RL & VSWR

#RF_Interview_Question :
What is the Return loss and VSWR? How are they related, pls. explain?
#Answer :
Both return loss and VSWR is used as a measure of reflection of E-M waves over coaxial cable or RF cable or microstrip line.

It gives how much power is reflected and how much power is absorbed at various points specially at terminating and source points and at places of impedance discontinuities. Return Loss in a coaxial cable having Z0 as characteristic impedance and ZL as terminating or load impedance can be expressed as follows:

Return Loss (dB) =
                  20*Log10((ZL-Z0)/(ZL+Z0))
Where Z0= (L/C)0.5

Both Return loss and VSWR are related as mentioned in the following expression.

Return loss = 20 log ((VSWR+1) / (VSWR-1))
VSWR ranges from 1 to infinity.

For instance, a 20 dB system return loss measurement is considered very efficient as only 1% of the power is returned and 99% of the power is transmitted. If the return loss is 10 dB, 10% of the power is returned. While different systems have different acceptable return loss limits, 15 dB or better is a common system limit for a cable and antenna system.

While an antenna system could be faulty for any number of reasons, poorly installed connectors, dented/damaged coax cables, and defective antennas tend to dominate the failure trends.

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