This chapter describes the Distance Domain feature in the Vector Network Analyzer. General descriptions, key concepts, and examples are presented for distance measurements for both coaxial and waveguide media. The Distance Domain was previously Option 501, but is now standard in newer firmware. The firmware revision must be equal to or greater than the following versions to have Distance Domain as a standard feature:

The Distance Domain feature provides the ability to transform the native frequency domain data (that is measured by the Vector Network Analyzer) into distance domain information. Some typical applications are: help in determining the location of impedance discontinuities, distance‑to‑fault (DTF) in cables, characterizing antennas, isolating and analyzing a desired response in a one‑port or two‑port network.

The relationship between the frequency‑domain response and the distance‑domain response of a network is described mathematically by the Fourier transform. The Vector Network Analyzer makes measurements in the frequency domain, then transforms that data into its distance‑domain response, which can be displayed. This computational technique benefits from the wide dynamic range of the instrument (and its measurement data) and from the error correction of the frequency‑domain data.

The transformation technique that is used by the Vector Network Analyzer (in most cases) is the chirp‑Z transform of the available frequency domain data for that parameter. Because the transform simply treats the frequency domain values as input data, any S‑parameter can be transformed. The chirp‑Z transform is (in a macro sense) very similar to the Fast Fourier Transform with the exception that the output range can be variable. This permits you to zoom in on a specific range of interest for the data display.

Two of the fundamental properties of distance‑domain conversion are resolution and maximum (alias‑free) range. Resolution is the ability to resolve one discontinuity from another. Resolution is limited by the frequency span of the measurement. Maximum range defines how far you can see discontinuities on the media you are measuring. Beyond the maximum range, the data just repeats itself, and you start seeing the same discontinuities from closer ranges. The maximum range is determined by the frequency step size.

For more details about distance domain fundamentals, refer to the following application notes: