Reflections back toward the input end of a cable assembly are caused by variations in impedance along the length of a cable assembly and can cause energy loss by interaction with incident wave. Typically, the connections and the cable/connector interference will be the major contributors to reflection loss. Cable manufacturing variations and assembly techniques may also affect such loss and will show up at a specific frequency as a spike. (Figure 1)
(Figure 1) Click to enlarge
(Table 1) Click to enlarge
The magnitude of reflection can be expressed by the voltage standing wave ratio, VSWR, which is defined as the ration of the sum to the difference of the incident and reflected voltages. A low VSWR is indicative of uniformity along the length of a cable, good design and attachment of connections, and proper compensation in the connectors for transitions in the cable/connector interfaces.
An equivalent parameter is either the reflection coefficient or the return loss, which compares the power in the reflected wave with that in the incident wave, expressed in decibels. Return loss can be calculated from VSWR, and vice-versa. (Table 1)
Note that as VSWR increases with greater reflection, the equivalent return loss value decreases. Typical microwave cable assembly VSWRs range from 1.1 to 1.5, or return losses of 26.4 to 14.0 dB, respectively, indicating power transmission efficiencies of 99.8% to 96.0%.