Rf Propagation And Path Loss - Nortel 2330 Solution Manual

Appendix 81
Bandwidth
The term bandwidth simply defines the frequency range over which an antenna meets a certain set of
performance criteria. The important issue to consider regarding bandwidth is the performance tradeoffs
between all of the properties described above as the bandwidth is widened to cover more area.
Summary of Related Performance Tradeoff Issues
The antenna properties described above are all related in varying degrees. In selecting or designing an antenna
system, the tradeoffs between all of these performance properties must be considered in order to ensure
optimum system performance. These factors must also be weighed against factors beyond the actual antenna
properties such as RF propagation and path loss.
In all cases, the required or specified performance properties of an antenna can only be achieved over a limited
frequency bandwidth. The extent of this realizable bandwidth is a function of the performance requirements,
antenna type selected and the antenna size relative to the operating wavelength. Typically, optimum antenna
performance is achieved at the center frequency of the operating band.
In the case of VSWR, system performance is typically degraded at the edges of the operating frequency band.
The overall effect on system performance is a decrease in antenna gain due to impedance mismatch losses. The
antenna's radiation patterns, beamwidth, directivity (gain) and bandwidth are more directly related. As with
VSWR, performance is usually degraded at the edges of the operating band.
The 3-dB beamwidth and directivity (gain) are inversely proportional. As the antenna beamwidth decreases,
the directivity increases. These parameters are also related to the antenna size or aperture. As the antenna
aperture or size increases with respect to the operating wavelength, the radiation beamwidth decreases.
Therefore, to achieve increased antenna gain, it is required that the antenna size increase.
For linearly polarized antennas, polarization is generally not a function of bandwidth. However, this is not
always the case for circularly polarized antennas. In most cases, the polarization axial ratio for a circularly
polarized antenna increases at the edges of the operating frequency band.

RF Propagation and Path Loss

In addition to the antenna properties and wireless system hardware, other factors can influence the overall
performance of the WLAN system. These factors affect the propagation of RF signals through the air medium
and need to be considered when architecting a WLAN.
Path loss is encountered when an RF signal propagates through the air. The relative signal strength of the
propagating wave diminishes as a function of distance traveled from the transmitting antenna.
The free space propagation loss factor is described by the following equation,
Nortel WLAN 2300 Series Outdoor Solution Guide