Antennas in WiFi (WLAN)

Some Radio network interface cards (NICs) and access points have integrated antennas that you can't change. For example, laptops such as Apple iBook integrate the antenna within the cover or body of the device, which is not visible or changeable by the user. Some radio NICs and access points also use permanently mounted antennas. With these types of products, you have no choice but to use the antenna the vendor supplies.

Other wireless LAN devices have antennas that are interchangeable. In fact, it's a good idea to purchase access points with removable antennas. These allow more flexibility by enabling the selection of an antenna having characteristics better suited for your specific application. The more common antenna types for wireless LANs have omni-directional and directional radiation patterns. Omni-directional antennas propagate RF signals in all directions equally on a horizontal plane (i.e., throughout the facility), but limit range on the vertical plane. This radiation pattern resembles that of a very large doughnut with the antenna at the center of the hole.

Omni-directional antennas, having gains ranging up to 6 dB, apply to most applications inside buildings. Omnis provide the widest coverage, making it possible to form somewhat circular overlapping cells from multiple access points located throughout the building. Most access points ship standard omnis having relatively low gain.

References:

[1]-http://www.wi-fiplanet.com/tutorials/article.php/1144391/Antennas-The-Key-to-Maximizing-RF-Coverage.htm

[2]-http://www.wlanantennas.com/index.php

MIMO Technology

The radio environment on electrically small platforms is changing rapidly. Until recently one radio was used in isolation and was usually connected to only one antenna. The situation today is very different: there is usually more than one radio used at once for example a handset may have 4 cellular bands, GPS and BluetoothTM. Sometimes WLAN radios are also present. This means that more RF filtering of signals is necessary. It is also becoming common for each radio to use more than one antenna in order to create diversity or for MIMO applications.

Antenna diversity is already used with WLAN radio in order to counter multipath, reduce outages and improve the quality and reliability of the communications link. Generally three types of diversity are used, two antennas can be deployed as far apart as possible to create some spatial diversity, they can be oriented orthogonally to give polarisation diversity or they can have different beams patterns. Diversity in current WLAN systems is usually restricted to two antennas for each radio as this is enough to ensure that if one antenna is in an RF null, the other is generally not, thereby providing better performance in multipath environments. Only one radio is present and so the receiver listens to one antenna at a time and a RF switch is used to select the antenna giving the best signal.

Reference:

Antenova, "Antenna Designs for MIMO Systems", Queen Mary University of London 2004.