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Fundamentals of RF

16. Absorption

So absorption is what we would say happens to a signal when it doesn't bounce off, which will be reflection or moving around an object, as we'll see here with some of the other terms that you'll see. But instead, it doesn't bounce off; it doesn't move around. But if it cannot move through the object, then we would say we had 100% absorption. So, imagine that I had a wall and even a door—wooden or metal—it wouldn't matter. I'm hoping I've got some perspective going on with this office building, so that'd be the sidewall, and then, of course, we'd have the rest of the floor, but I don't think I'm going to try to make a 3D object.

But the idea is, if I have this antenna that's going to start radiating, in this particular case, if it cannot get through the wall, then we would say 100% absorption. But, when we look at that signal, and perhaps it's better to make it look like a sine wave, we have a high amplitude signal, and when it hits that wall, it has a lower amplitude when it comes out. And that's because the wall managed to take some of the power out of the amplitude. And so we still have the signal going through, but we would say that some of that signal has been absorbed. As an example, with a 2.4 GHz signal, if it goes through a brick wall, we'd be at about 116 percent of the original power. The same would apply to a brick wall or drywall, right? Just regular brick, not a large, solid brick. We would say we would lose half of the original power. So, as I attempted to illustrate, the change in the amplitude of that signal as it moves through an object is just one example.

17. Reflection

Now, reflection is basically the signal bouncing off a smooth surface, and it has to be a surface that is larger than the actual wave. And that just means it will change the actual direction of the wave. So let me make a little bigger office building here as an example. And this will lead into another issue called "multipath."

So, if I have my antenna in one part of the room and my laptop over here that wants to connect to this wireless network, as that signal is radiating, and remember, it's radiating in many directions as it moves out, It is possible, but that signal might bounce off this wall and get to my laptop, while at the same time, I'm going to have a direct signal. And maybe I have another one bouncing off of here. And so I'm going to technically see that signal in this little picture three different times—the same signal, the same information. The reason I'm going to see it three times is because these travellers are obviously the light.

The distance travelled by the first reflection to reach me was equal to or greater than the direct distance that connects to me. Even though we might measure it in nanoseconds or even potentially in milliseconds—depending again on how much time has passed or how much reflection we have—I'm still seeing the same signal multiple times. Eventually, that's going to move us into this idea of multipath. Now, one of the things you might notice is that there is what we call a "sky wave reflection." And for frequencies that are less than a gigahertz, like an AM radio, you might have noticed at certain times of the day that you can hear a radio station that is hundreds or even thousands of miles away on a clear night. And so what we're thinking here is that if I'm talking about planet Earth, and planet Earth has an atmosphere around it, when I'm transmitting that signal, it will bounce off that atmosphere and come to a different location.

As an example, where I live in the Pacific Northwest area, I can hear channels at night from neighbouring states where I know people, like in one case when I was in Seattle and was able to pick up an AM radio station in a city that was 600 miles away. And that was the idea of that signal bouncing off of that atmosphere. The skywave microwave reflection will be seen between gigahertz and 300 gigahertz. So when we see this again, remember that the reflection is what we said: when it hits an object that is bigger than the wave, then we will see this reflection. And in this case, right, the signal will be faster as we measure them from one to 300. That means that this signal could bounce off of smaller objects, like a metal door.

18. Scattering

So scattering is another thing that we have to look at with our waves. And you could actually think about it as being more than one reflection at a time. If ever you've seen a movie or been somewhere where they have these really cool things, what we called disco balls, and they've got all these different, different-shaped mirrors, not just flat ones, but ones facing different directions, And if somebody is shining a light onto this ball, we start seeing different colours come out, which is the multiple reflections occurring. And the way it's reflected helps us see colour differences; when a signal passes through a medium of small particles, we see scattering. Now, another example is if I put planet Earth here again, and now I'm not going to try to draw continents or anything else like that.

And we have an atmosphere around us, and technically, that atmosphere is made up of very small molecules: oxygen and nitrogen. I'm not a scientist when it comes to chemicals, but anyway, and then we have the sun out here. The sun, yes. I draw like my four-year-old, what can I say? And obviously, it's emitting its own part of the spectrum, the visible light that we see. And as that comes into our atmosphere, these small particles like my disco ball will change, basically due to the do scattering of that signal, which is at least one explanation that I remember being given for why, when I go outside, the sky looks blue. It looks blue because of the scattering effect of the light that's being sent.

It's not that the sun is sending a frequency that emits blue light; it's just how it scatters and what we see. And, of course, as the sun appears to us anyway, as our planet rotates and the sun appears to us to be setting, that angle shifts. And so then you start to see those really cool oranges and reds and different colours from that scattering.

19. Refraction

Now, refraction is where a signal basically gets bent. We're not going to change the frequency of that signal. You could see, again, some absorption, but the idea here is, you know, in this case, I've got these two microwave towers trying to communicate between them, and depending on where they're placed and how tall they are, we might actually see some other bending. But the idea is that when we have something like just cool air, well, that air, like I just said, with scattering, could cause some problems, but usually we have no problem.

But if we had water vapor, when that signal hits, that water vapor, because it's a little more dense and has more to it than just the regular dry air, could make that signal or that light bend, and that would be refraction, as maybe you've noticed if you've ever had a glass of water. So I got some water in here, and when you put a straw into that water, it looks like the straw suddenly bends in that water because, as the light passes through this medium, the signal gets bent going through that different medium. And of course, the straw is really not bent; it just appears that way. And that, again, would be an example of that refraction.

20. Diffraction

Now, diffraction is like reflection. That means we are bending the signal. But in this case, it's going around an object and not passing through it. So this is the greatest picture I could give you. You know, if I put this big rock in the middle of my river here, the water, which is the signal, wouldn't be able to get through. And so it's basically bent around the rock. And so it differs from refraction in that it does not pass through the medium, such as a glass of water, but rather goes around it.

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