Reduction of multiple reflection influence in microwave free-space measurements

The influence of the multiple reflections is one of the main error sources in the microwave free-space measurements. In this study, the influence is analyzed for the conventional free-space method, and a new method which uses absorbing material inserted between the antenna and the sample to reduce the influence of the multiple reflections is proposed. The effectiveness of this approach is verified by measuring the attenuation of timbers. It is showed that the maximum error caused by the multiple reflections in the improved free-space set-up is 0.4 dB, while that in the conventional free-space method is 1.5 dB.