Voltage Standing Wave Ratio and Return Loss

Voltage Standing Wave Ratio

Voltage Standing Wave Ratio (VSWR) is another parameter used to describe an antenna performance. It deals with the impedance match of the antenna feed point to the feed or transmission line. The antenna input impedance establishes a load on the transmission line as well as on the radio link transmitter and receiver.

To have RF energy produced by the transmitter radiated with minimum loss or the energy picked up by the antenna passed to the receiver with minimum loss, the input or base impedance of the antenna must be matched to the characteristics of the transmission line. The VSWR of a PCS antenna should be less than 1.5:1.

Return Loss

Return loss is the decibel difference between the power incident upon a mismatched continuity and the power reflected from that discontinuity. Return loss can be related to the reflection coefficient (VSWR) as follows;

RLdB = 20 log (1/p) Where p = VSWR-1/VSWR+1
VSWR = Vmax/Vmin

In other words, the return loss of an antenna can be considered as the difference in power in the forward and reverse directions due to impedance mismatches in the antenna design. All other things being equal, the higher the antenna return loss, the better the antenna.

The system engineer should choose an antenna with a return loss of 14 dB or better. Note that 14 dB corresponds to a VSWR of 1.5:1 as per the following example;

VSWR = 1.5/1 = 1.5 p = 1.5 VSWR-1/VSWR+1 = 0.5/2.5 = 0.2
RLdB = 20log (1/0.2)
RLdB = 13.979 dB

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