Return loss & VSWR

Return loss measures how much of a signal bounces back from a port instead of going in — and despite the name, bigger numbers are better. It is the ratio of incident power to reflected power, in decibels: 20 dB means only 1% reflects (99% accepted), while 3 dB means fully half bounces back, flagging a badly matched port. It is simply the magnitude of S11 written as a positive dB value. VSWR (voltage standing-wave ratio) is the same mismatch seen from the cable: when reflected and forward waves add and cancel along the line, VSWR is the ratio of the biggest voltage to the smallest, running from 1:1 (perfect) to ∞ (total reflection).

Why obsess over a small reflection? Reflected energy is wasted, it can damage a transmitter's output stage, and the standing waves it sets up distort the signal. RF systems typically demand 10–15 dB of return loss (under ~10% reflected), with high-performance antennas and filters pushing past 20 dB. The two views map onto each other directly: 20 dB return loss is a VSWR of about 1.22:1, and the field engineer's common acceptance limit of VSWR 2:1 corresponds to roughly 10 dB return loss (about 11% reflected).