19 - Voltage Standing Waves (VSW)

 In the previous discussion, we explored how AC voltage is applied directly to two dipole antennas, creating wireless waveforms. However, in practical applications, we usually don't connect our power source, such as a radio transmitter, directly to the antenna. There's a series of components, including connectors, cables, and other elements, that can introduce losses and affect how cleanly the signal is delivered to the antenna. In this blog, we will delve into the concept of voltage standing waves, which will set the stage for our discussion on the voltage standing wave ratio in the next blog. This ratio will play a significant role in understanding the losses incurred when delivering power to antennas.

So, let's dive in. Imagine we have an AC power source, and we want to deliver this power to the two antenna elements within our dipole antenna. However, we can't simply connect it directly, as there are various components, such as cables, connectors, and lightning arrestors, in the signal's path from the power source to the antenna. These components introduce resistance, known as impedance, into the circuit. For instance, the cable might have an impedance of 200 ohms, while a connector could have an impedance of 75 ohms.

 

The issue arises when these impedance values are mismatched. To illustrate this, let's consider an analogy.

Sure, let's continue. The analogy I'd like you to consider is thinking about water flowing through a pipe. Imagine we have a large pipe that allows a substantial amount of water to flow through, much like low resistance in an electrical circuit. Now, as we reach an abrupt change in the pipe size, the water is forced into a smaller pipe. This represents a change in impedance from an electrical perspective.

In the world of water, some of the water will impact the wall of the smaller pipe and bounce off it. This results in a combination of backflow in the system and a smaller amount of water being pushed through the narrower pipe. The purpose of this analogy is to illustrate the reflection concept. Just as water reflects off the change in resistance in the pipe, electricity behaves similarly. When an electrical signal encounters different impedance levels, a portion of the signal is reflected, creating what we call a voltage standing wave.

So, what is a voltage standing wave? It's essentially the interaction of two different voltages working against each other. One voltage is our input voltage, which is represented as a sinusoidal waveform. Let's say this AC voltage fluctuates between positive 5 volts and negative 5 volts. This waveform is moving in a certain direction. However, when this voltage encounters impedance changes in the circuit, some of it gets reflected back. We'll label this reflected voltage as V-reflect.

Now, let's explore the properties of this reflected voltage. First, the reflected voltage has a much smaller amplitude compared to the input voltage. Let's say, for our example, it's approximately plus or minus 1 volt.

However, a crucial point to understand is that the reflected voltage maintains the same frequency as the input voltage. This means that these two waveforms directly impact each other, similar to the concept of wireless interference when the frequencies align.

Here's where it gets interesting. We effectively add these two waveforms together, just as we discussed with wireless interference. The result is a standing voltage pattern, which means it's not traveling in any direction. This standing wave is essentially a pulsating wave, varying with time. The pulsations depend on whether the crests of the waves align with each other or if a crest lines up with a trough.

Remember, the individual waves are still moving in their respective directions, but the combined standing wave doesn't move. Sometimes, the amplitude might reach positive 6 volts when the crests align, adding the 1 volt to the 5-volt input. Other times, it could subtract from the 5 volts, resulting in a pattern representing 4 volts. This back-and-forth pattern is what creates the pulsations in the standing wave.

The phase and frequency of the standing wave remain constant, which is why it's called a "standing wave" – it doesn't move. Instead, it pulsates between positive 6 and positive 4 volts (at least on the positive side of the origin) and bounces between negative 4 and negative 6 volts on the other side of the origin.

Describing a standing wave without visual aid can be challenging. I recommend conducting a Google search for "voltage standing waves animation" to find some enlightening animations. These animations will illustrate how the waves pulsate, with the maximum amplitude varying slightly above and below the input voltage.

What's important to understand is that the standing wave ratio quantifies how much the waveform pulsates. We'll delve deeper into this in the next blog. The overarching goal of this entire course is to grasp voltage standing wave ratios because they directly relate to signal loss in our transmission lines. Minimizing this pulsation is crucial, and this can be achieved through matched impedances.

The root cause of this situation is the varying impedances along the electrical path, which result in reflected behavior, similar to the water pipe analogy. Imagine a scenario where a radio is connected to a connector represented by a pipe of a certain size, which then connects to a cable with the same impedance, leading to another connector with matching impedance. In this ideal scenario, the electrical signal, the AC voltage, flows seamlessly without any reflection.

However, it's important to note that achieving this level of impedance matching is mostly theoretical. In real-world scenarios, it's nearly impossible to achieve perfect impedance matching. There are several reasons for this:

1. Cable Length: The cable length is a variable that can't always be cut to an exact length to match different connector types.
2. Variance: Even if two components are both rated at 50 ohms, for example, there may be slight variances due to manufacturing or other factors. One component might have an impedance of 50.5 ohms, while the other might be at 49.7 ohms, leading to a mismatch.

So, despite our efforts to buy components with the same rated impedance, there can still be impedance variations that lead to reflections and voltage standing waves. This variance is one of the challenges faced in real-world RF systems. In the next blog, we'll delve deeper into the Voltage Standing Wave Ratio (VSWR) to understand how these reflections affect our systems.

And so, in summary, it's generally considered impossible to perfectly match all impedances within a complex RF system due to the various components involved, such as connectors, cables, arrestors, attenuators, and signal boosters. These components have their own impedance levels, and achieving perfect matching in a real-world scenario is highly challenging.

As we've discussed, this impedance mismatch results in reflected voltage and the formation of a standing wave. The input voltage and the reflected voltage combine, and as they move in opposite directions, they cancel each other out, creating a standing wave. The wave fluctuates between higher and lower amplitudes, depending on how the waveforms align at any given moment.

While achieving perfect impedance matching is often unattainable, it remains a goal in wireless system design to minimize impedance mismatches as much as possible. This is critical for reducing reflected voltage and standing waves, which can impact the performance and efficiency of RF systems.