Dave Mast

Wireless Network Antenna - The Basics of Selecting an Antenna

Overview

The selection and deployment of access point antenna equipment affect network performance and availability. The signal strength or amount of energy radiated from an antenna has to do with antenna type and access point transmit specifications. It is expressed as effective isotropic radiated power (EIRP) and expressed as the sum of access point transmit power plus antenna gain minus cable loss. Cable length will increase signal attenuation or loss and should be minimized. Each antenna type will have a horizontal and vertical beam width specified. The antenna type, gain, cable length, number and mounting is all key to the design. For instance a directional antenna with high gain, no external cable, mounted at proper height will give best performance in an outside deployment with a lot of interference.

The wireless coverage cell on inside deployments can be extended with external antennas that plug into connectors on the access points. Most antennas can be mounted on the ceiling or wall and outside antennas often use a mast to increase height for line of sight.

Effective Isotropic Radiated Power = Transmit Power (dBm) + Gain (dB) - Cable Loss (dB)

Omni-Directional

Omni-directional antenna sends a horizontal radiated pattern of 360 degrees and a vertical pattern of 50 - 70 degrees from its source. From a practical perspective the pattern isn't circular as much as being elliptical. It is a multi-homing antenna that sends signals to several clients in proximity causing some multipath fading that is minimized with antenna diversity. The gain values with 2.4 GHz antenna range from 2.2 dBi - 12 dBi. The higher gain antenna is deployed outside for the most part. Some Cisco access points have an integrated omni-directional dipole antenna or " rubber ducky" that is standard with each device.

Directional

The directional antenna sends out a directed radiated pattern connecting with a distant antenna. The antenna purpose is to send traffic between antennas and not used for multi device homing. There are 3 specific directional antennas: Yagi, Patch and Dish. The Dish antenna has highest gain and narrowest radiation angle beam while Patch antenna has lowest gain and widest radiated beam. Gain values with 2.4 GHz antennas range from 6dBi - 21 dBi. Most antenna from this group are implemented in buildings with there is a lot of signal attenuation such as manufacturing, warehouses, and structures with a lot of steel, concrete, angled designs etc.

Diversity 

The diversity antenna implement dual antenna on the access point receiving signals on both. The access point determines what antenna has best gain and transmits on that antenna.  Diversity antennas are designed using either omni-directional, directional or dipole styles. Multipath fading occurs in buildings that have a lot of signal reflection. The signal finds multiple paths from source to destination and signal becomes distorted at receiver. This is minimized or eliminated with diversity antennas. 

External Antenna

Some Cisco access points such as 1200 series have connectors available for an external antenna. This is available with deployments in environments where the access point is at a maximum 300 feet from the wired switch or signal distortion is excessive. The external antenna with a specified coax cable of 3 - 100 feet allow for placement of the antenna at optimal positioning. Minimizing the cable length will decrease signal attenuation before transmission. Implementing the higher gain diversity patch antenna fixes the problem of clients being out of range. The company could have deployed a repeater as an alternative solution with an 1100 series access point that doesn't support an external antenna.

Building material and structural design will contribute to signal attenuation. The following describes from best to worst building material used with walls, floors and ceilings. Steel and concrete with several feet of construction is most difficult.

• Wood

• Concrete

• Reinforced Concrete   

• Steel

Antenna Mounting

Access points should be ceiling mounted if possible at least 17 - 19 feet in height optimizing horizontal and vertical beam width. In some cases the access points will have to be mounted on the wall. Antenna mounting is important and should be implemented with instructions from the hardware installation guide. The Cisco access points utilize the standard RP-TNC 50 ohm type connector.

Outside Bridge Mounting

Companies have implemented a lot of Cisco wireless bridges that connect buildings in a campus network. The line of sight must be available or use a repeater bridge to extend the network where it isn't available. Outside antennas deal with environmental issues and longer distances that require higher gain directional antennas. Mounting is usually with a mast or flat against a wall. Distances can extend around 250 - 500 feet or much further with higher wattage bridges, proper mounting of antenna and deploying repeaters. 

Point to point topology is implemented a lot with outside wireless deployments. Point to multipoint topology is implemented both with inside and outside wireless deployments. The root bridge or root access point in a multipoint topology is homed with traffic from several non root bridges or access points. That topology is implemented with several spoke buildings and a hub or core office that must be networked outside. The spoke offices have a point to point connection with the hub office and the hub office has a multipoint connection with all spoke offices. The hub office will implement an omni-directional antenna with a lot of beam width while spoke offices will use a directional antenna.

The inside standard access point designs won't have a group of directional antennas. Most will implement omni-directional antennas and use directional antennas where needed as a means of addressing distances. The outside implementations for the most part imply greater distances and environmental concerns making directional antennas an effective selection. 

Cisco Wireless Network Design Guide available at amazon.com and eBookmall.com

Shaun Hummel is an author of various technical books and has a web site focused on information technology job search solutions and certifications.

http://www.networkjobsolutions.com

About the Author

Shaun Hummel, CCNP, is a Senior Network Engineer with 11 years experience in enterprise network planning, design, and implementation. He has worked for various private and public companies in Canada and the United States improving infrastructure, security, and management. He has written Network Planning and Design Guide, Cisco Wireless Network Design Guide and Network Assessment Guide. www.networkjobsolutions.com

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