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Uncertainties associated with many-port (>4) S-parameter measurements using a four-port vector network analyzer

While four-port vector network analyzers (VNAs) are now commercially available, there is an increasing need for S-parameter measurements of devices with more than four ports. Test sets and algorithms for performing many-port measurements have been known for some time, although extensions for four-port and larger VNAs have not been seen widely. This paper addresses a measurement approach and a method of obtaining approximate uncertainties for these measurements. It is well known that disregarding the load match of unused ports can have serious effects, but it has been found that these match terms can be handled reasonably well even when intra-device isolation levels are low. The impact of mismatch uncertainty on the impedance transforming algorithms themselves is also examined. Measurements using a nine-port test set connected to a four-port VNA are employed to illustrate the results.