Basic RF Antenna Concepts - 3

WLAN Antenna Types

Three common types of antennas for use with wireless LANs are:

• Omnidirectional / Dipole antennas
• Semi-directional antennas 
• Highly directrional antennas

Omnidirectional Antennas

Omnidirectional antennas are very common on most access points. An omnidirectional antenna has a horizontal beamwidth of 360. The vertical beamwidth will vary depending on the antenna's gain. As the gain of the antenna increases, the horizontal radiation pattern will increase, providing more horizontal coverage. However, the vertical radiation pattern will decreases, therefore providing less vertical coverage.

The shape of the radiation pattern from an omnidirectional antenna looks like a donut, as shown in the following picture.

 

Above picture from CWAP

Omnidirectional antennas are one of the most common type of antennas for indoor wireless LAN deployments. Most of this type antenna typically has a low gain of 2 dBi or 3 dBi and connects directly to an access point. 

Azimuth and elevation charts are usually available to allow  visualization of the RF radiation pattern emitted from the antenna, as shown in the following picture. 

Semidirectional Antennas

Semidirectional antennas take power from the transmitting system and focus it into more specific pattern than an omnidirectional antenna offers. Semidirectional antennas are available in various types, including patch, panel, sector and Yagi. 

Patch/Panel

In the wireless LAN world, the terms patch and panel are commonly used to describe the same type of antenna. A patch/panel antenna can have a horizaontal beamwidth of as high as 180, but usually the horizontal beamwidth is between 50 and 80. The following picture shows a 2.4Ghz flat patch antenna. 

above picture from CWAP

Azimuth and elevation charts are available for patch/panel antennas, as shown in the following picture. 

Sector

Sector antennas are often used for base station connectivity for point-to-multipoint connectivity. Sector antennas have an azimuth that varies from 90 from 180. The following are pictures for sector antennas.

above pictures from CWAP.

Yagi 

Yagi antennas are designed to be used indoors in long hallways and corridors, or outdoors for short-range bridging. Yagi antennas have vertical and horizontal beamwidths range from 25 to 65. The following are pictures for Yagi antennas.

Highly Directional Antennas

Highly directional antennas are typically parabolic dish antennas used for long-range point-to-point bridging links. Some manufacturers of parabolic dish antennas advertise ranges of 25 miles or more. Parabolic dish antennas have very narrow horizontal and vertical beamwidths. This beamwidth can range from 3 to 15. The following are pictures for Highly directional antennas.

   

Basic RF Antenna Concepts - 2

Gain

The gain of antenna provides a change in coverage that is a result of the antenna focusing the area of RF propagation. Amplification of an RF signal will result in gain. An antenna is a device that can change the coverage area, therefore propagating an RF signal further. Antenna gain is measured in decibels isotropic (dBi),which is a change in power as a result of increasing the isotropic energy. Isotropic energy is defined as energy emitted equally in all directions. The sun is a good example of isotropic energy, emitting energy in a spherical fashion equally in all directions. The following shows a drawing of a wireless LAN system with 100 mW of power at the antenna. Because of gain, the antenna emits 200mW of Power.

Above the picture from CWAP

Antenna Polarization 

Antenna polarization describes how a wave is emitted from an antenna and the orientation of the electrical component or electric field of the waveform. To maximize signal, the transmitting and receiving antennas should be polarized in the same direction or as closely as possible. 

If the polarization of the transmitter and receiver are different, the power of the signal will decrease depending how different the polarization is. 

above picture from CWAP

Basic RF Antenna Concepts - 1

The terminology for characteristics of antennas is listed here:

• RF lobes - Shape of the RF patterns
• Beamwidth - Horizontal/Azimuth and vertical/elevation measurement angels
• Gain - Changing the RF coverage pattern
• Polarization - Horizontal or vertical 

RF lobes 

In radio frequency technology term, lobe refers to the shape of the RF energy emitted from an antenna element. RF lobes are determined by the physical design of the antenna. Antenna design also determines how the lobes project from an antenna element. Antennas may project many lobes of RF signal. The type of antenna utilized - omnidirectional, semi directional, or highly directional parabolic dish will determine the usable lobes. The following picture shows highly directional parabolic antenna. 

Above pictures from CWAP

Beamwidth

The patterns of energy emitted from an antenna are known as lobes. For antennas, the beamwidth is the angle of measurement of the main RF lobe measured at the half-power or -3 db point. Beamwidth is measured both horizontally and vertically, in degrees.

Azimuth and elevation charts available from the antenna manufacturer will show the beamwidth angles. The azimuth is the view from above or the bird's eye view of the RF pattern. The elevation as a side view, if you were to look at a mountain form the side view.

Above pictures from CWAP