Power Density And Electric Field Calculations; Localized Field Source - HP 11941A Operation Note

HP 11941 OPERATION NOTE
3-6. Use the data included in figure 1 for interpolating antenna factors between the five calibration
frequencies included in the probe's calibration table.
3-7. Power Density and Electric Field Calculations
3-8. The HP 11941A measures magnetic field strength. An estimate of the magnitude of the associated
electric field can be made if a field impedance is assumed. An example is given below using a 377-ohm field
impedance. Radiated power density can also be estimated, assuming that the fields are in phase and that the
field impedance is 377 ohms.
E-Field Strength (dB Vsa + AF + K + 20 log(377)
" *** AF + K + 51.5
Power Density
7
^
Vsa +AF + K -104.3
EXAMPLE:
\ cm^
where
VsA = voltage in dBfxV measured on analyzer
AF = HP 11941A Antenna Factor in
K * loss or gain in dB between HP 11941A and spectrum analyzer
(Positive value if loss, negative value if gain.)
At 10 MHz, measured Vsa " 65.3 dB/itV. Let K * 0 and AF * 39
then
E 65.3 +39+ 51.5= 155.8 dB^V/m ^ 61.7 V/m
s(dB^)= 65.3 + 39- 104.3 = 0dB2^ = 1 ^
\ cmy cm^ cm*
3-9. Localized Field Source
3-10. The probe can also be used as a localized magnetic field source for locating susceptible components
on a circuit board. It is a reciprocal device: that is, a current fed into the probe will generate a magnetic
field at the dual-loop sensor. The specified antenna factors cannot be used to estimate the magnitude of the
output field. Table 2 lists characteristic field strengths developed 1 mm (0.040 in.) away from the probe tip,
using an incident power of 0 dBm into 50 ohms.
CAUTION
Power applied to the probe must not exceed the maximum Input power
listed In table 1 or damage to the probe will result.
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