Measurement of Characteristic Impedance of RF Coaxial Cables

RF coaxial cables are widely used as transmission lines in communication systems. Characteristic impedance is the primary electrical parameter to consider when designing and selecting RF coaxial cables. This article will analyze the measurement of this parameter.

I. Definition of Characteristic Impedance

When electromagnetic waves propagate along a cable, there are usually incident waves propagating forward and reflected waves propagating backward. The incident waves and reflected waves superimpose to form standing waves. The ratio of the total voltage to the total current at any point on the transmission line is defined as the input impedance looking toward the load at that point. In general, the input impedance of a transmission line is related not only to the line length but also to the frequency. However, when the transmission line is infinitely long, there are only forward-traveling waves (traveling waves) on the line. At this time, the input impedance at any point on the transmission line is independent of the line length and is equal to a constant value Zc, which is called the characteristic impedance of the transmission line.

In addition, when the terminal of the transmission line is connected to a constant pure resistive load, the input impedance at any point on the transmission line is the same everywhere and independent of the line length. This constant resistance value is the characteristic impedance of the transmission line. The characteristic impedance Zc of an RF coaxial cable depends only on the diameters of the inner and outer conductors of the transmission line and the equivalent dielectric constant of the filling medium between them, and is independent of the line length.

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II. Measurement Methods of Characteristic Impedance

The characteristic impedance of RF coaxial cables can be measured by frequency-domain methods or time-domain methods.

The frequency-domain method generally uses a vector network analyzer to test cable performance. Due to the use of band-pass filters and digital filters in vector network analyzers, which have very low background noise, they can accurately measure the characteristic impedance of cables. According to the different transmission directions of the test signal, the frequency-domain method can be further divided into transmission measurement and reflection measurement.

Among the commonly used current measurement methods for the characteristic impedance of RF coaxial cables, the transmission phase method, transmission phase difference method, open-circuit or short-circuit resonance method, etc., belong to transmission measurement in the frequency-domain method, while the newer single-connector measurement method belongs to reflection measurement in the frequency-domain method.

The voltage standing wave ratio (VSWR) of a coaxial connector is mainly caused by the internal impedance inhomogeneity of the connector and the deviation from the characteristic impedance of the cable. Since the characteristic impedance of coaxial connectors is relatively easy to control (e.g., (50±0.5) Ω), the VSWR caused by their internal impedance inhomogeneity, including discontinuous capacitance generated by sudden dimensional changes, is extremely small. Below low frequencies (e.g., 200MHz), the VSWR of the connector is generally only about 1.005, which is much smaller than that of the cable assembly. Therefore, the VSWR of the connector can be ignored. However, the reflection caused by the internal impedance inhomogeneity of the tested cable cannot be ignored, and this influence should be eliminated during testing. Thus, the main reflection source in a single connector comes from the deviation between the cable impedance and the standard impedance. Finally, the characteristic impedance of the tested cable can be directly obtained by measuring the VSWR of the single connector.

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In the RF band, the characteristic impedance of coaxial cables is independent of frequency. Therefore, it is only necessary to measure at any frequency within the 30～200MHz frequency range specified in the national standard GB 4098.3 "Test Methods for Characteristic Impedance of RF Cables" using the transmission phase method.

It should be noted that due to the large error of the transmission phase difference method, its use is not recommended. The single-connector method is simple to operate, the measured data is accurate, and it is directly linked to the VSWR, which has strong practicability. It is a convenient and practical method for measuring the characteristic impedance of coaxial cables and is recommended as the preferred method.