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ENVIRONMENTAL EVALUATION APPENDIX A: ELECTROMAGNETIC ENERGY INFORMATION
FIGURE A-4 EXAMPLE OF NEAR FIELD/FAR FIELD CROSSOVER
The data in Figure A-4 represent the crossover for a 6 dBd gain omni antenna that is 2.74 m (9 ft.) in
length. As the aperture length and gain change, the crossover point will also change greatly. For the
example above, the crossover point is approximately 9.14 m
(30 ft.). For an antenna with 10 dBd of gain and an aperture length of 3.96 m (13 ft.), the crossover point
will be over 30.5 m (100 ft.). A 3 dBd gain antenna with an aperture length of 457 mm (1.5 ft.) will have
a crossover point of only 914 mm (3 ft.).
A.2.3.1.1CYLINDRICAL MODEL
For vertical antennas, with omnidirectional horizontal patterns, the power density in the near field can
be estimated using the circular radiation pattern and the height of the antenna. The area of a cylinder
placed over this antenna is assumed to be uniformly exposed to the power radiated by the antenna; no
RF energy emanates from the top or bottom. This power density is approximately the same as the
average power density an individual of a specific height would be exposed to when standing next to the
antenna. This formula is referred to as the Cylindrical Model because it utilizes a cylinder for the
modeling (see Figure A-5).
FIGURE A-5 CYLINDRICAL MODEL
S =
--------------
P
2πDL
S = Power Density (mW/cm 2 )
P = Total Power into Antenna (mW)
D = Distance from Antenna (cm)
L= Length of Antenna Aperture (cm)
A-8 68P81089E50-B 9/1/05