In order to be able to communicate between two or more transceivers, the radio frequency (RF) signal must be radiated from the antenna of the transmitter with enough power so that it is received and understood by the receiver.
The installation of antennas has the greatest ability to affect whether the communication is successful or not. Antenna installation can be as simple as placing an access point in the middle of a small office, providing full coverage for your company.
Or it can be as complex as installing an assortment of directional antennas, kind of like piecing together a jigsaw puzzle. Do not look at this as something to be afraid of; on the contrary, with proper understanding of antennas and how they function, successfully planning for and installing them in a wireless network will become a skillful and rewarding task.
We will discuss on the different categories and types of antennas and the different ways that they can direct an RF signal. Choosing and installing antennas is like choosing and installing lighting in a home.
When installing home lighting, you have many choices: table lamps, ceiling lighting, narrow or wide beam directional spotlights. In previous article you were introduced to the concept of antennas focusing RF signal.
In this article, we will learn about the different types of antennas and their radiation patterns and how to use the different antennas in different environments. We will also learn that even though we often use light to explain RF radiation, there are differences between the way the two behave.
We will learn about aiming and aligning antennas, and we will learn that what you see is not necessarily what you will get. In addition to learning about antennas, we will learn about the accessories that may be needed for proper antenna installation.
In office environments you may only need to connect the antenna to the access point, and you are done installing the antenna. In outdoor installations you will need special cable and connectors, lightning arrestors and special mounting brackets. We will introduce you to the components necessary for successfully installing an antenna.
To summarize, we will gain the knowledge that will allow to properly select, install, and align antennas. These skills will help to successfully implement a wireless network, whether it is a point-to-point network between two buildings or a network providing wireless coverage throughout an office building.
Active and Passive Gain
In the previous article, we learned that we can increase the signal that is radiated out of the antenna (EIRP) by increasing the output of the transmitter, which in turn increases the amount of power provided to the antenna (Intentional Radiator) and thus the amount of power from the antenna (EIRP).
When the power is increased by some type of electrical device, such as the transmitter or an amplifier, the increase is referred to as active gain . Another method of increasing power is to direct or focus the power.
When power is focused, the amount provided to the antenna does not change. It is the antenna acting like a lens on a flashlight that increases the power output by concentrating the RF signal in a specific direction.
Since the gain from the antenna was created by shaping or concentrating the signal, and not by increasing the overall power, this increase is referred to as passive gain .
When trying to decide whether gain is active or passive, determine whether the gain is due to a total increase in power caused by an electronic device (active gain) or whether it is due to the power being focused or directed (passive gain).
Azimuth and Elevation Chart
There are many types of antennas designed for many different purposes, just as there are many types of lights designed for many different purposes. When purchasing lighting for your home, it is easy to compare two lamps by turning them on and looking at the way each disperses the light.
Unfortunately, it is not possible to compare antennas in the same way. Actual side-by-side comparison requires you to walk around the antenna with an RF meter, take numerous signal measurements, and then plot the measurements either on the ground or on a piece of paper that represents the environment.
Besides the fact that this is a time-consuming task, the results could be skewed by outside influences on the RF signal, such as furniture or other RF signals in the area.
To assist potential buyers with their purchasing decision, antenna manufacturers create azimuth charts and elevation charts , commonly known as radiation patterns, for their antennas.
These radiation patterns are created in controlled environments where the results cannot be skewed by outside influences and represent the signal pattern that is radiated by a particular model of antenna. Figure 1 shows the azimuth and elevation charts of an antenna.
The azimuth chart, labeled H-plane, shows the top-down view of the radiation pattern of the antenna. The elevation chart, labeled E-plane, shows the side view of the radiation pattern of the antenna.
There is no standard that requires the antenna manufacturers to align the degree marks of the chart with the direction that the antenna is facing, so unfortunately it is up to the reader of the chart to understand and interpret it.
Here are a few statements that will help you interpret the radiation charts:
- In either chart, the antenna is placed at the middle of the chart.
- Azimuth chart = H-plane = top-down view
- Elevation chart = E-plane = side view
The outer ring of the chart usually represents the strongest signal of the antenna. The chart does not represent distance or any level of power or strength. It only represents the relationship of power between different points on the chart.
One way to think of the chart is to consider the way a shadow behaves. If you were to move a flashlight closer or farther from your hand, the shadow of your hand would grow larger or smaller. The size of the shadow does not represent the size of the hand.
The shadow only shows the relationship between the hand and the fingers. With an antenna, the radiation pattern will grow larger or smaller depending upon how much power the antenna receives, but the shape and the relationships represented by the patterns will always stay the same.
Many flashlights have adjustable lenses, allowing the user to widen or tighten the concentration of light that is radiating from them. RF antennas are capable of focusing the power that is radiating from them, but unlike flashlights, antennas are not adjustable.
The user must decide how much focus is desired prior to the purchase of the antenna. Beamwidth is the measurement of how broad or narrow the focus of an antenna is and is measured both horizontally and vertically.
It is the measurement from the center, or strongest point, of the antenna signal to each of the points along the horizontal and vertical axes where the signal decreases by half power (–3 dB), as seen in Figure 2.
These –3 dB points are often referred to as half power points. The distance between the two half power points on the horizontal axis is measured in degrees, giving the horizontal beamwidth measurement. The distance between the two half power points on the vertical axis is also measured in degrees, giving the vertical beamwidth measurement.
It is important to realize that even though the majority of the RF signal that is generated is focused within the beamwidth of the antenna, there is still a significant amount of signal that radiates from outside of the beamwidth and from what is known as the antennas side or rear lobes.
As you look at the azimuth charts of different antennas, you will notice that some of these side and rear lobes are fairly significant. Although the signal of these lobes is much less than the signal of the main beamwidth, they are dependable, and in certain implementations very functional.
It is important when aligning point-to-point antennas that you make sure they are actually aligned to the main lobe and not a sidelobe. Table below shows the different types of antennas that are used in 802.11 communications.
|Antenna Types||Horizontal Beamwidth (in degrees)||Vertical Beamwidth (in degrees)|
|Omni-directional||360||7 to 80|
|Patch/panel||30 to 180||6 to 90|
|Yagi||30 to 78||14 to 64|
|Sector||60 to 180||7 to 17|
|Parabolic dish||4 to 25||4 to 21|