What is Insertion Loss?

Insertion Loss is a term commonly used in microwave electronics, radio frequency engineering and telecommunications to describe the reduction in signal power or energy when a component, device, cable or system is inserted into a transmission path.

It is typically expressed in decibels (dB) and represents the difference in signal strength before and after the insertion of the component.

What causes Insertion Loss?

Insertion loss can occur due to various reasons. For example when a signal passes through a medium like a cable, optical fiber, or waveguide, some of its energy may be absorbed, scattered, or otherwise attenuated, leading to a reduction in signal power.

Here are some examples of practical situations where insertion loss occurs:

  • Passive RF Components: Components such as connectors, switches, filters, and couplers can introduce insertion loss due to their inherent characteristics. For example, a cable may not perfectly transmit a signal over its entire frequency range, causing some signal power to be lost.
  • Filters and Attenuators: These components are designed to intentionally reduce signal power within specific frequency ranges, and their insertion loss is a measure of how effectively they achieve this attenuation. Note that the pass band of every practical filter has a finite amount of insertion loss.


Importance of Insertion Loss

Insertion loss is an important parameter to consider in the design and analysis of communication systems and networks.

Lower insertion loss is generally desirable* because it means that the component or system is more efficient at transmitting signals without significant degradation.

Conversely, higher insertion loss indicates a greater reduction in signal strength, which may require compensation or amplification in the system to maintain the desired signal quality.

*An exception to this is in the case of filters. The blue plot below shows the S21 of a band pass filter. The absolute value of S21 is equal to the insertion loss. The absolute value should be high in the out-of-band region. This is in fact the case below where signals outside of the 2.4 to 2.6 GHz region are rejected.

Measuring Insertion Loss

Insertion loss can be measured using either:

  • a Signal Generator with a Spectrum Analyzer or
  • a Network Analyzer

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