CWNP CWNA – Components and Measurements of RF Part 2

11. Watt Part1

So when we start off with the measurement of a watt, and remember, we’re going to be talking about milliwatts, or one thousandth of a watt, it is a basic unit of power. And some of the ideas here are to say, you know, if I had this hose, this water hose, that was going to be used for whatever reason. And when we talk about the water that’s flowing through the hose, we would call that the current that we’re sending. What we’ll do is talk about the pressure generated by the power washer.

Again, a power washer is one that’s going to take that water, which normally when it flows out of the hose is pretty benign, right? I mean, the water comes out, and it’s not under high pressure. It depends on how big the hose is, I guess, and how much you turn on the water to go through there. But it by itself isn’t going to clean your house, your deck, or your car.

So what we do is talk about the pressure generated by that machine, and that pressure in electrical systems is going to be what we call voltage. Instead of water flow, electrical systems now have current, which indicates how much water is flowing through there. We measure them in amps, and the wattage is equal to the voltage multiplied by the amps. All right? So if the amps are going to be, let’s say, uniform with this water hose, I have it on full blast. The hose is filled to capacity; water is coming out. Then there’s one of those little nozzles that looks like a gun when I add it on. It’s a pressure washer-type thing. And now that stream is certainly much more powerfully coming out; that’s more pressure with the same amount of current.

We are still having the same amount of water flow through that hose, but more is coming out because of the pressure from that nozzle, which changes the way in which we would measure it anyway as far as how powerful it is. So the watts then are going to be the volts—the pressure, or in this case the focusing, if you will—of that water times the amps, which is how much water is flowing through. So if I turn off or turn half off the water going through the hose, then the current is going to be lower, which means that even with the pressure we’ve added, the total amount of wattage is going to be lower. So there is a relationship between them.

12. Watt Part2

Now, as I said, when we talk about our WiFi, the RF, we’re going to measure everything in milliwatts. And that, again, is still a unit of power. As I said, it’s just one 1000th of a watt. Now, you have to be concerned with milliwatts because most of the indoor 8211 equipment that you’re going to be using is going to transmit power between one milliwatt and 100 milliwatts. I know it doesn’t sound like a lot, but when it comes to radio frequency, we obviously see that it works very well.

13. Decibel (dB) Part1

So let’s talk about the decibels. One of the first things you need to know is that it is a unit of comparison, not a unit of power. Again, we’re just comparing or trying to find a way to compare two different frequencies or two different sets of power to each other. It’s kind of a ratio. So we’re just trying to show the difference between two values. So really, then, a decibel is a relative expression, and it’s just that measurement in the change of power. More often than not, decibels refer to the amount of power at this location of the antenna that begins to radiate the radio frequency versus being 20 feet or 40 feet away.

Then what is that power level by the time it reaches you? And that’s what the decibels will do, which is give you an expression of how that power has changed from where you were to where you’re going. And, of course, we discussed everything that contributed to the change in power, including free path loss. So that’s all we’re measuring. So it’s just a comparison or the difference or the loss of the EIRP output from that transmitter’s antenna to the amount of power that was received by the receiver’s antenna. Enough.

14. Decibel (dB) Part2

Now, the unit bell Bell is actually defined as a one-to-one ratio of the powers of two sounds. So, as an example, if I had an access point that was transmitting data at 100 milliwatts, and then laptop one received the signal from the access point but at a power level of ten milliwatts, then we would say that the ratio of what was sent at 100 to what was received at ten would be a ratio of ten to one. Or, in that case, it would be measured as one bell.

15. dB Mathematics

in order to get an understanding of the mathematics. And the reason we use this idea of a bell is because the mathematics behind it is close enough for our measurements, if not completely accurate. If we did the mathematics with the actual amount of AC current that’s received, that would be very difficult. So the idea is that part of the actual equation deals with logarithms. Now, I don’t know if you remember about logarithms from times you were in math class, but the term “log” with the subscript “X” just means that we’re using a base-ten system.

A base-ten system means that we have those digits right from zero through nine in our numbering system. Whereas if we used a base-2 system, which we call binary, the only digits are zero and one. If we used hexadecimal, it’s a base of 16. As a result, it employs the numbers 0 through F. So it’s about how many digits we have. And base ten is what we’re used to in most of our numbering systems. And because we’re doing a ratio of ten to one, we’re going to be using base ten for the logarithms. The idea is that if we look at a number like ten to the first power, we know that ten to the first power isten, or at least I hope we do.

And so the logarithm that we would use to figure out what the exponent of this setup was would be ten. The number we had multiplied by the letter “O” to the base ten of ten. So the log in base ten of ten basically asks us what that number, that exponent was. So if we had ten to two, which is 100, the same thing would have happened. And by the way, it’s not ten times a log. I’m sorry about that. That’s just the number ten; converting, in this case, ten to the second power is 110 squared. So then if we said what is in base ten, the logarithm of the number 100, then we would get the exponent, and it continues to go that way, right? If it was ten to the third, that actually starts off as 1000.

And so we keep going through that. That rings a bell. That’s how we’re doing this comparison for the number of bells; a DB, or decibel, excuse me, is one hundredth of a bell. And the way in which we calculate those is to actually use the number ten multiplied by the log of base ten of the initial power versus the received power. So if you think about it this way, in that last comparison, we were talking about the initial power, which was 100, and when it was received, it was received at ten milliwatts. So these are milliwatts. And when we do that division, 1100 divided by ten, we get the answer ten.

