Return Loss is a relative measurement of the reflected signal on a cable. In an ideal system, no signal would be reflected, all of the power on a signal would be received by the load, and the return loss would be infinite. In real-life applications, there will always be at least some amount of the input signal reflected back on the cable, usually either at a point of discontinuity/ termination or in the case of a mismatched impedance on the electrical load; this leads the return loss to vary between frequencies.
VSWR (Voltage Standing Wave Ratio) is a ratio measuring the strength of the reflected signal relative to that of the original signal sent into the cable. A high value of return loss results in a low value of VSWR, meaning that a VSWR value closer to 1 is the result of a very small reflected signal.
Lectromec uses a spectrum analyzer with an internal tracking generator to make return loss measurements. The spectrum analyzer is equipped with a VSWR bridge. The cable under test is attached to the DUT (Device Under Test) input on the bridge, with the other end terminated to the system’s nominal impedance. A signal is sent into the cable to the resistive load, and any reflected signal returns back through the VSWR bridge. The spectrum analyzer will measure the return loss and VSWR directly at specified locations on the reflection curve.