Return Loss is the decrease of forward energy in a system when some of the power is being reflected back toward the transmitter. What will cause high return loss in an RF transmission system, including the radio, cables, connectors and antenna?
A.
The use of 50 ohm cables longer than one meter in the RF system
B.
High output power at the transmitter and use of a low-gain antenna
C.
A significant impedance mismatch between components in the RF system
Return loss is a measure of how well the components of an RF system are matched in terms of their impedance. Impedance is the opposition to the flow of alternating current in a circuit, and it depends on the frequency, resistance, capacitance, and inductance of the components. When the impedance of the source, the transmission line, and the load are not equal, some of the power is reflected back to the source, causing a loss of forward power. This loss is expressed in decibels (dB) as return loss. The higher the return loss, the lower the reflection and the better the impedance matching. Conversely, the lower the return loss, the higher the reflection and the worse the impedance matching.
VSWR (Voltage Standing Wave Ratio) is another way of expressing the same concept. It is the ratio of the maximum voltage to the minimum voltage along a transmission line due to the interference of the incident and reflected waves. A VSWR of 1:1 means that there is no reflection and perfect impedance matching. A VSWR higher than 1:1 means that there is some reflection and impedance mismatch. The higher the VSWR, the higher the reflection and the lower the return loss.
Therefore, a significant impedance mismatch between components in an RF system will cause high reflection, high VSWR, and low return loss.
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