ADTRAN 6000 Series Antenna Beam Patterns, Fresnel Zones, Earth Curvature, and Antenna Heights

TRACER 6000 Series Integrated System Manual Section 2 Microwave Path Engineering Basics

Antenna Beam Patterns

Antennas used with the TRACER 64x0 system have a particular beam shape, determined in part by the physical construction and geometry of the antenna. The antenna beam patterns are characterized by a dominant main lobe, which is the preferred lobe to use for point-to-point communications, and several side lobes, as shown in Figure 2. When setting up a microwave link, antenna alignment is nothing more than steering the main lobes of both antennas until the main lobe of one transmitter is centered on the receiving element of the receiving antenna.

main lobe

side lobes

Figure 2. Typical Antenna Beam Pattern

Antennas are also designed to radiate RF energy efficiently for a specific range of frequencies. Please consult the data sheet for your particular antenna make and model to ensure that it is specified to operate in the 2400 MHz to 2483.5 MHz (TRACER 6410) or 5725 MHz to 5850 MHz (TRACER 6420) frequency bands for your TRACER 64x0 system.

Fresnel Zones, Earth Curvature, and Antenna Heights

Fresnel zones correspond to regions in the microwave path where reflections of the intended signal occur and combine in both constructive and destructive manners with the main signal, thereby either enhancing or reducing the net power at the receiver.

In general, the odd numbered Fresnel zones (1, 3, 5, ...) add constructively at the receiver, while the even numbered Fresnel zones (2, 4, 6, ...) add destructively at the receiver.

The first Fresnel zone corresponds to the main lobe, 60 percent of which must be free of physical obstructions for the path calculations to be valid. Since the main lobe contains the vast majority of the microwave energy, this zone is typically used to determine proper antenna heights when placing antennas on towers or buildings.

The curvature of the Earth becomes a legitimate obstruction for path lengths of 7 miles (11 kilometers) or greater, and must also be accounted for when determining minimum antenna heights.

The aggregate expression for minimum antenna height that incorporates both the 60 percent first Fresnel zone and the Earth’s curvature is given by

h = 72.1	d	+ 0.125d	2	(feet)
	----
	4f

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