Wide range radiation monitor

A unique diode sensor adapted for responding with pre-established sensitivity over a range of relatively low frequencies. A diode is connected to a dipole antenna having both conductive and resistive portions. Lumped impedances are connected in parallel with the diode and the values of these impedances are selected in coordination with the characteristics of the diode and antenna to control the sensitivity of the sensor as a function of the frequency of the illuminating energy.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to radiation monitors and more particularly, to 
measuring instruments suitable for monitoring radiation of electric fields 
within a frequency range extending to the vicinity of 100 GHz. 
2. Description of the Prior Art 
The essential components of instruments capable of monitoring radiation 
over a range of frequencies, are the electrical field sensitive elements. 
Their characteristics, structure, and interconnection are selected to 
achieve a sensitivity to the field that is relatively flat over the 
frequency ranges of interest. Thermocouples and semiconductor diodes are 
common sensing elements. It has been recognized that each type of element, 
while having inherent advantages and limitations, also exhibits frequency 
limitations. Thus for example, diode sensors have been shown to operate 
most effectively at lower frequencies, e.g. below 1.5 GHz, while 
thermocouple sensors operate most effectively at higher frequencies, e.g. 
above 300 MHz. 
The characteristics of various sensors also have shown that diodes are most 
effective when applied in conjunction with the high reactance of 
electrically short dipole antennas. In contrast, the best sensitivity of 
thermocouple elements is achieved when low resistivity materials are 
employed in a thin-film configuration in conjunction with low reactance 
dipoles. There are other recognized characteristics which lead to 
corresponding limitations. At higher frequencies the reactance of diode 
sensors may be dominated by shunt capacitance effects which severely limit 
their high frequency response. At the other extreme, thermocouple sensors 
are limited in their application to the low frequency region due to 
difficulties in developing thermocouples of sufficiently high film 
resistivity to operate in conjunction with the higher reactance dipoles 
and with sufficient sensitivity to provide adequate signal levels. 
One form of effective field measuring equipment is shown in the inventor's 
U.S. Pat. Nos. 3,641,439 and 3,794,914, issued on Feb. 9, 1972 and Feb. 
26, 1974, respectively. These patents disclose a near-field radiation 
monitor utilizing thin-film thermocouples positioned in quadrature to 
measure relatively high frequency electric fields. 
As further described in the inventor's article Broad-Band Isotropic 
Electromagnetic Radiation Monitor, published in the November 1972 I.E.E.E. 
Transactions on Instrumentation and Measurement, one may utilize sensors 
of the general type described mounted along three mutually orthogonal axes 
in order to achieve isotropic performance. On the other hand, to date, no 
suitable equipment is known to be available that is capable of operating 
over the broad frequency range contemplated herein. 
SUMMARY OF THE INVENTION 
The present invention uses thin-film thermocouple sensors and diode sensors 
mounted on a single probe for monitoring an ultra-broadband of 
frequencies. Within the lower frequency range, the diode sensors furnish 
the monitoring capability. Within the upper frequency range, thin-film 
thermocouple sensors furnish the monitoring capability. The various 
sensors are mounted within a single housing, yet they are maintained 
essentially independent in order to prevent interaction. The outputs of 
the sensors are summed to provide a true indication of power density over 
the complete wide range of frequency being monitored. 
An object of the present invention is to provide a portable ultra-broadband 
radiation detector. 
Another object of the invention is to provide a portable radiation detector 
having an isotropic response. 
Yet another object of the invention is to provide a portable radiation 
detector having pre-established sensitivity to impinging radiation 
throughout both low and high frequency regions. 
Still another object of the invention is to provide a portable radiation 
detector utilizing both diode and thin-film sensing elements. 
From one aspect, the invention resides in a unique diode sensor adapted for 
responding with pre-established sensitivity over a range of relatively low 
frequencies. From another aspect, the invention resides in the utilization 
of separate sensors of diverse type, for responding over a broad range of 
frequencies, which exceeds the operating capabilities of either type of 
sensor, if used alone. 
In the first embodiment, the energy sensing means includes a diode 
connected to a dipole antenna having both conductive and resistive 
portions. Lumped impedances are connected in parallel with the diode and 
the values of these impedances are selected in coordination with the 
characteristics of the diode and antenna to control the sensitivity of the 
sensing means as a function of the frequency of the illuminating energy. 
In the second embodiment, the aforedescribed diode sensing means is 
arranged along a common axis with one or more thermocouple dipoles, though 
separated therefrom. In this embodiment, the thermocouple component or 
components form a sensing means designed to respond with constant 
sensitivity to illuminating energy over the higher substantially adjacent 
frequency range which exceeds the controllable response of the diode 
sensing means. 
