Nortel 2330 Solution Manual page 79

Appendix 79
If we double the length of the aperture, the gain of the antenna is doubled, which means the gain increases by
3 dB.
An antenna's efficiency is a measure of the ratio of the gain of an antenna to its directivity. If an antenna were
100% efficient, it would have a gain equal to its directivity. The VSWR provides an indication of how closely
the impedance of an antenna matches the impedance of the connecting access point. If an impedance mismatch
exists, a reflected wave will be created and will reduce the level of forward energy transferred from the access
point into the antenna. This effectively reduces the total level of radiated energy and the effective gain of the
antenna.
Effective Radiated Power
Effective Isotropic Radiated Power (EIRP) is a figure of merit for the net radiated power in a given direction. It
is equal to the product of the power supplied to an antenna and the antenna gain in a given direction relative to
an isotropic radiator. It can be expressed in watts or dBm.
Polarization
The polarization of an antenna describes the orientation and sense of the radiated wave's electric field vector.
All radiated waves are defined as elliptically polarized. In this general case, the antenna's total electric field
(E-field) has two components that lie in the same plane. These two E-field components may be of different
strength and are oriented at different angles.
The most common cases of elliptical polarization are:
Circular, in which the two E-field components are equal in magnitude, 90 degrees out of phase and spatially
oriented at 90 degrees from one another, and
Linear, in which the wave has a single E-field component. In a linear polarized antenna, the E-field is either in
a vertical line, a horizontal line or a slanted line, such as 45 degrees from the horizontal.
The term axial ratio is used to define the relative strength of the two E-field components in an elliptically
polarized wave. For pure circular polarization the axial ratio is 0 dB and for linear polarization the axial ratio is
infinite.
The important performance issue relative to signal polarization is that maximum energy transfer between a
transmitting antenna and a receiving antenna will only occur if both antennas have identical axial ratio,
identical polarization sense and the same spatial orientation. It is assumed that nothing in the propagation path
causes the signal polarization to change, which is known as polarization distortion.
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