Unmatched load connected with transmission line causes reflected waves and following phenomenon.

1) transmission loss by reflection 2) frequency dependency of transmission characteristics 3) standing wave caused by interference between incident and reflected wavesGenerally following parameters are used to evaluate the amount of reflection. We must pay attention that all the parameters other than "reflection coeeficient" have no information about direction of reflection.

## reflection coefficient

- This is the ratio of reflected wave to incident wave at point of reflection. This value varies from -1 (for short load) to +1 (for open load), and becomes 0 for matched impedance load.
## reflection coefficient in power

- This is a squere of the eflection coefficient which means the ratio of the reflected power to the incident power.
## Voltage Standing Wave Ratio (VSWR)

- This is the ratio of maxmum voltage to minimum voltage in standing wave pattern. It varies from 1 to (plus) infinit.
## reflection loss

- This is a ratio of reflected power to incident power in dB.
## return loss

- This is the dB value of absolute reflection coefficient. It is rather curious concept for transmission engneering. This loss value becomes 0 for 100% reflection and becomes infinite for ideal connection.

The reflection coefficient can be calculated from the "normalized value" of the load impedance.

r = (Zn - 1)/(Zn + 1) Zn = Z/Z0 where r = reflection coefficient Zn = normalized load impedance Z0 = characteristic impedance of line (Ohm) Z = load impedance (Ohm)

Following relations stand for these parameters.

reflection coefficient in power = r^2 VSWR = (1 + abs(r))/(1 - abs(r)) reflection loss = -10*log10(1 - r^2) [dB] return loss = 10*log10(r^2) [dB] r = reflection coefficientThe "abs(x)" replesents absolute value of x. The "log10(x)" replesents the common (base of 10) logarithm of x.