ADTRAN 12804108L1A System Manual

Section 2 Microwave Path Engineering Basics TRACER 4108/4208 System Manual22 © 2003 ADTRAN, Inc. 612804208L1-1BFresnel Zones, Earth Curvature, & Antenna HeightsFresnel zones correspond to regions in the microwave path where reflections of the intended signal occurand combine in both constructive and destructive manners with the main signal, thereby either enhancingor reducing the net power at the receiver.In general, the odd numbered Fresnel zones (1, 3, 5, ...) add constructively at the receiver, while the evennumbered Fresnel zones (2, 4, 6, ...) add destructively at the receiver.The first Fresnel zone corresponds to the main lobe, 60% of which must be free of physical obstructionsfor the path calculations to be valid. Since the main lobe contains the vast majority of the microwaveenergy, this zone is typically used to determine proper antenna heights when placing antennas on towers orbuildings.The curvature of the Earth becomes a legitimate obstruction for path lengths of 7 miles or greater, and mustalso be accounted for when determining minimum antenna heights.The aggregate expression for minimum antenna height that incorporates both the 60% first Fresnel zoneand the Earth’s curvature is given bywhere f is in GHz and d is in miles.Table 5 tabulates minimum antenna heights for given path lengths.Table 5. Minimum Antenna Height for Given Path LengthsPath Length(miles)Min. Antenna Height@ 2.4 GHz(ft)Min. Antenna Height@ 5.8 GHz(ft)2 33 224 48 326 61 418 73 5010 85 6014 111 8116 124 9218 138 10420 153 11722 169 13124 185 14526 202 16128 220 17730 239 19532 259 21334 279 23236 300 252h 72.1 d4f----- 0.125d2+= (feet)