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Sometimes it is necessary to know the actual field intensity or power density at a given distance from a transmitter instead of the signal strength received by an antenna. | FIELD INTENSITY and POWER DENSITY Sometimes it is necessary to know the actual field intensity or power density at a given distance from a transmitter instead of the signal strength received by an antenna. Field intensity or power density calculations are necessary when estimating electromagnetic interference EMI effects when determining potential radiation hazards personnel safety or in determining or verifying specifications. Field intensity field strength is a general term that usually means the magnitude of the electric field vector commonly expressed in volts per meter. At frequencies above 100 MHZ and particularly above one GHz power density PD terminology is more often used than field strength. Power density and field intensity are related by equation 1 E2 E2 E2 m PD ------- --- 1 D Z0 120b 377 where Pd is in W m2 E is the RMS value of the field in volts meter and 377 ohms is the characteristic impedance of free space. When the units of Pd areinmW cm2 thenPD mW cm2 E2 3770. Conversions between field strength and power density when the impedance is 377 ohms can be obtained from Table 1. It should be noted that to convert dBm m2 to dB iV m add 115.76 dB. Sample calculations for both field intensity and power density in the far field of a transmitting antenna are in Section 4-2 and Section 4-8. Refer to chapter 3 on antennas for the definitions of near field and far field. Note that the term before m m2 andcm2 inTable1 mean per i.e. dBmperm2 not to be confused with the division sign which is valid for the Table 1 equation P E2 Zo. Rememberthatinordertoobtain dBmfromdBm m2 given a certain area you must add the logarithm of the area not multiply. The values in the table are rounded to the nearest dBW dBm etc. per m2 so the results are less precise than a typical handheld calculator and may be up to A dB off. VOLTAGE MEASUREMENTS Coaxial cabling typically has input impedances of 50 75 and 93Q 2 with 50Q being the most common. Other types of cabling include the .