ProSoft Technology ILX34-AENWG - Manual (Page 70)

PSW-PWD ♦ ProSoft Software Antenna Selection Considerations
ProSoft Wireless Designer User Manual
Page 70 of 91
ProSoft Technology, Inc.
July 8, 2009
Beam width is an angular measurement of how strongly the power is
concentrated in a particular direction. Beam width is a three dimensional quantity
but can be broken into two-dimensional slices just like the antenna pattern. The
beam width of an omnidirectional pattern is 360 degrees because the power is
equal in all directions.
8.1.2 Antenna Polarity
Antenna polarity refers to the direction that the electromagnetic field lines point
as energy radiates away from the antenna. In general, antenna polarization is
elliptical. The simplest and most common form of this elliptical polarization is
simply a straight line, or linear polarization. Of the transmitted power that reaches
a receiving antenna, only the portion that has the same polarization as the
receiving antenna polarization is actually received.
For example, if the transmit antenna polarization is linear pointed in the vertical
direction (vertical polarization), and the receive antenna also has vertical
polarization, the maximum amount of power possible will be received. On the
other hand, if the transmit antenna has vertical polarization and the receive
antenna has horizontal polarization, theoretically, no power would be received. If
the two antennas have linear polarizations orientated at 45 degrees to each
other, half of the possible maximum power will be received.
8.1.3 Voltage Standing Wave Radio (VSWR)
VSWR represents the degree with which an antenna is matched to the radio
system impedance. Modern day antennas do not require tuning for optimum
performance. VSWR is one of the easiest parameters to measure and VSWR
meters are becoming very popular antenna installation tools. Most modern
antennas, receivers, and transmitters are designed for peak performance when
operating into a 50-Ohm transmission line. If the VSWR is too high, the
transmitter power may be reduced as well as the strength of the received signal.
The typical commercial standard for maximum allowable VSWR across the entire
bandwidth of a system is 1.5:1. This means that the antenna impedance must be
somewhere between 37.5 and 75 Ohms. A VSWR of 2:1 or greater is usually
considered unacceptable (for receiving purposes only) since it increases losses
in the transmission line. Incidentally, decreasing your VSWR below 1.5:1 will
often be expensive and will have little noticeable performance improvement. You
should specify the maximum VSWR and operating frequency bandwidth when
specifying your antenna.