Cisco AIR-ANT2420V-N= Datasheet download pdf (Page 3)

Unlike isotropic antennas, dipole antennas are real antennas. Dipole antennas have a different radiation pattern

compared to isotropic antennas. The dipole radiation pattern is 360 degrees in the horizontal plane and 75 degrees

in the vertical plane (assuming the dipole antenna is standing vertically) and resembles a donut in shape. Because

the beam is “slightly” concentrated, dipole antennas have a gain over isotropic antennas of 2.14 dB in the

horizontal plane. Dipole antennas are said to have a gain of 2.14 dBi (in comparison to an isotropic antenna).

Some antennas are rated in comparison to dipole antennas. This is denoted by the suffix dBd. Hence, dipole

antennas have a gain of 0 dBd (= 2.14 dBi).

Note that the majority of documentation refers to dipole antennas as having a gain of 2.2 dBi. The actual figure is

2.14 dBi, but is often rounded up.

Types of Antennas

Cisco offers several different styles of antennas for use with access points and bridges in both 2.4-GHz and 5-GHz

products. Every antenna offered for sale has been FCC-approved. Each type of antenna will offer different

coverage capabilities. As the gain of an antenna increases, there is some tradeoff to its coverage area. Usually

high-gain antennas offer longer coverage distances, but only in a certain direction. The radiation patterns below will

help to show the coverage areas of the styles of antennas that Cisco offers: omnidirectional, Yagi, and patch

antennas.

Omnidirectional Antennas

An omnidirectional antenna (Figure 1) is designed to provide a 360-degree radiation pattern. This type of antenna

is used when coverage in all directions from the antenna is required. The standard 2.14-dBi “Rubber Duck” is one

style of omnidirectional antenna.

Figure 1. Omnidirectional Antenna

Directional Antennas

Directional antennas come in many different styles and shapes. An antenna does not offer any added power to the

signal; it simply redirects the energy it receives from the transmitter. By redirecting this energy, it has the effect of

providing more energy in one direction, and less energy in all other directions. As the gain of a directional antenna

increases, the angle of radiation usually decreases, providing a greater coverage distance, but with a reduced

coverage angle. Directional antennas include patch antennas (Figure 2), Yagi antennas (Figure 3), and parabolic

dishes. Parabolic dishes have a very narrow RF energy path, and the installer must be accurate in aiming these

types of antennas these at each other.

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Cisco AIR-ANT2420V-N= Datasheet Page 3