20 - Voltage Standing Wave Ratio (VSWR) and Return Loss

 So now that we have a grasp of what a voltage standing wave is, the next step is to understand the degree of its impact. Is it minor bouncing or something more significant? To measure this, we use the Voltage Standing Wave Ratio, or VSWR, as our metric. This will also tie into our discussion of line loss.

As we previously discussed, a voltage standing wave assumes a sinusoidal pattern, with bouncing effects causing fluctuations in amplitude around the origin, typically from positive to negative voltages. In our example, this ranges from +6 volts to -6 volts, and +4 volts to -4 volts around 0 volts.

To quantify the severity of this bouncing, we calculate the VSWR using a straightforward mathematical equation. It involves dividing the maximum voltage on the line by the minimum voltage. In our example on the positive voltage side, it's 6 volts divided by 4 volts, which equals 1.5. Since VSWR is expressed as a ratio, the denominator is typically 1 to keep it simple, resulting in a 1.5 to 1 VSWR ratio. This ratio provides a measure of how pronounced the voltage standing wave is and its impact on the system.

In our example, a VSWR of 1.5 to 1 is quite good, considering that achieving a perfect 1 to 1 ratio, with no reflected voltage, is an ideal but practically unattainable scenario. A VSWR of 1.5 to 1 falls between the ideal and a very good real-world situation, making it favorable.

However, we need to set our expectations based on some common VSWR values. The best-case scenario is around 1.1 to 1, which represents an outstanding VSWR. A pretty good VSWR is about 2 to 1, and a satisfactory value could be a 3 to 1 ratio. Ideally, we aim to keep the VSWR below the 2 to 1 threshold. Anything better than this is acceptable.

The significance of maintaining a lower VSWR becomes apparent when we consider the amount of reflected energy, which directly correlates with power loss. With a 2 to 1 VSWR, about 11% of the total power is reflected back towards the source. This may not seem favorable, but it's considerably better than a 3 to 1 VSWR, which reflects approximately 25% of the power. Thus, striving for a 2 to 1 VSWR is essential to keep reflected energy and power loss within acceptable limits.

Now, let's delve into the concept of return loss, which provides a more comprehensive perspective on power loss due to voltage standing waves. Return loss is a mathematical expression that quantifies this loss.

We're now transitioning from discussing VSWR to exploring the concept of return loss, which provides a more comprehensive perspective on power loss due to voltage standing waves.

Return loss is a mathematical expression that quantifies the loss experienced when power is reflected. To calculate return loss, we start with the input power and subtract the reflected power. At this point, we're converting the power values into dBm (decibel-milliwatts), which is a common way to express power levels.

For instance, let's say our transmission power is 20 dBm, which is similar to how we assess the transmission levels of access points. We then subtract the power reflected back towards the source. If, in our case, the reflected power is -5 dBm, the return loss would be 25 dB.

In return loss, a higher value is better. We want the return loss to be as high as possible. Generally, in the industry, we aim for return loss values greater than 20 decibels. Anything above 10 decibels is considered acceptable. If the return loss falls below 10 decibels, it becomes a significant concern.

As we conclude this discussion on electrical RF concepts, our primary takeaways are to understand the importance of minimizing power loss due to VSWR and the role of return loss in quantifying this loss. While we don't need to be electrical engineers for this exam, having a grasp of these concepts and their significance in wireless networking is essential.

 

In summary, let's revisit some key takeaways regarding voltage standing wave ratio (VSWR) and return loss, which have been the focal points of this skill.

• VSWR Goals:

Aim for a VSWR of 2 to 1 or better. A lower VSWR indicates better performance and less power loss.

Matching impedances between components is crucial. Components with similar impedance values minimize signal loss.

Ensure connectors are properly tightened and the system is correctly installed to avoid increased resistance and impedance mismatches, which can lead to unexpected power loss.

• Return Loss:

Return loss quantifies power loss due to reflected waves in the system.

The higher the return loss value, the better. In general, aim for a return loss greater than 20 decibels (dB), with anything above 10 dB considered acceptable.

Return loss is determined by comparing the input power level to the reflected power level..       

Understanding and managing VSWR and return loss are essential aspects of ensuring the efficiency and performance of wireless communication systems. These concepts help us assess the quality of our RF transmission lines and make informed decisions for optimal signal transmission.

In summary, here are the practical takeaways to ensure optimal performance in wireless communication systems regarding voltage standing wave ratio (VSWR) and return loss:

• Match Impedances:

Whenever possible, aim for components with similar impedance values. This minimizes reflected power and helps reduce signal loss.

• Secure and Tighten Connections:

Ensure that all connectors, cables, and other system components are securely tightened and properly installed. Loose or poorly connected components can lead to unexpected power loss and impedance mismatches.

• Focus on Outdoor Designs:

These concepts are particularly relevant in outdoor wireless deployments. Pay special attention to impedance matching when designing and installing outdoor systems to achieve optimal performance.  

By carefully designing and maintaining your wireless communication systems, you can effectively manage VSWR and return loss, ensuring that your systems operate at their best and minimize signal loss. Thank you for your attention, and I hope you found this information valuable.