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

Polarization, Antenna Gain and Diversity Considerations 6-3
fade versus the low gain fade over the low gain fade interval of 0 to 15 dB. The data
points were found to follow the linear relation
A ( HG)
= 1.
133⋅
A(
LG)
+ 0.
51,
(6-3)
where A(HG) and A(LG) represent the high and low gain fades (in dB), respectively.
Agreement between relation (6-3) and the data points for A(HG) is within 0.2 dB RMS.
Table 6-1: Summary of pertinent antenna characteristics [Vogel et al., 1992].
Characteristic Low Gain High Gain
Type Crossed Drooping Dipoles Helix
Gain (dB) 4 14
Nominal Pattern (El.) 15°-70° 45° (Principal Planes)
Nominal Pattern (Az.) omni-directional 45°
Polarization LHCP or RHCP RHCP or LHCP
The high-gain antenna system consistently experienced slightly more fading than the
low-gain system. For example, at 3 and 14.5 dB (of low-gain fades), the high gain fades
were 4 and 17 dB, respectively, which represents 33% and 17% increases. This slight
increase in attenuation for the high-gain case occurs because less average power is
received via multipath from surrounding obstacles as the associated antenna beam is
narrower. In contrast, the azimuthally omni-directional low gain antenna receives more
scattered multipath contributions resulting in an enhanced average received power. It
should be emphasized that negligible ground specular backscatter was received by either
antenna because of the gain filtering characteristics at low elevation angles. The slight
increase of signal for the lower gain azimuthally omni-directional antenna came from
diffuse scatter from surrounding tree canopies. It is important to note that because the
high-gain antenna has 10 dB more gain associated with it, the net power received by it is
still significantly higher than that received for the low-gain case. Even at the 15 dB fade
level (low-gain receiver system), the net received power for the high gain mode is larger
by 7.5 dB.
Mayer [1996] examined the effects of antenna gains for low elevation angle
measurements (8°) at 20 GHz in Alaska through measurement and analysis of
transmissions from the Advanced Communications Technology Satellite (ACTS). He
compared clear line-of-sight multipath effects for aperture antennas with gains of
approximately 16 dB, 22 dB, and 28 dB. Because of the low elevation angle, ground
multipath is more likely for the lower gain antennas. The 28 dB gain antenna clearly
showed the smallest clear line-of-sight multipath fading, giving magnitudes between 1 to
2 dB. The smaller fading is due to reduced specular scattering from the road. This
compares to multipath fading of approximately 10 dB for the lowest gain antenna.
However, tracking of the satellite with the higher gain antenna was found to be more
difficult. Tracking errors for the higher gain antenna was found to result in substantial
fading of the satellite signals vis-à-vis the lower gain antenna.