MoPro Chapter 6.3

6.3 Distributions from Low- and High-Gain Receiving Antennas

During the Australian campaign by Vogel et al. [1992], a number of repeated runs were executed in which high- and low-gain antennas were employed. The characteristics of these antennas are given in Table 6-1. Figure 6-2 shows a plot of the high gain receiver 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

(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 since 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 azimuthal 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.

Figure 6-2: Fades measured by high- and low-gain systems at equi-probability levels.

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