Handbook of Propagation Effects for Vehicular and ... - Courses

11-4
Propagation Effects for Vehicular and Personal Mobile Satellite Systems
−4
B e = 0.
43⋅10
(Webers/m 2 ), (11-6)
polarization rotations of 142.7° and 48.0° occur at f = 870 MHz and f = 1.5 GHz,
respectively. It is apparent that at UHF frequencies, significant signal loss due to
polarization mismatch may occur. As mentioned, this is normally avoided by
transmitting and receiving circular polarized signals since the receiving antenna is
insensitive to the same polarization shifts of the orthogonal linear components
comprising the circular polarized wave. At L-Band and higher frequencies, Faraday
rotation may generally be ignored.
11.2.3 Ground Specular Reflection
This type of specular reflection is generated on the ground near the vehicle, where the ray
from the reflection point to the antenna is below the horizontal. This coherent reflection
comes from an area around the intercept point the size of approximately the first Fresnel
zones. Its strength, relative phase shift, and polarization depend on the roughness and
dielectric properties of the ground and are elevation angle sensitive. In a system utilizing
a low-gain antenna (e.g., a dipole) which can geometrically see the specular point and
also has gain in that direction, destructive interference between the specular reflection
and the direct wave can produce deep fades [ITU-R, 1986a (Report 1008); Flock, 1987].
The antennas contemplated for use in LMSS are either low-cost, medium gain,
fixed pointed or higher-cost, high gain, tracking antennas. A typical medium gain
antenna is a crossed drooping dipole, which has azimuthally omni-directional gain of
about 4 dB from 15° to 60° elevation. At lower elevation angles, its gain decreases
rapidly, thus providing protection against specular reflections from the ground near the
vehicle and multipath from elevated objects at larger distances. A high-gain antenna,
typically a mechanically or electronically scanned array, achieves even greater rejection
of multipath power and a concomitant narrowing of the Doppler spectrum. Placing the
antenna on the center of the vehicle roof, which acts as a ground plane, helps to direct the
pattern upwards. This further enhances the isolation from ground specular scatter.
For circular polarization, rejection of the reflected multipath energy occurs when
the grazing angle is greater than the Brewster angle. At these angles the sense of rotation
of the reflected energy is reversed and hence opposite that of the receiving antenna.
These grazing angles are in the range of 15° to 35° for very wet to very dry land,
respectively [Reed and Russel, 1966].
11.3 Empirical Regression Models
Empirical regression models are represented by fade distributions derived from
experimental measurements at different frequencies, elevation angles, vehicle headings,
sides of road, types of terrain, and extent of shadowing. They all have the common
advantage of being based on actual data and hence they may be used with a certain
degree of confidence for the prediction of fade distributions over similar types of roads.
Although they are derived from "time-series" of fading events, this information is lost in