NARDA 8718B, User Manual

Page: 128 / 137

127 Appendix B Theory of Operation
This provides for the independence of probe orientation relative
to the polarization of the field. The instantaneous charge
distribution on adjacent cold junction elements produces a
potential difference across the thin-film resistive thermocouples
and a resultant dissipation of energy in these films. As the
frequency increases, the phase difference between the
potentials developed in adjacent junctions also increases the
open circuit voltage.
The Model 8764D operates from 100 kHz to 300 MHz. It contains
three orthogonal displacement energy sensors with diode
detectors. This unique design has much lower impedance than
traditional dipole designs. The major advantage is the greatly
reduced interaction between the meter and probe, the human
body making the measurements, and the RF field.
The Model 8782D uses an “active antenna” design to accurately
measure RF fields as low as 3 kHz over a 60dB dynamic range.
These probes are responsive to the H-Field component. Each
of the three mutually perpendicular coils in every probe has a
diameter of 3.5 inches (8.9 cm), consists of multiple turns and is
series-resonated somewhat below the low frequency end of the
band. The RF current induced by the “H” field dissipates power
in the thermocouple elements, heats the hot junctions and
provides a DC output voltage proportional to the square of the
induced current. Circuitry is added to minimize high frequency
out-of-band responses typical in many H-field probe designs.
Low Frequency
Electric Field
Probes
Low Frequency
Magnetic Field
Probes