configuration for two wheeled robots were evaluated to estimate their posture from floor-installed RFID tags. RFID systems where IC tags are installed under/on floors have widely been utilized in recent years as the next positioning infrastructure. The reader antennas should be properly placed on a robot so that such an environment can give full play to its potential capabilities of positioning the robot. This problem calls for guidelines in designing the configuration of reader antennas. Experiments using actual robots cannot offer sufficient data because of time and physical limitations, which prevent helpful and reproducible evaluations. We overcame this problem by constructing a simulation environment using a localization model and by evaluating the effects of configurations on positioning accuracy using computations. Of particular note, we found a "forward-backward configuration effect" from the results and had a detailed a discussion on how it occurred. Finally, a simple experiment using an actual robot validated the effect.