When a load is not matched to the characteristic impedance of a transmission line, part of the incident wave is reflected back from the interface between the transmission line and the load.

Reflection Coefficient: shows what fraction of an incident signal is reflected when a source drives a load.

Standing Wave Ratio (SWR): is the ratio of the maximum to minimum values of the "standing wave" pattern that is created when signals are reflected on a transmission line. This measurement can be taken using a "slotted line" apparatus that allows the user to measure the field strength in a transmission line at different distances along the line.

Return Loss: shows the level of the reflected signal with respect to the incident signal in dB. The negative sign is dropped from the return loss value, so a large value for return loss indicates a small reflected signal. Example: a return loss of 26 dB is roughly equivalent to a reflection coefficient of 0.05.

Mismatch Loss: is the power lost relative to the incident power, expressed in dB. This is the power lost due to mismatch between the source and load impedances.

Tip: Try entering a value between 0 and 1 into the Reflection Coefficient field at right, then press 'enter.' The other reflection parameters are automatically calculated for you, and the animation updates to show the relative sizes of the transmitted wave, the reflected wave, and the standing wave pattern.

Update: fixed a problem with inputting impedances. Inputting a short circuit impedance (zero Ohms) now produces the correct pattern.

To learn more about mismatch, reflection, and standing waves we recommend the following courses:

Technical: Applied RF Techniques I, RF Measurements: Principles and Demonstration

Semi-technical: RF Productivity Improvement

Non-technical: RF and Wireless Made Simple, RF and Wireless Made Simple CD ROM

Check Microwaves101.com for a very good tutorial on VSWR.

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