The invention will be more thoroughly understood and appreciated from the 
following description which is made in conjunction with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The probe 10 shown in FIG. 1, serves to mount the sensors of the invention 
and also modifies the signals generated therein for transmission over a 
line 17 to a metering instrument. This is explained in greater detail 
later in the specification. In essence, three sets of sensing elements 
11a, 11b, and 11c are supported at one end of probe 10 along mutually 
orthogonal axes. 
By proper selection of sensor elements, a particular probe provides the 
sensitivity response characteristics shown graphically in FIG. 2. Proper 
handling of the signals generated with this sensitivity, affords a variety 
of monitoring opportunities. It will be recalled that the low frequency 
sensitivity of the monitor described herein is determined by diode sensors 
while the high frequency sensitivity is determined by thermocouple 
sensors. In FIG. 2, the low frequency region is shown to extend generally 
from 0 through 1.5 GHz and the high frequency region extends generally 
from 1.5 GHz to 40 GHz and beyond. 
Before considering in greater detail the specific characteristics of the 
various sensing elements, a visual appreciation of the elements and their 
interrelationship will be available from FIG. 3. This figure shows an 
array of series-connected thermocouple dipoles 18, making up the high 
frequency sensors, axially aligned with a dipole antenna 19, which serves 
as the input source for the low frequency diode sensor. Three such sets of 
sensing elements provide a complete unit for isotropic response of an 
ultra-broadband of frequencies. 
By utilizing separate sensing elements for different portions of a broad 
frequency range, one is able to select the sensing element characteristics 
best suited to either the portion of the range of interest, or best suited 
to the measurement of interest within that portion of the range. In 
addition, one is able to trim each sensing element to match the type of 
response desired within each portion of the broad range. It might be the 
designer's intent to produce constant sensor sensitivity, or response, 
across the entire range. It is also possible to tailor the response to 
predetermined criteria, as illustrated with the embodiment disclosed 
herein. 
The low frequency diode sensor element is illustrated in FIGS. 4A and 4B, 
and the lumped equivalent circuit for such a sensor is shown in FIG. 5. 
The equivalent inductive and capacitive reactance of the dipole segments 
is illustrated separately as L.sub.a1, L.sub.a2 and C.sub.a1, C.sub.a2, 
respectively. When considering the combined effect of such reactances, 
reference will be made to L.sub.a and C.sub.a, respectively. Similarly, 
the voltage induced in the dipole responsive to illuminating energy is 
illustrated by separate sources, e. The equivalent resistance of the 
dipole is illustrated as R.sub.a. 
In one specific embodiment of the invention, beam lead Schotky diodes were 
used to minimize connecting loops that might couple to the field and 
introduce erroneous signals. In addition, the size of the resistive and 
capacitive components was minimized by use of chip components. Typical 
values of the components shown in conjunction with the diode sensing 
element were (with a total dipole length of 3.75 inches): 
C.sub.1 =12 pfds. 
C.sub.2 =200 pfds. 
C.sub.a1 =0.1 pfds. 
C.sub.a2 =0.2 pfds. 
R.sub.2 =330 ohms 
R.sub.a =800 ohms 
L.sub.a1 +L.sub.a2 =0.036 mh 
In the frequency range from 30 to 300 MHz, the equivalent circuit 
simplifies to that shown in FIG. 8 wherein the dipole resistance R.sub.a 
and dipole inductive reactance of L.sub.a are negligible compared to the 
dipole capacitive reactance of C.sub.a. Within this range, reactance of 
C.sub.2 is selected to be very low, and resistance R.sub.2 is selected to 
be extremely high relative to the reactance of shunting capacitor C.sub.1. 
The dipole capacitance C.sub.a and the shunting capacitance C.sub.1 act as 
a capacitive voltage divider having a uniform output with frequency over 
this range, the capacitance C.sub.L having been shunted across the diode D 
to obtain the desired sensitivity. 
Descending in frequency and commencing at 30 MHz, the shape of the response 
is principally controlled by the resistance R.sub.2 in conjunction with 
the reactance of shunting capacitor C.sub.1, and the dipole capacitance 
C.sub.a. The equivalent circuit then reduces to the one shown in FIG. 7. 
The radio frequency voltage across the diode D varies at a 6 db per octave 
rate as the frequency decreases to 3 MHz. The diode DC output, having a 
square law characteristic, provides an output proportional to the square 
of the radio frequency voltage across the diode. The diode output 
sensitivity therefore decreases as the square of frequency, f.sup.2. 
