Wireless Antenna

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The most commonly used wireless antenna on both wireless access points (WAPs) and wireless devices is an omnidirectional (or omni) antenna. Another type of antenna is a directional antenna. If you’re planning on taking the Security+ exam, you should have a basic understanding of the different types and specific power measurements of wireless antennas.

For example, can you answer this question?

Q. You need to provide connectivity between two buildings without running any cables. You decide to use two WAPs and a high-gain directional antenna. Which of the following antennas is the BEST choice to meet this need?

A. Yagi

B. Omni

C. Isotropic

D. Dipole

More, do you know why the correct answer is correct and the incorrect answers are incorrect? The answer and explanation is available at the end of this post.

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Types of Wireless Antenna

The most commonly used wireless antenna on both WAPs and wireless devices is an omnidirectional (or omni) antenna. Omnidirectional antennas transmit and receive signals in all directions at the same time. This allows wireless devices to connect to a WAP from any direction.

Another type of antenna is a directional antenna. A directional antenna transmits in a single direction and receives signals back from the same direction. Because the power of the antenna is focused in a single direction, the directional antenna has greater gain than an omni antenna, and it can transmit and receive signals over greater distances.

Although omnidirectional and directional antennas are generic, there are some specific types of antennas used and referenced with wireless devices and networks. They include:

  • Isotropic. An isotropic antenna is a theoretical concept where an antenna has a perfect three-dimensional radiation pattern of 360 degrees vertically and horizontally. In other words, if you measured the power level of the signal 50 feet from any direction, it would have the same power level. Dipole and other omnidirectional antennas attempt to mimic an isotropic antenna.
  • Dipole. A dipole antenna is an actual antenna. Assuming the antenna is standing vertically (such as a pencil standing straight up balanced on the eraser), it has a radiation pattern of 360 degrees horizontally, and about 75 degrees vertically. Some variations of a dipole antenna, such as a folded dipole and half-wave dipole, increase the power gain and overall covered area of the antenna. Most omnidirectional antennas used in wireless networks are a type of dipole antenna.
  • Yagi. A Yagi (also called Yagi-Uda) antenna is a common type of directional antenna. Yagi antennas typically use a dipole, folded dipole, or half-wave dipole combined with additional elements such as a reflector or director element. These additional elements focus the antenna in a single direction while also increasing the gain and reducing the radiation pattern.

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The following figure provides a generic view of the radiation patterns of different types of antennas. Notice that the omni and dipole antennas don’t provide perfect 360-degree coverage. You can also see how the Yagi/directional antenna narrows the radiation pattern while also increasing the gain in a single direction.

Wireless Antenna

Directional and omnidirectional antennas

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Antenna Power Capabilities

Wireless access points and antennas include specific measurements to identify their power capabilities. Most devices and antennas include a variation of a decibel (dB) value to express power as a ratio. Three common terms are:

  • dBi. Decibels-isotropic (dBi) identifies the gain of an antenna and is commonly used with omnidirectional antennas. It references an isotropic antenna that can theoretically transmit the signal equally in all directions. Higher numbers indicate the antenna can transmit and receive over greater distances.
  • dBd. Decibels-dipole (dBd) identifies the gain of an antenna compared with a type of dipole antenna. Just as with dBi, higher dBd numbers indicate the antenna can transmit and receive over greater distances.
  • dBm. Decibels-milliwatt (dBm) identifies the power level of the WAP and refers to the power ratio in decibels referenced to one milliwatt. Higher numbers indicate the WAP transmits the signal over a greater distance.

You cannot modify the dBi or dBd gain of an antenna without changing its physical properties. However, many WAPs include power settings allowing you to increase or decrease the power levels. For example, you can decrease the power level to reduce the coverage. Administrators do this occasionally to restrict access to a small area such as a conference room, or to prevent wireless users from connecting from the parking lot or somewhere else outside the building.


 

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Q. You need to provide connectivity between two buildings without running any cables. You decide to use two WAPs and a high-gain directional antenna. Which of the following antennas is the BEST choice to meet this need?

A. Yagi

B. Omni

C. Isotropic

D. Dipole

Answer is A. A Yagi antenna is a high-gain directional antenna with a very narrow radiation pattern and is an ideal choice for this scenario.

An isotropic antenna is theoretical and indicates the signal goes in all directions equally.

Omnidirectional and dipole antennas attempt to mimic an isotropic antenna, but have stronger gains horizontally then vertically, assuming they are standing vertically.

See Chapter 4 of the CompTIA Security+: Get Certified Get Ahead: SY0-401 Study Guide for more information on securing wireless networks.

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