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Questions about free space GRL calibration

Question asked by zhouron on Nov 29, 2006
Latest reply on Jun 12, 2008 by AndréK.
Greetings everyone!!

I am trying to do the free space GRL calibration. I have tried several times, but it never worked.

Here is how I do the free space GRL calibration:
1. Calibrate the VNA(85058E) to the end of the cables using SOLT calibration.
2. Connect the cables to the antennas. Transform frequency domain to time domain. Use gating option to gate the reflection between the cable and the surface of the sample, which is O11 and T11 when transformed to frequency domain.
3. Test the metal plate standard and empty fixture standard. Then use this tested data to solve for the O12, O21, O22, T12, T21 and T22.

The problem is that the computed magnitude of O22 and T22 are larger than 1. I even use the software 85701E to solve this problem. The software also gives me s-parameters larger than 1.

During the testing, I find that the reflection of the antenna is much larger than the reflection of the metal plate in time domain. How to choose the gating time in this case? When the metal plate is removed, the difference in time domain is less than 20dB. Is this because the metal plate is not large enough? And another question is how to determine the minimum size of the sample.

I am stuck in this for a long time:cry: . I hope you experts can help me out. Thanks a lot for your advice.