Insertion Loss, Return Loss and Attenuation
In order to understand losses, study the figure above.
Microwave power is sent down a transmission line from the left and it reaches the component. This power is the incident power. When it reaches the component, a portion is reflected back down the transmission line where it came from and never enters the component. Rest of the power gets into the component. There some of it gets absorbed and the remainder passes through the component into the transmission line on the other side. The power that actually comes out of the component is called the transmitted power, it is less than the incident power for two reasons: (1) some of the power gets reflected. (2) some of the power gets absorbed inside the component.
The ratio of transmitted power to incident power, in dB terminology, is the insertion loss. The ratio of reflected power to the incident power, in dB terminology, is the return loss.
Insertion Loss= – (Transmitted power (dBm) / Incident power (dBm))
Return Loss = – (Reflected power(dBm) / Incident power(dBm))
Here, – (negative) sign is used because “Loss” term is expressed in opposite sense.