Introduction to Antenna Radiation Patterns

This article will cover the following topics:

• What is an antenna?
• How do you read and understand antenna patterns?
• What do the antenna parameters mean?
• What are the different antenna types? (Omni vs. Directional)
• How do you use antenna patterns to make decisions in wireless deployment?

What is an antenna?

An antenna converts electric power into radio waves, which radiates out from the antenna in a three-dimensional space. Wireless devices transmit data over the radio waves, which are commonly known as a “signal”.

Antennas are indispensable for any wireless device. Some devices contain internal (or built-in) antennas, which cannot be seen from the outside. Other devices do not have internal antennas, therefore external ones need to be attached to them.

How do you read and understand antenna patterns?

Briefly, an antenna pattern (or a radiation pattern) shows the intensity of radio waves which the antenna delivers in each direction. However, there are various terms and aspects that should be considered, which we will delve into below.

■   Isotropic Radiator

Other than directions and the antenna’s own properties, there are several factors related to the radiation intensity; such as the power conducted to the antenna and how far radio waves travel. To eliminate the effect of those factors, we need to introduce a reference antenna called an “Isotropic Radiator”. An Isotropic Radiator is an ideal antenna which radiates energy equally in all directions. To make out a given antenna’s own properties, we compare how it performs relative to how an Isotropic Radiator would in the same circumstance. Any changes of those factors that cause the same effect to both the antenna and the Isotropic Radiator, cancel out.

■   Directivity

An antenna delivers radio waves in three-dimensional space. A given antenna can perform differently in different directions. Therefore, each time we talk about the intensity of radio waves, we need to designate the direction first. One can easily figure out which direction the signal is stronger or weaker in from the antenna patterns.

■   Operating Frequency

An antenna can operate on different frequencies and perform differently when it transmits radio waves of different frequencies. Wi-Fi has two frequency bands, namely 2.4 GHz and 5 GHz. Therefore, antenna patterns are often shown separately in different frequency bands.

■   Gain

Gains measure the radiation intensity of antennas in any given direction. We commonly express gains in the unit “dBi”. The more dBi an antenna has in a particular direction, the more intensely it transmits in that direction. For detailed information about gain, refer to “Gain" in the “What do the antenna parameters mean?” section.

■   Types of Antenna Patterns

An antenna delivers radio waves in the three-dimensional space. As a result, it can be a little tough to characterize antenna performance in each direction with a single pattern. There are three-dimensional antenna patterns, however, we can also draw the patterns in a two-dimensional way. Typically, the manufacturer provides a set of patterns, which fall into three categories depending how they are drawn and presented. We will subsequently cover a few types of patterns, explain how to read and understand them, and discuss the pros and cons of each type. For consistency, all antenna patterns here characterize the same antenna.

• Spatial 3D Patterns

An example of spatial 3D patterns is shown in Figure 1-1. The colored patterns form a curved shape. In the three dimensional space, a fixed origin point (Point O) is located inside the shape. Each direction away from the origin point indicates a real spacial direction, and the color in that area indicates the gain in that direction. The color scale on the right indicates the value of gain that each color stands for using dBi as the unit of measure. Additionally, the distance between any point in the graphic and the origin point indicates the gain in that direction.

This is the most easily visualized type of antenna pattern. However, a 3D pattern cannot be fully displayed in the 2D plane of the screen or paper. In other words, we can only draw and view a part of it from a certain perspective.

Figure 1-1  A Spatial 3D Pattern

 

• Principal Plane Patterns

Principal plane patterns are derived by simply slicing through the Spatial 3D Pattern. There are two principal planes, namely the azimuth plane and the elevation plane. We slice through a horizontal plane of the spatial 3D pattern in Figure 1-1 to get the azimuth plane pattern in Figure 1-2, and the vertical plane  to get the elevation one in Figure 1-3. In each pattern, the angles around the circle show different directions in the plane. To tell the value of gain in any direction, draw a line from the center in that direction to meet the contour of the pattern. The distance between the point of intersection and the center indicates the gain in that direction. Moreover, a greater distance indicates a higher gain. Therefore, points in the same concentric circle indicate the same gain. To be more specific, the Gain (dBi) scale next to the pattern shows the gain value each concentric circle stands for.

This type of antenna patterns is useful when you want to look into the antenna performance in the horizontal or vertical plane. However, patterns in two principal planes cannot contain the full information in the three-dimensional space.

Figure 1-2  An Azimuth Plane Pattern

Figure 1-3  An Elevation Plane Pattern

• Plane 3D Patterns

A plane 3D pattern interprets the spatial 3D pattern in a single plane, as shown in Figure 1-4. The center of the image is the origin point. Each line indicates a specific horizontal direction. Each circle indicates a difference in the vertical plane. In this case, the vertical direction angles range from 0° (right above the antenna) to 90° (at the level of the antenna) in an increment of 30 degrees for each concentric circle. The color scale on the right indicates the value of gain that each color stands for using dBi as the unit of measure.

Figure 1-4  A Plane 3D Pattern

 

What do the antenna parameters mean?

The key antenna parameters, including gain and beamwidth, are derived from antenna patterns.

