Return Loss, Reflection Coefficient and VSWR

At high frequencies, the actual voltages, currents and phase relationships that define an impedance are difficult to measure directly. Instead, we use steadystate measurements of the traveling waves in transmission lines, which can be separated by a directional coupler into those traveling in “forward” and “reverse” (or “reflected”) directions. Return loss, reflection coefficient and voltage standing wave ratio are different numerical values for these radio frequency transmission measurements. Although they are related mathematically, each refers to a different means of measurement or analysis. Reflection coefficient (Γ) is the fraction of a forward traveling wave that is reflected from a mismatched load, expressed in polar coordinates (|Γ|(θ). The magnitude has no units, since it is a ratio, and is also referred to as rho (ρ). The phase angle may be given as 0 to 360 degrees or +180 to –180 degrees, depending on the analytical system that will use the data.