A modified time-domain gating calibration method for free-space material measurement

Microwave network s-parameters calibration plays an important role in free-space material measurement. A calibration method based on time domain gating technique gives a simple way to calibrate free-space measurement system for engineers. This paper presents a modified free-space time-domain gating calibration method to increase the calibration accuracy of two-port free-space measurement systems. Based on the s-parameters calibration principle of two-port microwave network, the free-space error model is established which includes ten error terms because the isolation error terms are added into the classic calibration error model. Two simple standards measurement, which are thru and reflect, are implemented by measuring the empty sample holder and a metal plate. Then the relationship between s-parameters of standards and error terms is established. The time-domain gating technique is employed to get the directivity errors in order to obtain enough equations to solve for the error terms. After the error terms are calculated from these equations, the transmission s-parameters of the thru standard measurement are used to correct the value of error terms. Finally, the error terms are used to calculate the real s-parameters. From the testing result, it is can be seen that better calibration results are obtained by using this modified calibration method than using the classic time-domain gating based method. And the dielectric constant results agree with expected value by using the proposed method.