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Return loss is the reflected power expressed in decibels below the incident power: rho = 10^(-RL/20). Higher return loss means a better match — 20 dB corresponds to 1% reflected power and an SWR of 1.22:1, while 10 dB is 10% reflected and 1.92:1.
Return loss
rho = 10^(-RL/20); VSWR = (1 + rho)/(1 - rho); Preflected = Pfwd x rho^2
By convention it is quoted as a positive number of decibels of loss. Network analysers display S11 as a negative dB figure, which is the same information.
10 dB (1.9:1) is usually acceptable, 15 dB (1.4:1) is good and 20 dB (1.2:1) is excellent for most antenna and filter work.