Descending still further in frequency, at approximately 3 MHz the reactance 
C.sub.2 increases to the magnitude where it exceeds the resistance R.sub.2 
and the equivalent circuit reduces to approximate the schematic of FIG. 6. 
Hence, the circuit again performs as a capacitive divider having a 
constant sensitivity with frequencies below 3 MHz. 
Within the frequency range above 300 MHz and to 1.5 GHz, the dipole 
characteristics are selectively modified to achieve controlled roll-off. 
The dipole is constructed of conductive and resistive portions, as 
illustrated in FIG. 4A. The conductive portion connects a resistive film 
portion to the terminals of the diode. The extent and location of the 
resistive film portion of the dipole is selected to achieve the desired 
characteristics. When the frequency increases above 300 MHz, the 
resistance of this film predominates over the reactance of that portion of 
the antenna. If the entire dipole had been constructed of resistance 
material, the sensitivity would decrease at a 6 db per octave rate, or 
1/f.sup.2. To approximate the desired 1/f, 3 db per octave rate, a portion 
only of the dipole is made resistive, the resistivity in the present case 
being chosen to start roll-off at 300 MHz. That portion of the conductive 
dipole contributes a signal to the diode that is constant over the 
frequency range thereby achieving the desired reduction in sensitivity to 
approximate 1/f. The diode operates in its square law region and the 
induced voltages are made approximately equal for both conductive and 
resistive portions. 
In the low to high frequency transition 300 MHz to 1.5 GHz range, the 
equivalent circuit of FIG. 5 reduces to that of FIG. 9, where the 
resistance R.sub.2 is much greater than the shunting reactance of C.sub.1 
and the dipole reactance L.sub.a is negligible. 
Under these circumstances the diode output voltage V.sub.d becomes: 
##EQU1## 
The response of the diode sensor thus decreases as the frequency increases, 
but the addition of the output from the higher frequency sensor elements 
as described hereafter, will produce additional correction. This technique 
can be modified further by using discrete resistances strategically placed 
along the dipole length. FIG. 4B illustrates this structure with a single 
resistive pad R.sub.P on each leg of the dipole. 
The resistivity of the dipole and the concomitant decreasing sensitivity, 
prevents affecting the response in the high frequency region as the dipole 
approaches resonance. The values of capacitance and inductance for the 
sections of the dipole can be calculated from the average characteristic 
impedance for each portion of the dipole. The equation for characteristic 
impedance Z.sub.0 is: 
##EQU2## 
where H.sub.1 and H.sub.2 define the limits of each portion of the dipole, 
and a is the effective radius of the dipole. The equation can then be 
integrated over the portion of the antenna in question. 
Using the expression of L.sub.a and C.sub.a : 
EQU L.sub.a =Z.sub.0 (8f.sub.0).sup.-1 
EQU C.sub.a +(.pi..sup.2 f.sub.0 Z.sub.0).sup.-1 
The thermocouple sensing elements used for the high frequency region, are 
described and explained structurally in the inventor's aforementioned U.S. 
Pat. No. 3,641,439 and in his I.E.E.E. article. The probe may consist of 
three mutually perpendicular broadband probe elements as illustrated in 
FIG. 1. Broadband characteristics are obtained by distributing resistive 
thermocouple dipoles along the length of the high frequency sensing 
element at spacings that will permit no resonant length over the range of 
frequencies within which the probe is intended to operate. The spacing is 
less than one-quarter wavelength of the highest frequency to be measured. 
In effect, the high frequency sensing element may be viewed as a group of 
series-connected small resistive dipoles or as a very low Q resonant 
circuit. 
Each thermocouple dipole element and/or connected set provides a DC output 
signal that is proportional to the square of the electric field strength 
tangential to the element. The elements are preferably thin-film 
thermocouples that provide true square-law outputs. The DC signal is 
proportional to the power dissipated in the thermocouple elements and 
indicates the average energy density in the volume in which the elements 
are contained. The summation of the DC signals from the three orthogonal 
sensing elements provides a measure of the total energy or power density, 
independent of direction or polarization of the RF signals. 
A lumped equivalent schematic representation of a thermocouple sensing 
element is shown in FIG. 11. L.sub.3 and C.sub.3 are the lumped equivalent 
inductance and capacitance of the element determined from the average 
characteristic impedance Z.sub.0. 
