Specifications

5-27
5.9 Cable measurements
The characteristic impedance (Z(Ω)) capacitance per unit length (C (pF/m)), and the propagation
constants α (dB/m) and β (rad/m) are parameters commonly measured when evaluating cables.
Figure 5-37 shows a measurement setup for coaxial cable using an auto-balancing bridge type
impedance analyzer and the 16047E test fixture. Note that the High terminal of the test fixture is
connected to the outer conductor of the cable. This measurement setup avoids the effects of noise
picked up by the outer conductor of the cable and is important to regard when the cable length is
long. The characteristic impedance and propagation constants are determined by measuring the
impedance of the cable with its other end opened and shorted (open-short method), and calculating
using the equations shown in Figure 5-37. The I-BASIC programming function of the impedance
analyzer facilitates the calculations required. Figure 5-38 demonstrates an example of measured
characteristic impedance versus frequency.
Figure 5-37. Coaxial cable measurement setup and parameter calculation