NARDA 8718B User Manual

Page 127

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126

Appendices

The 872xD series probes operate from 300 MHz to >50 GHz.

They contain resistive thermocouples distributed along the

length of the dipole. The spacing prevents resonance over the

operating frequency range of the probes. The dipole may be

viewed as a group of series connected small resistive dipoles or

as a very low Q resonant circuit. Element sensitivity decreases

with frequency. At frequencies above12 GHz the dipole begins

to align along the Poynting Vector with the cold junctions

oriented tangential to the electric field. This is the traveling wave

mode of operation.
Each of the three mutually orthogonal elements contains four

resistive dipoles with the cold junctions oriented at right angles.

This provides for the independence of probe orientation relative

to the polarization of the field.
These probes operate from 300 kHz to 50 GHz. Two distinct

sets of isotropic sensors are used to cover this very broad

bandwidth. The output from each set of detectors is processed

separately by separate amplifier circuits in the probe handle.

The two signals are summed within the 8718B meter. Models

include the Model 8741D that has a flat frequency response and

the x8722D series that have shaped frequency response

characteristics.
In the lower frequency region below 1500 MHz, a dipole with

diode detector and both distributed and discrete components

are used. Three orthogonally mounted conductive dipoles, each

terminated in diode detector, form the low frequency detection

circuit. These diode detectors are operated in their square law

region. This region is defined as that in which the DC diode

current is proportional to the square of electric field tangential to

the dipole. The elements that are used in the higher frequency

region are thin-film thermocouples. They provide true square law

output and function in two modes. Between 1.5 and 12 GHz, they

are resistive dipoles. As a resistive dipole, each probe contains

three mutually perpendicular elements. Above 12 GHz, it utilizes

the phase delay of a traveling wave to produce additional output.

In this mode, the dipole is aligned along the Poynting Vector with

the cold junctions oriented tangential to the electric field. Each of

the three mutually orthogonal elements contains four resistive

dipoles with the cold junctions oriented at right angles.

Microwave Band

Electric Field

Probes

Ultra-broadband

Electric Field

Probes

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