User Manual

Table Of Contents

TRACER 4205 System Manual Section 2, Microwave Path Engineering Basics

Antenna Beam Patterns

Directly related to the subject of antenna alignment is the topic of antenna beam patterns. Antennas being

used with the TRACER 4205 system will 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 on page 19. The antenna alignment step to setting up a microwave link is in fact steering

the main lobes of both antennas until the main lobe of one transmitter is centered on the receiving element

of the receiving antenna.

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 5725 MHz to 5850 MHz frequency band.

Fresnel Zones, Earth Curvature, & Antenna Heights

The 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, and must be at least 60% 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 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% first Fresnel zone

and the Earth curvature is given by

where f is in GHz and d is in miles.

main lobe

side lobes

h 72.1

----- 0.125d

+= (feet)