User's Manual

SEL-3031 Instruction Manual Date Code 20091009
Installation
Overview
2.2
Radio Path
Spread-spectrum radios operating in the 900-MHz ISM band are limited by line-of-
sight. Radio line-of-sight is longer than optical line-of-sight, due to the bending of the
radio wave toward the surface of the earth. This radio horizon is typically 30% longer
than the visual horizon. The longer the communications path, the taller the antennas
must be in order to maintain the line-of-sight.
Obstructions in the line-of-sight will impact the performance of the radio, as the
strongest radio signal is communicated directly along the radio line-of-sight.
The line-of-sight between two antennas is shaped like an ellipse (called the Fresnel
zone). The point exactly halfway between the two antennas is the widest part of the
ellipse, as shown in Figure 2.1. At 900 MHz and 1000 feet apart, the Fresnel zone is
16 feet in diameter. At 20 miles apart, it is 170 feet in diameter. Anything within the
Fresnel zone will obstruct and reduce signal strength and availability (the ground,
buildings, vegetation, etc.). Table 2.1 depicts the maximum Fresnel zone diameter and
path loss for some typical path distances.
Figure 2.1 Fresnel Zone
The formula used to calculate the widest distance of the Fresnel zone is as follows:
where:
b = radius of the Fresnel zone in meters
d = distance between transmitter and receiver in kilometers
f = frequency transmitted in GHz
d
b
b17.32d4f=