Using Directional and Omni-Directional Antennas in Wireless Networks

This post gives an introduction to the two types of Antennas used in Wireless Networks – Directional Antennas and Omni-directional Antennas. We also have a look at in-built antennas and external antennas and the options available with Access Points for the same. We discuss the type of coverage provided by either and particular situations where their usage is recommended.

In-built Antennas and External Antennas:

We have seen a lot of Wi-Fi access points, but we rarely see antennas attached to them. That is because, most of the access points come with in-built antennas and are sufficient for most of the indoor applications. But there are certain access points that come with a slot to attach external antennas (Generally, one antenna per radio is required). The external antennas are required either to increase the power and coverage of single access points in a coverage based access situation (Where longer coverage is more important than the bandwidth available for individual users, like in lawns or parks) or in special cases where directional coverage is required (Focusing the antenna coverage within a certain angular direction where coverage is required only in a certain region – like corridors etc).

Directional and Omni-Directional Antennas:

Omni-directional antennas (In-built antennas and certain external antennas are of this type) provide equal coverage in all directions. The coverage pattern looks like a sphere (In 2D, this is simplified to a circle). In-built omni-directional antennas provide mid to high gain performance and detachable omni-directional antennas are selected if higher level of gain performance is required. The coverage pattern for an omni-directional antenna is ideally spherical, but they are generally made to decrease the coverage in the vertical directions and increase the coverage in the horizontal directions, resembling the shape of a donut.

Where to use Omni-directional antennas:

¤ Indoor implementations are generally preferred with omni-directional antennas.
¤ Omni-directional antennas perform better with radio management, adjusting power levels etc. as their patterns are uniform and quite predictable.
¤ Location tracking and live radio frequency coverage maps can be visualized better with omni-directional pattern of coverage.
¤ Usage of directional antennas require a lot of planning and improper planning can cause problems like ‘hidden node’ etc. which are avoidable with omni-directional antennas.
¤ Omni-directional antennas are used in places where uniform coverage is required over larger areas.

Directional Antennas point RF energy in a specific direction for RF concentration within a targeted area. The gain is higher for that area or in that direction. The high gain directional antennas are good for sites requiring a directed coverage in specific area or wireless back-haul extensions where two wireless access points are connected with each other to extend the wireless network, rather than connecting them to the wired backbone.

Where to use Directional antennas:

¤ Directional antennas are required in areas like long corridors where a directional coverage is required over an extended but narrow area, and it would be inefficient to use omni-directional coverage with a lower gain in such cases.
¤ Directional antennas are required in areas where the RF attenuation is high for certain surrounding materials in certain directions. In such cases, the RF signals would be needed to be diverted in the direction of least amount of attenuation. Example: Warehouses with metal racks and holdings and libraries with metal shelves etc.
¤ In areas where there are limited access point mounting options, like large rooms with open ceilings, could do better with directional antennas at the corners of the rooms (focusing on 90/120 degree sectors, for example).
¤ Directional antennas are also used in certain dense deployments with concentrated coverage within a certain region, due to higher activity level in that region.

Please note: Antennas come with their own frequency ranges that they support, and generally a single antenna may not support both 2.4 Ghz and 5 Ghz dual radio deployments.

Antenna position/direction:

When built-in antennas are used or external antennas are used along with access points, they need to be pointed perpendicular to the floor for optimum coverage. If the access points are mounted parallel to the floor on ceilings, then their antennas need to be turned 90 degree’s to point perpendicular to the floor. If the access points are mounted on the walls, then their antennas need to be in the same direction as the access points so that they would point perpendicular to the floor.

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