RF Antennas

An RF antenna is a device used to convert high frequency (RF) signals on a transmission line (a cable or waveguide) into propagated waves in the air. The electrical fields emitted from antennas are called beams or lobes. There are three generic categories of RF antennas:

• Omni-directional
• Semi-directional
• Highly-directional

Each category has multiple types of antennas, each having different RF characteristics and appropriate uses. As the gain of an antenna goes up, the coverage area narrows so that high-gain antennas offer longer coverage areas than low-gain antennas at the same input power level. There are many types of antenna mounts, each suited to fit a particular need. After studying this section, you will understand which antenna and mount best meets your needs and why.

Omni-directional (Dipole) Antennas

The most common wireless LAN antenna is the Dipole antenna. Simple to design, the dipole antenna is standard equipment on most access points. The dipole is an omnidirectional antenna, because it radiates its energy equally in all directions around its axis. Directional antennas concentrate their energy into a cone, known as a "beam." The dipole has a radiating element just one inch long that performs an equivalent function to the "rabbit ears" antennas on television sets. The dipole antennas used with wireless LANs are much smaller because wireless LAN frequencies are in the 2.4 GHz microwave spectrum instead of the 100 MHz TV spectrum. As the frequency gets higher, the wavelength and the antennas become smaller.

Figure 5.1 shows that the dipole's radiant energy is concentrated into a region that looks like a doughnut, with the dipole vertically through the "hole" of the "doughnut." The signal from an omni-directional antenna radiates in a 360-degree horizontal beam. If an antenna radiates in all directions equally (forming a sphere), it is called an isotropic radiator. The sun is a good example of an isotropic radiator. We cannot make an isotropic radiator, which is the theoretical reference for antennas, but rather, practical antennas all have some type of gain over that of an isotropic radiator. The higher the gain, the more we horizontally squeeze our doughnut until it starts looking like a pancake, as is the case with very high gain antennas.

The dipole radiates equally in all directions around its axis, but does not radiate along the length of the wire itself - hence the doughnut pattern. Notice the side view of a dipole radiator as it radiates waves in Figure 5.2. This figure also illustrates that dipole antennas form a "figure 8" in their radiation pattern if viewed standing beside a perpendicular antenna.

If a dipole antenna is placed in the center of a single floor of a multistory building, most of its energy will be radiated along the length of that floor, with some significant fraction sent to the floors above and below the access point. Figure 5.3 shows examples of some different types of omni-directional antennas. Figure 5.4 shows a two-dimensional example of the top view and side view of a dipole antenna.


High-gain omni-directional antennas offer more horizontal coverage area, but the vertical coverage area is reduced, as can be seen in Figure 5.5. This characteristic can be an important consideration when mounting a high-gain omni antenna indoors on the ceiling. If the ceiling is too high, the coverage area may not reach the floor, where the users are located.

Usage
Omni-directional antennas are used when coverage in all directions around the horizontal axis of the antenna is required. Omni-directional antennas are most effective where large coverage areas are needed around a central point. For example, placing an omnidirectional antenna in the middle of a large, open room would provide good coverage. Omni-directional antennas are commonly used for point-to-multipoint designs with a hub-n-spoke topology (See Figure 5.6). Used outdoors, an omni-directional antenna should be placed on top of a structure (such as a building) in the middle of the coverage area. For example, on a college campus the antenna might be placed in the center of the campus for the greatest coverage area. When used indoors, the antenna should be placed in the middle of the building or desired coverage area, near the ceiling, for optimum coverage. Omni-directional antennas emit a large coverage area in a circular pattern and are suitable for warehouses or tradeshows where coverage is usually from one corner of the building to the other.