What is meant by combiner loss?

Combiner loss is a term that refers to the theoretical loss that is incurred in a power combining situation. In a 2:1 combine, the loss is 3dB or 50% of the total input power. In a 3:1 combine, the loss is 4.8dB or ~67%. In a 4:1 combine, the loss is 6dB or 75%, and so on. Insertion loss is the theoretical combiner loss plus the dissipative loss.