L1 Wireless concepts (RF & Power)

Wireless
1

Hello, everyone.

I unfortunately did not find a matching lesson, so I’m creating a new topic. NetworkLessons lacks lessons for the following topics:

3.3.a Describe Layer 1 concepts, such as RF power, RSSI, SNR, interference, noise, bands, channels, and wireless client devices capabilities
3.3.b Describe AP modes and antenna types

I have questions regarding watts and decibels. Fortunately, it’s only theory since I understand the calculations.

OCG says the following:

[image removed due to copyright]

I get what’s happening here but not the explanation behind it? If we use the logaritmic comparison function for dB, we will find out what the loss is in dB, which is -65. So what are they trying to tell us in the later paragraphs? What exactly is a better approach? Like they’ve just converted the watts into dBm which makes it easier to work with but what is this page trying to say? I am not quite wrapping my head around it.

The second pages say the following:
[image removed due to copyright]I don’t get the following:

By itself, an antenna does not generate any amount of absolute power. In other words, when an antenna is disconnected, no milliwatts of power are being pushed out of it. That makes it impossible to measure the antenna’s gain in dBm. Instead, an antenna’s gain is measured by comparing its performance with that of a reference antenna, then computing a value in dB.

Nothing generates power when it’s disconnected.. Are they trying to say that the antenna doesn’t produce any power on its own but it rather radiates the power the transmitter sends to it? So we use a different unit here, dBi?

[image removed due to copyright]

Does it matter which term we use, then? They just calculated the overall power at the EIRP by adding dBms and dBis together.

Also, maybe I am just confused, but when is dB used? If dBm is a power unit relative to the watts, dBi is a unit used to represent gain/loss, what is dB?

That’s all, thank you.
David

2

Hello David

First of all, I had to remove your images of the OCG due to copyright restrictions. Sorry about that… :man_shrugging: My response however should still be understandable in the context…

I will let Rene know to take a look at these.

If you have a transmitter and receiver, you can calculate the signal loss between the two by directly examining the power values at the transmitter and the receiver. The signal loss would be measured in dB. dB essentially gives you a ratio between the two, and it only has meaning if you know the actual power measured at either the transmitter or the receiver.

Now if you use a reference power, it gives you more information about what you’re measuring. That’s where the dBm unit comes in. dBm essentially measures the power and compares it to a reference power of 1mW. So dBm gives you the ratio of the power measured as compared to the reference power.

So if the transmitter has 20 dBm, and the receiver measures -65dBm, from that information we can determine the actual power in W measured at both locations as well as the signal loss between them very easily. It’s a matter of more convenient mathematics.

What it is saying is that antennas are a passive component of the whole arrangement. They don’t generate signal, but what they do is they redistribute the signal through space, concentrating it in particular directions.

To understand this, we imagine a hypothetical isotropic antenna, which is essentially a point in space from which the transmitted power is sent out in equal power in all directions. This is the isotropic antenna. A real physical antenna takes that signal and redistributes it in space in a particular manner. Depending on the antenna type, it concentrates that signal in a particular direction. The resulting increase in power in that direction is called the gain.

So the dBi is the ratio between the power you would measure at a particular point in space compared to what you would have measured if the physical antenna were replaced with an isotropic one. And the point in space that is used for such a comparison is the location where that particular antenna has a maximum concentration of signal.

So in much the same way that the dBm is used, the dBi is also used to measure gain, a comparison of the isotropic antenna with the real one at the maximum point of signal concentration.

Remember, both dBm and dBi indicate ratios in power (Watts), but in different things. Since they both use the same units, they can be used like this in formulas. It matters what term you use from the point of view of understanding which value comes from which component.

I think at this point you should have already realized that dB is just a ratio, it has no reference to units. dBi and dBm are ratios but they are ratios of specific units: Watts.

I hope this has been helpful!

Laz

3

Hello Laz.