EQU C.sub.3 =2(.pi..sup.2 f.sub.0 Z.sub.0).sup.-1 
EQU L.sub.3 =Z.sub.0 (8f.sub.0).sup.-1 
f.sub.0 is the resonant frequency of a dipole of the same length as the 
element. R.sub.3 equals the total resistance of the probe element less 
R.sub.4. C.sub.4 is the shunt capacitance across a small dipole and can be 
determined from the geometry of FIG. 10 as 
##EQU3## 
The DC output of the small thermocouple dipole is proportional to the 
power dissipated in it. 
The resistive dipoles are composed of thin films of overlapping dissimilar 
resistive films 30, 31, deposited upon a thin plastic substrate. The 
geometry creates alternate cold and hot junctions. As shown in FIG. 10, 
the hot junctions are formed at the center 32 of the narrow strips having 
relatively high resistance thereby allowing for the dissipation of energy 
and the resultant increases in temperature. The wider sections 33, 34 have 
a low resistance and thus function as cold junctions, the low resistance 
allowing little energy to be dissipated within these sections. In 
addition, the broad area distributes the energy and conducts heat rapidly 
into the substrate so that very little temperature rise occurs. The 
resultant DC output voltage is directly proportional to the energy 
dissipated in the resistive portion of the thermocouple. 
The spacing "D" between the cold junctions is a small fraction of a 
millimeter. The close spacing minimizes zero drift due to ambient 
temperature, since only a very small temperature gradient can occur due to 
the variation in ambient temperature. Variation in the ambient temperature 
will cause variation in sensitivity that is less than 0.05 
percent/.degree.C., which will not degrade the basic accuracy of the 
instrument even over wide temperature ranges. The leads that carry DC 
outputs from the probe elements to the metering instrumentation are 
high-resistance films and present a high resistance near the probe 
elements resulting in low current to minimize any interaction of the DC 
leads and the probe elements. 
The total effect of the structure described is to approximate the condition 
of the high frequency sensing elements being suspended in space because 
the leads are transparent to the RF fields. The extremely light coupling 
into the field, results in very little perturbation of the RF field being 
measured due to scattering phenomena. A break point of the frequency 
sensitivity curve is provided at 1.5 GHz at the low frequency end and 
above 40 GHz at the high frequency end. This yields extremely flat 
frequency response from 1.5 GHz, while below 1.5 GHz the response 
decreases at a 6 db/octave rate. 
To minimize cross coupling between the resistive high frequency 
thermocouple dipole and the low frequency diode connected dipole, each 
element pair is placed on the same axis, with some spacing between the 
adjacent ends. FIG. 3 shows the spacial relationship. 
With an appreciation of the structure and arrangement of the low and high 
frequency sensor elements of the invention, it will be understood that 
monitoring instrumentation of various forms can be adapted to cooperate 
with a properly designed probe. FIG. 12 is a block diagram of one such 
form of instrumentation. 
DC signals from a housing 11 containing suitably mounted high frequency 
thermocouples and low frequency diode elements, are delivered to a 
pre-amplifier in the handle of the probe 10, over a high resistivity 
transmission line 12. A first two-wire DC transmission line 13 is used for 
the three high frequency elements which are connected in series, as 
previously explained. A second two-wire DC transmission line 14 is used 
for the three diode elements which are connected in parallel, as also 
previously explained. The leads are held rigidly in place to prevent cable 
modulation. A high resistivity film is advantageously applied over the 
probe to provide shielding from static charges. The pre-amplifiers utilize 
balanced instrumentation amplifiers and the resistive transmission line 
leads are matched to further reduce common mode and static charge induced 
signals. 
The pre-amplifier contains two sections. One section 15 conditions the 
diode signal and provides temperature compensation and such linearity 
correction as may be required at the upper extreme of the operating power 
density range. A gain setting control may be provided for calibration. The 
conditioning amplifiers 16 for the high frequency thin-film thermocouple 
sensors need provide for calibration only, inasmuch as temperature 
compensation and linearity correction are not necessary. The signals from 
the pre-amplifiers are transmitted over a conductive cable 17 to a 
suitable metering instrument. 
Within the metering instrument shown in FIG. 12, the two signals from the 
probe are first combined in a summing amplifier 20. The resultant signal 
is further processed by a maximum hold amplifier 21. The output of this 
latter circuit feeds an amplifier 22 which in turn drives a D'Arsonval 
meter movement 24. 
A particular ultra-broadband radiation monitor has been described. It is 
recognized that modifications will be apparent to those skilled in the 
art. All modifications coming within the teachings of this disclosure are 
intended to be covered by the following claims.