The return loss being the difference between the incident power and the reflected.ĩ. The VSWR is a measure of the standing waves set up in a feeder as a result of a mismatch, whereas the return loss looks at the amount of power absorbed by a load when power from a source is sent to it. Causes of a high VSWR rating could be use of an improper load or something unknown such as a damaged transmission line. In an ideal system, there would be a VSWR of 1:1. If the VSWR is too high, there could potentially be too much energy reflected back into a power amplifier, causing damage to the internal circuitry. In an ideal system, 100% of the energy is transmitted. VSWR (Voltage Standing Wave Ratio), is a measure of how efficiently radio-frequency power is transmitted from a power source, through a transmission line, into a load (for example, from a power amplifier through a transmission line, to an antenna). Return loss is a measure in dB of the ratio of power in the incident wave to that in the reflected wave, and we define it to have a negative value. In this case the VSWR will be 1:1 and the voltage and the current will be constant over the whole length of the feed line. This means that a poor match between the feeder and antenna will result in a high SWR which causes the output to be reduced and hence a significant loss in transmitted power. The voltage fluctuations come about as a result of the voltage components from the forward power and the reflected power summing together.Īlthough the term VSWR is normally used, both the voltage and current standing waves can cause issues. The VSWR definition states that the VSWR is equal to the maximum voltage on the line divided by the minimum voltage. In this case, no power is reflected from the antenna, which is ideal. The smaller the VSWR is, the better the antenna is matched to the transmission line and the more power is delivered to the antenna. The VSWR is always a real and positive number for antennas. So when someone says that the antenna is poorly matched, very often it means that the VSWR value exceeds 2 for a frequency of interest. The antenna can be described as having a “Good Match”. A VSWR value under 2 is considered suitable for most antenna applications. The range of values for VSWR is from 1 to ∞. L M: Mismatch loss is a measurement describing the amount of power that will not be available at the load due to the reflected wave. L R: Return loss is a amount of power that is lost to the load and returns back to the system as a reflection typically expressed in dB. Return loss can be measured by using microwave and RF test equipment such as a signal generator, directional coupler along with a spectrum analyser or power meter, or alternatively by using a vector network analyser.
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