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# Insertion Loss

In a communication transmission system, the magnitude of the loss caused by the insertion into the system of a new network or device must often be known. This loss is determined by several factors, such as the degree of impedance match at the input terminals, the internal transmission loss in the device to be inserted, and the degree of impedance match at the output terminals.

By definition,1 the "insertion loss at a given frequency caused by the insertion of apparatus in a transmission system is the ratio, expressed in decibels, of the powers at that frequency delivered to that part of the system beyond the point of insertion before and after the insertion."

A generalized equation that will give the insertion loss of any line, circuit, or device becomes quite involved and will not be given.4,5 As an example, consider the insertion loss when a network, such as a few miles of telephone cable having a transmission loss (or attenuation loss, Chapter 7) within itself of LN decibels, is inserted between a generator and a load. For these conditions, four losses will be involved in determining the insertion loss. These are the reflection loss L1 at the junction of the network and the generator; the reflection loss L2 at the junction of the network and the load impedance; the loss LN within the cable (or network); and the reflection loss L3 which would occur if the load impedance were connected directly to the generator. For this example the insertion loss is L1 + L2 + LN - L3, the loss L3 being subtracted because it enters into determining the received power both before and after the insertion of the cable or other network.

Last Update: 2011-05-30