COKEVA 11940C, Operation Note

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Cokeva 11940C Operation Note
5
3 ‐ 8. Power Density and Electric Field Calculations
3 ‐ 9. The Cokeva 11940C measures magnetic field strength. Assuming a 377 ‐ ohm field impedance, an associated
electric field strength or a wave power density can be calculated from the magnetic field measurement. Refer to the
following equations:
E ─ Field Strength
(
dB
µ
m
)
= V
SA
+ AF + K + 51.5
= V
SA
+ AF + K + 51.5
Power Density
(
dB
mW
cm
2
)
= V
SA
+ AF + K ‐ 104.3
where
V
SA
= voltage in dBµ V measured on analyzer
AF = 11940C Antenna Factor in dB
(
µA/m
µV
)
K = loss or gain in dB between 11940C and spectrum analyzer
(Positive value if loss, negative value if gain.)
EXAMPLE:
At 100 MHz, measured V
SA
= 60.3 dBµV. Let K = 0 and AF = 44 dB
(
µA/m
µV
)
then
E
(
dB
µV
m
)
= 60.3 + 44 + 51.5 = 155.8 dBµV/m = 61.7 V/m
S
(
dB
mW
cm
2
)
= 60.3 + 44 – 104.3 = 0 dB
mW
cm
2
= 1
mW
cm
2
3 ‐ 10. Localized Field Source
3 ‐ 11. The probe can also be used as a localized magnetic field source for locating susceptibility problems on an
electronic circuit. It is a reciprocal device; that is, a voltage fed into the unit will create a magnetic field at the dual ‐
loop sensor. For this application, the maximum input power is 0.5W with a characteristic worst ‐ case VSWR of 3:1.
Table 2 lists characteristic magnetic field strengths developed at the probe tip using 0 dBm incident power.
Table 2. Characteristic Magnetic Field Strength
Frequency
(MHz)
Input Power
(incident)
Magnetic Field Strength
( H ), dB (µ Α /m)
30 0 dBm 101
100 0 dBm 98
500 0 dBm 94
850 0 dBm 93
1000 0 dBm 98