From the beginning:
Many of the questions that appear in this forum regarding VNA operation suggest the fundamental premise of the VNA is either missing, misunderstood, assumed, or ignored. Questions elicit a barrage of responses; most of which are appropriately motivated, but not always helpful or instructive.
Often; original posters become silent while threads evolve and drone on until eventually terminating open ended; with final posts sometimes unrelated to the question originally asked. This is common on forums, but it is not efficient toward establishing a knowledge base.
The scope of questions as well as most answers to them overlook the obvious but simple detail that all VNA's only produce one output result per measurement. That result can be presented to a user in a plethora of data formats, with the most common format being a complex number pair referred to as a voltage reflection coefficient or simply reflection coefficient, and only one VNA port is utilized for the measurement. Measurements between ports are called transmission coefficients and are used for insertion loss (gain), delay, isolation, etc. measurements. Armed with the measured coefficient(s), the frequency at which the measurement was made, and an accurately affixed known reference called the characteristic impedance or Z0, all other data formats are derived through mathematical manipulation.
By the above; the entire essence of a VNA is to make one or more complex voltage (amplitude and phase) measurements. It is precisely analogous to an AC voltmeter. Other than the operational tasks of physically connecting to, calibrating, and performing the measurements; knowledge of how the hardware acquires and presents its results neither adds to nor detracts from the instruments capability, utility, and usefulness of the device to a user.
What now becomes glaringly obvious is that beyond the powerful measurement capability and utility of a VNA, modern instruments include an equally powerful embedded feature that performs involved complex mathematical computations in real time with the measurements being made. Analogous to a pre-programmed, fully automatic graphing calculator, the VNA produces a family of tabular and graphical displays of the most common RF parameters and formats of interests to engineers and technicians.
A key point here is that the measured results presented by the VNA are precisely identical, no matter the format in which they are presented. Unless it has been determined that a bug exists in the VNA software/firmware; users can take it on faith that the parameters are accurate as presented; assuming the instrument is being utilized as dictated by its design.
Inasmuch the various data formats are used, related, manipulated, interpreted, etc., operation of the VNA should be and remain transparent to discussions regarding data format relationships. Motivated users can (and should) pursue an understanding of the underlying mathematics of scattering parameters, complex numbers, and linear algebra to the extent required to meet their needs.
In most cases, users are interested in a particular measurement detail; e.g. impedance, VSWR, gain, etc., and alternative data formats have little if any relevance. In other cases they may find utility in the convenience of format conversion; e.g. inductance to reactance, phase to delay, etc.; all being computationally derived from identical measurement.data.
For users who are just becoming familiar with VNAs and their applications; an intuitive understanding and acceptance of the fundamental premise of VNA measurements as described here should sufficiently allow easy access to most of the utility that VNAs provide, and inspire the pursuit of deeper understanding of the relationship and derivation of all the parameters used to define RF networks.
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73
Gary, N3GO