And we know that the inverse of ten, the number ten, will be one. And so it will be, and once again, you may say, “Wait a second.” You said that ten to one was one bell. Yes, I said it was one bell, but we are talking about decibels. So, once again, one hundredth of it. So that’s where we would say, “Okay, the log in this case would come out as ten,” and that would be ten decibels as opposed to how many bells it would be.

16. dBi Part1

Now, when we look at the calculations, one of the terms we had was “DBI,” which is the decibels of that IR, that isotropic antenna. And it’s important for us to be able to calculate the radiating power of the antenna so we can determine how strong a signal is at a certain distance from the antenna. So, again, why is this important to us? Well, if I’m looking at this office building that you have, and maybe the office building is 100 feet by 30 feet in size, that’s a pretty good-sized office building, I guess.

Where do you put the access point to be able to see all of the people that might be in the office? I mean, if you say, “Well, maybe I’ll put the access point here,” Okay, well, you need to be able to calculate the strength of that cell; we call them cells. But in other words, what is the area that that will cover to a point where the measurement of the strength in these decibels will be too weak for us to actually use that signal?

And so that’s where we use this information to make that decision about where to put these access points to get the kind of coverage that we want. And maybe I don’t want to have an access point in the corner that’s going to be radiating my signal out to the parking lot for everybody else to get onto. So that’s where we might also have to measure a different area with a directional antenna. And when we talk about antennas, at some point in the future, you’ll get an idea about some of the different types of directional antennas.

But again, that’s just helping us learn what our coverage area is going to be. That’s why we say the signal is at a certain distance from the antenna. In this case, the measurement is the DBI, which is expressed in decibels isotropic. Comparing measurements from an isotropic radiator, or, once again, the decibel gain relative to the radiator or the change in power relative to an antenna. The big thing to remember is that isotropy is basically non-directional, and we’re trying to determine what that area of coverage is going to be. It’s just a fancy way to say, “How far away from the antenna can I be and still have enough signal strength that I can receive?”

17. dBi Part2

Now, we talked about the power of antennas, and we measured that by this idea of gain. And you may recall that when we discussed these antennas. Here we go. Put a little base on there. Is it possible that some could be directional, while others could be omnidirectional? But between that and the amount of power from that transmitter, we’re going to see an overall gain.

Well, all right, we can figure out what that gain is because we know how much power we are using and we know the type of antenna that we have, so we can get that as an absolute measurement. But the problem is, again, that as this radiates out, we’re losing the power of that gain. And so this DBI is simply a measurement of the antenna’s gain in a measure that we can use as a relative change. Rather than doing the math to calculate the gain, we can measure the power at the beginning and then measure the power at the end. And again, it is a unit of comparison to compare what those two powers are.

So we start anyway at the strongest point or the focus point of the antenna signal, and then we can measure what that power is at some other location. And so we’re going to get a sense of how much of that power has been attenuated as a matter of comparison. And again, you could do the full mathematics, but the actual equations become a little more complicated when you’re dealing with one hundredth, one thousandth, and going on from there. And so, having a way of just doing a comparison gives us a good idea about the power loss of what we can measure and, again, the idea of the coverage area.

18. dBd Part1

Now DVD is again a measurement of relationship, but it’s considered dipole or decibel gain relative to a dipole antenna. So the first scale we did talk about for the measurements was the DVI, and now we’re going to look at the DVDs, or decibel gains, relative again to that dipole antenna. So the DVD, then, as we say here, is the increase in the gain of an antenna. When is it compared to the signal of a dipole antenna? Well again, how do we get that increase? Well, that’s where we might have an antenna that uses a little more directional type of signal so it gets more focused, and that would create more gain than we would have had if it was omnidirectional. And, of course, the AC power is still very relevant in that setup.

19. dBd Part2

What I just said about DBD seems simple enough, but how do you compare two antennas when one is represented with DBI and the other is with DBD? That’s a little more difficult. Well, it sounds difficult right now. We’re told it’s quite simple. So a standard dipole antenna has this DBI value of 2.14.

And then if I get this antenna that has a value of three DBDs, which are the DVDs again, the DVDs are the measurement of the gain versus the power coming out of that antenna. So I’ll draw my typical picture here. And again, you can just think about this as a flashlight on a flashlight. In some cases, you can make the beam very narrow by twisting the actual casing where the light bulb is. Or you could make it by twisting it a little bit wider, but it’s not as powerful, right?

It is less focused but covers a larger area. That’s what we’re measuring, so the idea of how much gain I have is based again on things like the antenna, the amount of power, maybe even some loss of power as the electricity goes through the cable, and maybe you have something to amplify it. There are so many choices here. But anyway, if the dipole antenna is 2.14, then all you have to do is add the DVD, the gain, to that 2.114. The power is the gain. And by adding it together, what you get is that you have a DBI of 5.114. because we’re basically adding three points to the power’s gain. That’s three points that could be attributed to the signal being focused in one direction.

20. dBm

Now, DBMS is a comparison to milliwatts. So instead of comparing the signal from one to the other, it’s used to compare the signal to what we call one milliwatt of power. So DBM is simply the decibels related to that one milliwatt. And we begin with the unit of one milliwatt, which has zero DBMS. So that means, basically, we’re at the starting point of what we would be looking at. Now, when we want to figure out the DBMS, you can see the equation for ten times the logarithm base ten of the power measured in milliwatts. So if I said, “Okay,” I’ve got ten times the log base 10 of 100 milliwatts. Well, that little rhythmic part comes out to two. And the ten, of course, multiplied, tells me that I’d be at 20 DBMS.