OPERATION GUIDE FOR PRODYN “AD” SERIES

DIFFERENTIAL FREE-FIELD D-DOT SENSORS

 

GENERAL DESCRIPTION

The PRODYN “AD” series free-field D-Dots are a range of broad band
differential output sensors that were designed to measure time rate of change
of electric displacement. Electrical and physical specifications for each
model can be found on PRODYN data sheet “Electric Field Sensor, Free Field”. 

The sensor consists of two asymptotic sensing elements mounted on each
side of a ground plate and held in position by dielectric supports. The sensing
elements are connected to transmission lines to the output connectors. 
In most cases the output connectors are spaced for direct hook-up to an
appropriate PRODYN balun.

THEORY AND PRINCIPLES OF OPERATION

Basic Theory

 It is recommended that the user review PRODYN application note number 895
before using the D-Dot sensors. PRODYN’s other application notes
(PAN 1195, 192 & 890) are referred to and have useful information on the
theory and operation of EM sensors. 

The “AD” type free-field sensor may be thought of as a current generator which
generates a current proportional to D-Dot. On each side of the sensor,
the generated current flows through the load impedance (cable termination)
and produces the output voltage:

Where

            R= total load impedance (100 ohms for free field sensors)

            Aeq= equivalent sensor area in square meters

            = time rate of change of displacement current in amperes/m2

 

The load resistance is shunted by the sensor capacitance, which by passes
part of the generated current and limits the high frequency response of the sensor. 
Current flows to ground through the load resistance and sensor capacitance. 
On each side, the current generated is picked off the sensing elements that are
connected to 50 ohm transmission lines at the center of the sensor. 
The transmission lines carry the signals to the load impedances. 
The load impedances are each 50 ohms to the ground plane, so the total load
impedance is 100 ohms and the total output voltage is balanced about the
ground plane.

Operation Description

 PRODYN’s free field D-Dot sensors are used in free space
to measure the time rate of change of the electric displacement vector.   

                                                             ,

The electric displacement vector (D) is related to the electric field intensity (E),
which is normally the desired measurement quantity; D=e0 E

      Where e0 is the permittivity of free space (8.85 x 10-12 Coulombs/V-m)

The sensor is normally positioned no closer than a few sensor diameters
from conducting surfaces and at a distance from the EM source where
g radiation will not affect its operation.

Equivalent Circuit

  

 

 Frequency Response of D-Dot Sensor

 

 

Sensor Response To Step Function

 Following is a typical expected sensor response to a step function. 
The time base will vary, depending on the sensor’s equivalent area.

  

 Sensor Directivity

The axis of greatest sensitivity is normal to the ground plate.  When measuring fields
with direction deviating from normal, the effective area Aeq becomes Aeq × Cosq,  
where q is the angle between the mounting plate normal and the incident.

 Positive voltage is developed at the output center conductor when the incident E-field
generates positive potential above the sensor (E-field pointing toward ground plate).

 Amplitude Range

 The sensor output voltage limitation typically is directly related to that of the output
connectors, which are rated by the connector manufacturer. 
The voltage rating is listed on the data sheet.

 Accuracy

  The accuracy of PRODYN’s free-field D-Dot sensors is based on precise
manufacturing tolerances (see Measurements Report) and assembly procedures. 
This relates to less than  ± 1% of the sensor equivalent area,
excluding high frequency effects.

Calibration

  The “AD” series D-Dot sensors are not encumbered by an energy transformation
mechanism, since they are not transducers.  Transducers depend on some
transformation mechanism, which usually has some nonlinearity and variability over time. 
The equivalent area of a sensor is not subject to change, so it only needs verification
of its equivalent area.  Once the equivalent area of a given model is established,
the variance between units is insignificant, so periodic calibration is unnecessary. 
Electrical integrity of a sensor can be checked using a time domain reflectometer. 
Pulse or sweep measurements from a sensor can be done in various types of test
fixtures, such as TEM cells.  It should be noted every test fixture will produce results
unique to itself and the measurement should be considered as typical.

 MAINTENANCE

 Environmental

 The PRODYN free-field D-Dots can typically be used over the temperature
range of -20°C to 55°C.  For continued outdoor use, models that are equipped
with gas pressure valves, it is recommended that a positive pressure of .5 to 1 psi
of dry air or nitrogen be maintained.  Since the connectors are not weatherproof,
they should be protected to maintain a dry condition.

Storage

 When not in use, PRODYN’s D-Dot sensors should be stored in a dry,
dust free area.  It is recommended that the sensors be stored in their original foam
packaging to protect the sensing elements from any impact or shock from dropping.
This is especially important for the high frequency models,
such as AD-20(R), AD-80(R), AD-70(R) and AD-55(R).