How to Improve Return Loss or VSWR

In this post we show a simple technique to improve the return loss [1] of an RF component like a mixer or amplifier.

A good VSWR to aim for is at least 1.5. This is equal to a return loss of 14 dB or greater.

The easiest way to improve return loss is to add an attenuator. In the example below an attenuator is added between the input and amplifier to improve its return loss.

As an example, if the return loss of the amplifier is 4 dB, use the calculator below to see that adding a 5 dB attenuator will improve the RL to 14 dB.



The attenuator will reduce the cascaded gain by 4 dB so that will have to be accounted for.

The amplifier can be substituted with a mixer. For instance the LO port can have a VSWR as high as 4. This is too high and presents a poor match. The RL is 4.44 dB, Adding a 5 dB attenuator will increase the RL to 14.44 dB and a VSWR of 1.47.


What is Return Loss?

Return loss is a measure of the amount of power loss in a system due to the reflection of a signal. It is expressed in decibels (dB) and represents the ratio of reflected power Pr to the incident power Pi at a specific frequency.

RL = 10*Log10(Pi/Pr)

Since Pr is always less than Pi, the return loss is always a positive number.

If the returned power is equal to the incident power, it represents a very poor match and RL = 10*Log10(1) = 0 dB

A high value in dB for return loss indicates a good match between the source and load impedance, meaning that most of the power is transmitted and little is reflected back. For example RL = 30 dB is a better match than RL = 20 dB.

On the other hand, a low value in dB for return loss indicates a mismatch between the source and load impedance, leading to a significant portion of the power being reflected back. The closer RL gets to 0 dB, the poorer the match.

The return loss is important in communication systems, especially for antennas, as it affects the efficiency of the signal transmission. For example, if an antenna RF input/ouput connector has a return loss of 20 dB, it means that only 1% of the power is reflected back and 99% is transmitted. Using the return loss to mismatch loss calculator tool.

Additionally, return loss affects the noise figure of the system and can impact the overall performance in a given frequency band. Therefore making an accurate return loss measurement is critical for analyzing and optimizing the performance of communication systems.

What is VSWR?

Voltage Standing Wave Ratio (VSWR) is a measurement used to determine how well an antenna is matched to the cable or transmission line that transfers energy to or from it. This is crucial for efficient signal transfer.

When an antenna is not properly matched to the transmission line, some of the power being delivered to the antenna is reflected back towards the transmitter instead of being radiated by the antenna.

This results in wasted energy and reduced overall system performance.

Mathematically, VSWR is a ratio of maximum voltage to minimum voltage along a transmission line [1]. A low VSWR indicates a good impedance match between the antenna and transmission line. The closer the value is to 1, the better the quality of the match.

By understanding and monitoring VSWR, engineers can ensure that the maximum amount of power is delivered to an antenna and therefore minimize the amount of power being reflected back to the transmitter. This allows for optimal signal transmission, longer range, and higher received signal level. Therefore understanding VSWR is important in order to maintain efficient and reliable communication systems.

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[1] VSWR on Wikipedia