A major component of a wireless LAN system is the antenna. There are several different types and they all have their place. However, there can be some confusion surrounding the language used to specify antennas as well as the basic function of each type of antenna. The purpose of this white paper is to dispel the confusion surrounding antennas and their function. This document is not meant to be an electromagnetic primer nor a deployment guide. Rather, it should be used as a dictionary of basic antennas and antenna terminology as well as a tutorial specifically covering antenna patterns and the parameters associated with those patterns. The focus is on many of the various antennas that might be encountered in a wireless LAN system.
We begin with a glossary of basic definitions and then progress through a discussion of some common antenna types and their properties. Along the way, the antenna patterns are shown and explained, including the 3-D radiation pattern from the antennas. Typical performance from each antenna type is described as well. Of course, there are plenty of exceptions to the “typical” antenna, as many antenna types can be designed to enhance one or more parameters. But it is often helpful to see a few examples and have some of these parameters highlighted.
In a WLAN system, commonly used antennas are dipoles, omnidirectional antennas, patches and Yagis. These antennas are shown in Figure 1. Although these antenna packages might vary somewhat from one manufacturer to another, these are typical packages for these types of antennas. The function of each of these antenna types is explained in some detail in this paper.
Figure 1. Various Antennas Commonly Found in WLAN Systems
Figure 2. Dipole Antenna with 3D Radiation Pattern, Azimuth Plane Pattern and Elevation Plane Pattern
Figure 3. 3D Radiation Pattern from 5.8 dBi Omnidirectional Antenna, Azimuth Plane Pattern and Elevation Plane Pattern
The goal of a dipole or any omni is to radiate energy equally in all directions in a plane. For dipoles and collinear arrays, the omnidirectional plane is intended to be the azimuth plane (the plane of the floor or the ground). For this reason, it doesn’t matter how the patterns are presented. It is understood that the elevation plane pattern is always orthogonal to the azimuth plane pattern. The orientation of the actual plot is largely dependent on the orientation of the antenna in the measurement system and that’s all there is to it.
Figure 4. Elevation Plane Demonstration
Patch Antennas
A patch antenna, in its simplest form, is just a single rectangular (or circular) conductive plate that is spaced above a ground plane. Patch antennas are attractive due to their low profile and ease of fabrication. The radiation pattern of a single patch is characterized by a single main lobe of moderate beamwidth. Frequently, the beamwidths in the azimuth and elevation planes are similar, resulting in a fairly circular beam, although this is by no means universal. The beamwidths can be manipulated to produce an antenna with higher or lower gain, depending on the requirements. An antenna built with a single patch will have a maximum gain of about 9 dBi or a bit less. The patch antenna in Figure 5 shows how simple these antennas can be. This is a simple rectangular patch built over a rectangular ground plane. The radiation patterns exhibit typical patch antenna characteristics. There is a single main lobe with a fairly wide beamwidth with shallow nulls pointing up and down from the antenna.
Figure 5. Single Patch Antenna with 3D Radiation Pattern, Azimuth Plane Pattern and Elevation Plane Pattern
Patch Array Antennas
A patch array antenna is, in general, some arrangement of multiple patch antennas that are all driven by the same source. Frequently, this arrangement consists of patches arranged in orderly rows and columns (a rectangular array) as shown in Figure 6. The reason for these types of arrangements is higher gain. Higher gain commonly implies a narrower beamwidth and that is, indeed, the case with patch arrays. The array shown here has a gain of about 18 dBi with an azimuth and elevation plane beamwidth of about 20 degrees. Notice that the back lobes are very small and that the front-to-back ratio is about 30 dB. The first sidelobes are down from the peak about 14 dB.
Figure 6. A 4×4 Patch Array Antenna with 3D Radiation Pattern, Azimuth Plane Pattern and Elevation Plane Pattern
Antenna patterns are frequently shown normalized to the peak gain. The peak gain (in dBi) is simply subtracted from the gain at all the points on the curve and the pattern is plotted with the new values. These patterns are expressed in dB with 0 dB corresponding to the peak gain. A normalized pattern is especially useful when the sidelobe levels and the depth of the nulls are of interest since it’s easier to read their respective levels. The patterns of the patch array shown here have enough lobes and features that a look at their normalized patterns in rectangular coordinates might be interesting. Figure 7 shows the azimuth plane in both polar and Cartesian (rectangular) coordinates. Figure 8 shows the elevation plane in both coordinate systems.