■   Gain

The radiation intensity in each direction is the most important property of an antenna. We can quantify this property with a parameter called “gain”. Gain is defined as the ratio of the antenna’s radiation intensity in a given direction to that of an Isotropic Radiator. Although you can simply express gain numerically, for example, you can say the gain in a certain direction is 5 times that of an Isotropic Radiator, it is much more convenient to introduce a new unit “dBi” (decibels relative to an Isotropic Radiator) for gain. Gain expressed in dBi is computed using the following formula: Gain_in_dBi = 10×Log (Numeric_Gain). To directly calculate gain in dBi is difficult, however, you can reference the conversion chart in Table 1-1.

When a single number is stated for the gain of an antenna without any direction mentioned, as is common among specs, it means the maximum gain among all directions. For example, if a spec says the gain is 3.5 dBi, it indicates the gain in the direction of the maximum radiation.

Table 1-1  Coversion Chart Between Numeric Gain and Gain in dBi

The followings are a few tips for you to understand this table better.

• If you know the numeric gain, which is exactly the same as a value in the table, you can quickly tell the corresponding gain in dBi and vice versa. For example, if the gain in dBi is 3.0, the numeric gain should be 2, namely the gain is 2 times that of an Isotropic Radiator.
• If you know the numeric gain is between two values in the first column, you can tell the gain in dBi is between the two corresponding values in the second column and vice versa. For example, if the gain in dBi is 6.0, which is between 4.8 and 7.0, the numeric gain should be larger than 3 and smaller than 5.
• If the numeric gain increases, the gain in dBi also increases, which means better antenna performance. For example, an antenna with a gain of 7.0 dBi in a direction performs better in that direction than another one with 6.0 dBi.
• Multiplying numeric gains results in adding gains in dBi. For example, in a certain direction, if the gain of antenna X is 2 times that of an Isotropic Radiator (that is 3.0 dBi), and the gain of antenna Y is 5 times that of X (that is 7.0 decibels relative to X), then you can calculate that the gain of Y is 3.0 dBi + 7.0 dBi = 10.0 dBi. This indicates a numeric gain of 10 according to the table (that exactly equals 2×5).
• Gain in dBi can turn out to be a negative value. Let’s assume +n dBi (n being a positive value) corresponds to a numeric gain of m (m being a number larger than 1). Then -n dBi indicates a numeric gain of the reciprocal of m, which is smaller than 1. For example, -3.0 dBi indicates a numeric gain of 1/2=0.5, which means the gain is half of that of an Isotropic Radiator.

■   Beamwidth

Beamwidth is the angular measure of the antenna signal coverage. Typically, beamwidth in each plane is defined as the angle between the points where the gain decreases by 50% (or -3 dBi) from the peak of the main lobe of the radiation pattern. We can define beamwidth in different planes. For example, elevation plane beamwidth indicates the beamwidth of the vertical plane.

Let’s illustrate how to calculate the beamwidth with Figure 1-5. The red line indicates the direction of the peak. The two orange lines indicate where the gain decreases by -3dBi from the peak. The beamwidth is the angle between the orange lines. In this example, the beamwidth is about 35°.

Figure 1-5  Antenna Beamwidth

 

What are the different antenna types? (Omni vs. Directional)

Can we simply equip all the devices with one type of antenna which can both provide sufficient gain and cover every direction without dead zones? It is not possible. An antenna never creates radiated power, although it does make gains of the power by transferring and redistributing power. Power needs to be concentrated to provide more gain. In other words, an antenna strengthens power in certain directions by sacrificing some of its ability to transmit power in other directions, which usually leads to a smaller coverage. An antenna designer has to find a balance between gain and beamwidth.

Antennas, whether built-in or external, fall into two categories; namely omni antennas and directional antennas.

■   Omni antennas

An omni antenna has well-proportioned moderate gains in each direction in a certain plane, such as the azimuth plane. Therefore, we don’t need to consider directivity and beamwidth in that plane. TP-Link Omada EAPs, for example, typically have built-in omni antennas, which are ideal for broadcasting Wi-Fi from the center of a location to fill the area.

■   Directional antennas

On the contrary, a directional antenna can have higher gains in certain directions but cover a smaller angle. Beamwidth becomes a critical parameter. Increasing the gain tends to reduce both the azimuth and elevation plane beamwidths, and typically increases the number of zones where the signal is weakened. This makes the antenna more sensible to displacement and orientation variation, and thus harder to deploy. TP-Link Pharos CPEs, for example, typically have built-in directional antennas, which are ideal for setting up point-to-point connections over a very long distance.

 

How do you use antenna patterns to make decisions in wireless deployment?

If you are deploying a wireless device with built-in antennas, you don’t need to bother with the choice of antennas. However, it can be somewhat challenging to optimize devices’ antenna performance. One practical principle is to make full use of the zones where the signal is strengthened and avoid any zones where the signal is weakened.

If you want to deploy a wireless device in a small room, it isn’t very useful to refer to the antenna patterns. This is because most products are well designed to handle this scenario as long as they are mounted correctly. Besides, signals are refracted by the ceiling, the floor, and the walls to cover the zones where signals are originally weakened. However, if the room is spacious, the effect of antenna performance cannot be ignored.

In the following example, we want to create a Wi-Fi setup with a ceiling-mount access point. For simple demonstration, the elevation antenna pattern is attached to where the access point is deployed. As shown in Figure 1-6, zone A and zone C are fully covered because antenna gains in these directions are considerable. Notice that gains in directions towards zone B are relatively lower, and that can form a zone where the signal is weakened. Considering Zone B is fairly close to the access point, the loss of signal is acceptable.

Figure 1-6  Indoor Hotspot Deployment