Before I knew anything about the language of radios, this was always puzzling to me. I always thought I understood it for those fancy, powered antennas. It always seemed to make sense that some powered antenna would have an amplifier in it that would enhance the signal going out or in. But the whole concept of a passive antenna was a mystery. So, for those of you that are in the dark, but afraid to admit it, the only thing you really need to do is to think in the language of radios.
A few blogs ago, I talked about the beauty of an isotropic antenna  both transmitter and receiver, and how they made the mathematics of radios really easy. If you recall, an isotropic antenna transmits and receives equally in all directions  be it up, down or side to side. Hence, the radiation pattern is a perfect sphere. In the world of math and geometry, we love spheres because the math is nice and easy. You just need to remember where to put the π. So to make it easy, we like to describe radios in terms of isotropic transmitters and receivers because we all like easy math. The problem is that there is really no such thing as an isotropic antenna. Picture this  an isotropic antenna would be the same as a person, who when speaking, would speak equally in all directions and also have ears that would hear equally in all directions. We all know that, while our mothers may have had eyes in the back of their heads, they couldn’t hear and speak that way.
Okay, so back to the world of antennas. To simplify things, we’ll focus on the world of transmitters first, knowing that the same principles apply to receiving as well, just in reverse. The key to the principle of antenna gain is to describe what the performance of an antenna as compared to an isotropic one. Let’s pretend for a moment that we have two antennas: antenna 1 is an isotropic antenna that transmits equally in a perfect, spherical pattern and antenna 2 is half of an isotropic antenna such that is transmits in a perfect semispherical pattern. Now, assuming the same power into the antenna, we can see that the semispherical antenna will transmit nothing on one side and twice the power on the other side. This is because the surface area of transmission is exactly half that of the isotropic or spherical antenna. This will be true at any distance and at any power. We might call this antenna a directional antenna because it sends the signal out in a specific direction.
Finally, in order to complete our understanding of antenna gain, we need to use dB or decibel math. I like to think that the notion of the decibel was created in order to avoid having to deal with really big numbers. So, for those of you that love logarithms, here is your chance to shine. The best way to think of it is that a value in dB is equal to 10 times the log of the number.
Original number

Number in dB

1

0

2

3

5

7

10

10

50

17

100

20

200

23

So, looking at the table above and putting the pieces together, we see that our semispherical antenna outputs twice the power in the desired direction so it is said to have a 3 dB gain. Now imagine an antenna that could focus a transmission into a tight beam, kind of like a flashlight does. We might very well find that this sort of antenna might reduce the area and hence increase the directed power by a factor of say 200 over its isotropic counterpart  often times antennas like this might resemble a plate or dish. In this case, we would say that it has a gain of 23 dB. Finally we need to add one additional letter, the letter “i” after dB. In this case the letter “i” means “relative to” or “increase over” so that we know the measure is relative. For our complete notation, we will describe an antenna in terms of dBi. All in the name of making the math easy and keeping the numbers small. So remember, the next time you see antenna described as 7 dBi, know that is just means that its transmission surface area is 1/5 the size of an ideal, not possible isotropic antenna.