March 8, 2016 | AccelTex Reading Radiation patterns can sometimes be a little tricky. But, have no fear, AccelTex is here to help you decipher these magical lines! Let’s start with this fun fact: antennas are based on mathematical theory. There are 4 electrical characteristics of antennas that influence each other: Frequency Gain Polarization Beamwidth When you change one of the electrical characteristics, it will affect the others. Every antenna manufacturer should list these electrical specifications on their datasheets. Here’s an example below: This antenna operates in 2 frequencies, 2.4 and 5 GHz. When an antenna operates in multiple frequencies, sometimes the other electrical characteristics will differ based on this frequency. So, in this example, the gain of the antenna is 4 dBi for the 2.4 GHz frequency and 6 dBi in the 5 GHz frequency. The horizontal beamwidth is the same in both frequencies, but the vertical beamwidth is 144 degrees for the 2.4 GHz frequency and 55 degrees for the 5 GHz frequency. This antenna is vertically polarized. Polarization describes the way in which the sine waves travel. Vertically polarized antenna sine waves travel up and down and horizontally polarized sine waves travel left and right. For a whole primer in dual polarized antennas and their benefits, read here. For the purposes of this post, just know that that if an antenna is dual polarized, you will possibly have additional beamwidths indicated on the spec sheet for each of the planes and the patterns may show a little difference in their degree values for the horizontally polarized elements versus the vertically polarized elements. Now we know the basic specs of the antenna, let’s move onto the radiation patterns portion of the program… Radiation patterns are a graphical representation of how an antenna radiates and receives energy in space. Space is 3 Dimensional and is hard to convey on a 2D spec sheet. So to give you the best representation of the radiating and receiving properties of an antenna, we take the pattern at its maximum value and split it in half vertically and horizontally. And that folks is how you get the radiation pattern you see on most spec sheets. Before we move on, we have to get through the disclaimers. Keep in mind: The radio, once connected to the antenna, will impact how the antenna performs AP Output Power will affect your coverage area Match up the pattern for your frequency with the desired coverage area Down tilt and installation angles will affect your coverage areas Environmental factors (both physical and electromagnetic) will affect your pattern Got it? Good, let’s move on to the fun stuff! There are two types of radiation patterns you may see. The first is based on mathematical theory and the 4 main characteristics we talked about earlier (frequency, gain, polarization and beamwidth). This theoretical representation will show the coverage pattern of the antenna when all the antenna elements are working in conjunction at the same time. See below for an example: The second type of radiation pattern is an actual antenna pattern that shows a tested data plot of each of the antenna elements within the radome. See the below image, this radiation pattern has 4 colored lines representing each of the individual elements within the radome. As you can see each one is a little bit different. This antenna was tested one element at a time in an anechoic chamber and then the patterns for the 4 elements were placed on top of each other to create this pattern. What’s an anechoic chamber? Well, here is a picture of our chamber. An anechoic test chamber is a non-echoing, non-reflecting room designed to completely adsorb all electromagnetic wave reflections. When an antenna is tested in this chamber, all outside noise and electromagnetic waves are isolated so the test results are accurate and describe only the intended test subject. Yay. So let’s begin talking about planes. And no, I don’t mean the planes that fly in the air. We are going to talk horizontal and vertical planes. We talked a little earlier about spherical antenna patterns being cut in half. You have to slice you sphere in two directions, one slice is horizontal and the other is vertical. Now you have the best 2D representation of a 3D object. When you do this, there is a horizontal plane and a vertical plane. See the below graphic. The horizontal plane (also called the azimuth) is like looking at the antenna pattern from the sky. The width of this angle is called the horizontal beamwidth. The vertical plane (also called the elevation) is like looking at the antenna pattern from a profile view. The height of this angle is called the vertical beamwidth. Got it? Good! Now we can move onto discussing some basic antenna theory and how the gain of the antenna will affect it’s beamwidth. First let’s start with omni (or omnidirectional) antennas. Omnis get their name because their patterns are round. Pretty simple. The H Plane of an omni really doesn’t change, it’s going to stay pretty round not matter what. But when you increase the gain of an omni, you will get a bigger circle. So a low gain omni will cover a smaller area than a high gain omni. The big change in omni patterns will occur on the V Plane when you increase gain. A lower gain omni will be pretty spherical, almost like a balloon. But when you begin to increase the gain of an omni antenna, the vertical beamwidth becomes more flat. See the image below. Or, for a better visual, think of an elephant squishing that balloon into more of a pancake shape. That’s what higher gain gets you, a pancake. Sometimes pancakes are good, but imagine if you put a pancake (high gain) omni 60 feet in the air in a warehouse. That signal is not going to reach the floor where the clients are. So all you have is a nice pancake omni in the air doing absolutely nothing. Bad pancake. Ok, enough about pancakes. Let’s talk pies now. Directional antennas (also called patch antennas and even high density patch antennas) are just that. Think of them as slices of pie. How big of a slice do you want? Well, that depends on how hungry you are. Pies can come in a variety of diameters. If you have a small pie and take 1/4 of the pie, the slice is going to be very wide, but short. This represents your low gain patch. If you have a larger diameter pie and want the same amount of pie as the 1/4 of the smaller pie, your slice is going to be skinnier, but longer. This represents your higher gain patch. You get the same amount of pie in both examples, even though your pieces physically look different. The H and V planes on Patch antenna operate very similarly. Lower gain patch antennas have a wide beamwidth, but short distance. As you add gain to an antenna, those beamwidths will become more narrow, but they will go farther. The best description of this is a flashlight. When you have the aperture on the flashlight open, you can see a wide area, but you can’t see too far. As you close the aperture, the field of view becomes more narrow, but you can see further. A patch antenna works just like that. So now you understand a little more about antennas, and how the relationships between antenna frequency, gain, polarization and beamwidth work. So let’s get down to the actual reading of the antenna patterns. We read antenna beamwidths at the 3 dB line. Here’s an example of an antenna radiation pattern. To read this, you will count the degrees to the left and right of the 0 degree line and stop where each one of those antenna element lines crosses that 3 dB line. I know, I know. You are thinking “Are you kidding me? I need my glasses.” But remember, the antenna vendor will tell you in plain English words on the spec sheet what that degree is. Phew, thank goodness. Each antenna should have a separate antenna pattern for each frequency and each H and V plane for that frequency. So a 2.4/5 GHz antenna will have a total of 4 patterns, 2 describing the H plane, one at 2.4 GHz and the other at 5 GHz and then another 2 patterns describing the V Plane, one at 2.4 GHz and the other at 5 GHz. So the last question you may have is why we use the 3 dB beamwidth line to read the beamwidths. Well, the 3 dB line indicates the beamwidth when the radio is at half power. Its a point in the middle of no power and full power that almost every antenna manufacturer has standardized on. The outer line of the radiation pattern shows the pattern when the radio is at full power. On the pattern below, when the radio is at full power, the beamwidth is very, very narrow. It almost comes to a point. At no power, you see there is almost no beamwidth either. So, if you increase the power, you can trace what the beamwidths would be at every power level. And that there fine folks is how you read radiation patterns.