Figure 7. Azimuth Plane Patterns of the 4 x 4 Patch Array in Polar and Rectangular Coordinates
Figure 8. Elevation Plane Patterns of the 4 x 4 Patch Array in Polar and Rectangular Coordinates
Yagi Antennas
A Yagi antenna is formed by driving a simple antenna, typically a dipole or dipole-like antenna, and shaping the beam using a well-chosen series of non-driven elements whose length and spacing are tightly controlled. The Yagi shown here in Figure 9 is built with one reflector (the bar behind the driven antenna) and 14 directors (the bars in front of the driven antenna). This configuration yields a gain of about 15 dBi with azimuth and elevation plane beamwidths that are basically the same, around 36 degrees. That is a common feature of Yagi antennas. Many times these antennas are designed so that they can be rotated for either horizontal or vertical polarization, so having the same 3-dB beamwidth in each plane is a nice feature in those instances.
Figure 9. Yagi Antenna Model with 3D Radiation Pattern, Azimuth Plane Patten, and Elevation Plane Pattern
Again, the Yagi antenna is a directional antenna that radiates its energy out in one main direction. Very often, these antennas are enclosed in a tube, with the result that the user may not see all the antenna elements. Their directional nature seems to be somewhat intuitive due to their common, tubular form factor. It is easy to visualize aiming these antennas much like a rifle.
Figure 10. Various 3D Radiation Patterns from a 90 degree Sector Antenna, Azimuth Plane Pattern and Elevation Plane Pattern
One of the problems encountered when deploying sectors, or omnidirectional antennas for that matter, is that there can be several nulls in the elevation plane. When the gain is higher, the number of nulls (and side lobes) generally goes up as well. When the antennas are used in offices or in low hanging outdoor deployments, this is seldom a problem. Signal strengths are generally high enough everywhere to guarantee service to all users with careful planning. But when the antennas are mounted high in the air on towers, these nulls can affect the performance of the system. Figure 11 illustrates the problem. Assume that the sector antenna is mechanically tilted down by 5 degrees. This effectively tilts the elevation plane pattern down 5 degrees as shown. This puts certain regions under the antenna in areas below the nulls in the pattern resulting in areas of low signal strength.
Figure 11. Coverage Gaps from Elevation Plane Nulls
System users “in the nulls” might have a problem depending on how much signal actually gets transmitted to the ground. The further out from the antenna, the worse the problem gets not only because the signal strength gets lower as the distance from the antenna increases, but also because the size of the low-signal area gets bigger. Notice too that many users are getting their coverage from the side lobes rather than from the main beam. This can be an important consideration. Some sectors are specifically designed to combat this problem with “null fill.” When the nulls are filled in, the distribution of energy to the various antenna elements in the array is changed so that more energy is radiated “below” the antenna. As a result, the peak gain of the main lobe is generally reduced. In order to preserve the peak gain, more elements must be added and the antenna gets physically larger. An example of a sector with “null fill” is shown below in Figure 14. This is actually the Cisco ® AIR-ANT2414S-R. The AIR-ANT2414S-R is a 14 dBi, 90-degree sector antenna. Many of the 90-degree sector antennas on the market for 2.4 GHz are shorter, but do not have the “null fill” property. This antenna was designed to keep the gain relatively high while filling in the nulls “under the antenna,” particularly the deep first null and second null that affect the coverage far away from the antenna.
Figure 12. A Cisco 90-degree Sector Antenna with Azimuth and Elevation Plane Patterns
Notice that the first two nulls in the elevation plane “under the antenna” are not as deep or seem to be gone altogether. This allows for increased signal levels to users who might otherwise be without coverage as illustrated in Figure 13. The figure shows that if the antenna is tilted down 5 degrees as in the previously illustrated case, there is no null pointed far away from the antenna. The nulls that still exist point to areas close to the tower, where total lack of coverage is less likely due to the shorter ranges involved.
Figure 13. Illustration of Reduced Coverage Gaps from a Sector Antenna with “Null Fill”
Ref: https://www.cisco.com/c/en/us/products/collateral/wireless/aironet-antennas-accessories/prod_white_paper0900aecd806a1a3e.html