t that the polarisations of transmitting and receiving antennas are matched, since a vertical polarised receiving antenna will be unable to receive a signal transmitted by a horizontally polarised transmitting antenna, and vice versa. It is equally important for antennas to be correctly mounted, as rotating an antenna 90° about the direction of propagation will change its polarisation by the same angle (e.g. from horizontal to vertical). Although WLAN operation in the 5 GHz band has developed more recently than in the 2.4 GHz band, a similar selection of antennas is available for the higher-frequency band. A variety of dual band omnidirectional and patch antennas are also available to operate in both WLAN bands. Smart Antennas The data throughput of a wireless network that uses a traditional antenna of the type described above is limited because only one network node at a time can use the medium to transmit a data packet. (Other so-called Wireless Network Physical Architecture Transmitter switches to beam 6 -9 dB to give maximum gain in the direction of the target station Target station direction Figure 3-17: Beam Pattern of a Six Element Switched Beam Array multiple access techniques will be described in the Section "Wireless Multiplexing and Multiple Access Techniques, p. 87"). Smart antennas aim to overcome this limitation by allowing multiple nodes to transmit simultaneously, significantly increasing network throughput. There are two varieties of smart antenna - switched beam and adaptive array. A switched beam antenna consists of an array of antenna elements each having a predefined beam pattern with a narrow main lobe and small side- lobes (Figure 3-17). Switching between beams allows one array element to be selected that provides the best gain in the direction of a target node, or the lowest gain towards an interfering source. The simplest form of switched beam antenna is the pair of diversity receiver antennas often implemented in wireless LAN access points to reduce multipath effects in indoor environmen