So decibels are a reference in general, to any type of dB meassurement. It’s a ratio, so it basically compares/defines a relationship between something and something (like in dBm where 1mW is 0 dBm and so on).

dBm is specific to power (it uses a reference value of 1mW from where further calculations are done)

dBi is used to meassure gain on antennas (where you just add the gain to whatever the power was before the signal was transmitted)

My further question is what do they mean when they say that an antenna gain is calculated by comparing it to a reference antenna - the isotropic antenna? I know what this antenna is but how do they even come up with the gain value by comparing it with something that’s supposed to be perfect? And how does this even differ if they compare it with an actual (dBd) antenna?

What they’re saying makes sense but I struggle to understand how all of this exists and comes together. When an antenna is manufactured, is it actually tested or compared against something that’s just.. theoretical? How do they come up with the gain, then?

And sorry if the posted material was copyrighted, I thought it would be okay to use one or two pages of the book to provide more clarity regarding the situation.

The entire Wireless L1 stuff is like a whole new world. It’s not just networking that’s involved, it’s electronics and physics as well.

David

4

Hello David

Yes, that is correct. Also, dB is unitless. dBm and dBi are still ratios, but they are referenced to predefined quantities with units—1 mW for dBm and an ideal isotropic radiator for dBi, also using Watts. Because they are referenced to specific values with defined units, the resulting measurements also have units and represent specific, calculable quantities. And this is more useful than just a ratio.

Remember, an antenna is a passive component. The only thing it does is take the signal and redistribute it in space in a particular pattern depending on the type of antenna it is. That means that at some points in space around the antenna, the measured signal will be stronger, and at other points, it will be weaker. To understand how an antenna redistributes the signal, we compare it to the isotropic antenna.

This is a theoretical antenna that distributes the signal evenly throughout space. So, if the signal from the transmitter was able to enter space without any redistribution, that is, with equal transmission in all directions, that’s what it would look like. This is the reference from which an antenna’s effect on the signal is measured. It is useful because you can compare all types of antennas to this same reference pattern.

Essentially yes. The gain is the ratio between the measured signal strength delivered by the real antenna at the point in space where it is at a maximum, and the calculated signal strength delivered by the isotropic antenna at that same point in space. This gives a very good indication of the directivity of an antenna, or the capability it has to direct signal in a particular direction.

No problem, it is easily fixed… :slightly_smiling_face:

Exactly, it is indeed physics and electronics. I think we got into it a bit more than is necessary for the certification, but it is very useful and interesting information in any case.

I hope this has been helpful!

Laz

5

Hello Laz.

Sorry for going back to this, but I unfortunately do not get how they calculate the gain while referencing a perfect antenna.

They calculate the gain by verifying the strongest point of the signal, correct? This strongest point only applies in a certain direction on a real antenna and other directions won’t share the same gain, because they aren’t isotropic.

So… when is this comparison done again? Why can’t they just calculate how much the antenna increases the signal (the strongest possible value) and then just leave the gain at that? Why do they compare it with an isotropic one? Actually, what does it even mean that they compare it? :smiley:

I am unfortunately not quite following with how they actually compare it during manufacture/testing.

Thank you.
David

6

Hello David

That is exactly what is done, but “how much the antenna increases the signal” requires an initial signal to compare it to, and that is the signal of the isotropic antenna.

I think the missing piece of information here is the fact that a theoretical isotropic antenna is one that provides 0 dBi gain. This value is assigned by definition. In other words, it represents what would happen if the signal that reaches the antenna were to be distributed throughout space uniformly.

An antenna will redistribute that signal in a different pattern, exhibiting directivity in a particular direction. The “comparison” or ratio between the power provided by an isotropic antenna and a real one, measured as gain, indicates the degree of directivity the antenna provides. Gain is thus the difference or the ratio between the measured power of a uniform distribution of the signal and the actual power at the point of maximum power.

The term “gain” is appropriate because it represents how much power the signal has “gained” by being sent through the antenna. This is the “gain” in power that is achieved by the antenna’s concentration of the signal in a particular direction.

Different antennas will deliver different distribution patterns, resulting in different areas of coverage. The choice of antenna depends on its usage, but I won’t go into those here. Suffice it to say that the directivity serves to increase the signal strength in a particular direction to serve the needs of that particular deployment. I hope this makes it clearer for you…

I hope this has been helpful!

Laz