Reference values give a good indication that the sources are operating as expected.
They DO NOT give any assurance that the reference test leads are clean and in good condition.As a guide, when setting a reference, you should expect better than:
DTX-MFM 62,5/125 µm
DTX-MFM 50/125 µm
where -19,5 dBm is better than -20,00 dBm.
You may be reading this article because you are experiencing negative loss readings or inconsistent test results. Your next step is to verify the quality of your reference leads. The following articles include a step to verify your Test Reference Cords.