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Fundamentals of RF

21. Attenuation

Now, attenuation is what we call the loss of a signal over distance. And again, if you go back to my attempt to draw a lake and, you know, basically I'll just make an antenna as that signal is close, I said that energy doesn't have as far to spread because as it spreads out, then there's more distance. And so that same amount of energy has to cover a larger area. And that then means that we see the appearance of the signal getting weaker because that same amount of energy has to cover a bigger area. As a result, we would refer to this as attenuation. It's basically spreading the same amount of energy over a greater distance. And at some point, that energy would be so low that the receiving antenna wouldn't be able to detect it. That's where we'd say, basically, it looks like we lost the signal. It doesn't mean the energy is doing anything other than just being there, and it's so weak that we can't hear it anymore. And that's what we call the freespace path lost, or the FSPL.

22. Multipath Part1

Now, multipath can be good or bad. I realise that sounds a little too political there. I'm taking both sides. But it's a phenomenon where they call it a propagation phenomenon, where it says that two or more paths of a signal arrive at the receiving antenna at the same time or within nanoseconds of each other. So this is where I talked a little bit about reflection.

Not too long ago, I said, "You know, I have an antenna that's going to radiate a signal." I've got somebody over here who wants to hear it. And that signal can, while it's radiating, be seen directly. But as I said before, it could bounce off and come back to the same device just nanoseconds later. And so that's an issue of reflection. But because I'm receiving the same signal in this picture twice, we would call that multipath. Now, most of the time, multipath is destructive, which means that we may be cancelling or weakening other signals regardless of whether we call down phase.

Remember that this will be apart from phase, right on that signal. It could also increase the amplitude, what we call the upfade, and it could also corrupt the data that we're sending just because I'm getting the same frequency, the same signal, and the same information, a little bit out of order, meaning the multipath issue. Now, where I was going to get into multipath being a good thing, when we get into some of the high throughput and very high throughput, we'll see where we're going to try to take advantage of having multiple signals. But in this case, we're talking about a single signal from the antenna that is reflecting and arriving in, causing it to appear twice or more at the receiver.

23. Multipath Part2

This is probably a better picture than the ones I've been trying to draw. But again, the idea is that, you know, from my laptop, as I'm emanating these signals, one is going to hit the access point directly. One could bounce off this back wall and come over here. It could literally bounce off other objects, right? Just the reflection So, for example, when we first looked at the design of an antenna for these antennas, and remember what we said at 2.4 GHz, we saw the distance of the wavelengths from one point to the other being about four; I forget what it is now. four, two, or something like that, inches in distance.

And so when you saw some of these original access points, they had two antennas that were about 4.2 inches apart. And the reason for that was to correct this multipath problem so that the access point could say, "All right, I'm getting this signal on both of my antennas, and they're coming at different times." One of them is stronger than the other. So then it picks an antenna to use for receiving and transmitting and tries to basically drop all of the reflected or multiple paths that are coming in.

24. Gain

Gain is known as the amplification or increase of amplitude. And we can use different devices to boost the type or amount of gain. So remember, my antenna is basically connected by a copper cable to a power source. And one of the things that we could use to increase the gain is to increase the amount of power that comes in from the power source to the antenna.

And now, active gain, it says, is usually caused by the transceiver or the use of an amplifier on the wire that connects the transceiver to the antenna. The transceiver is this little guy here, sending the power. So we could put an external object that could be an amplifier onto this, and that would boost the power as well. Another thing we can do with passive gain is focus the radio frequency signal using an antenna. In other words, rather than making it omnidirectional, we could make another antenna from it. Say that we're just going to send that signal in one direction.

So how does that work for us? Well, if you have ever worked with this little object we call a flashlight, what you're seeing is that because of the way in which we're focusing, there's a little light bulb in the middle. The way in which we are focusing that light makes it appear to be very bright as opposed to that same little light bulb. Well, it looks like a mushroom, but if that same little light bulb didn't have that little metal cone helping with the actual focusing, then it might not appear as bright because it's going all over the place. We didn't have to increase the power. We didn't have to add an amplifier. It was just the way in which the antenna was made that gave us that focus and increased the passive gain.

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