Method for measuring the temperature of a material by using microwave radiation

The invention relates to a method and a device for measuring the temperature T.sub.x of any material or object, by using a microwave radiation, as well as to an application of said method for determining the coefficient of hyperfrequency reflection of any material or object. According to the method, the microwave radiation emitted through an antenna (1) is captured and the signals received are directed towards signal processing means (3). Additionally, between the antenna (1) and said means (3), a given impedance line (4) is intercalated whose impedance is a function of the input impedance of said means (3), and with a length L much bigger than the wave length of processed signals, so that the correlation factor of said means (3) is negligible. Furthermore, the output voltage is processed by calculation of all parameters by cylindrically modifying the structure of said processing means (3).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to the determination of the temperature of a given 
material or object, as well as of its microwave frequency reflection 
coefficient. 
To determine the temperature of an object, it is known in the art to use 
measuring processes whereby the thermal noise signals emitted by this 
object in the microwave frequency range are picked up and a correspondence 
is established between strength of the signals picked up and the 
temperature of the object. 
In this connection, the term "object" is to be taken in a very broad sense 
as it can refer equally well to a material object or to a material, or 
even to living tissues. Any absorbent body in fact emits thermal noise 
signals directly related to its temperature. Such thermal noise signals 
are emitted over a very wide frequency range. 
2. Description of Background Information 
To carry out temperature measurements, there are also known other processes 
using signals emitted in the infrared range. However, the drawback is that 
the signal picked up are emitted primarily by the surface of the body to 
be measured and the surface temperature cannot then be measured. 
Another known measuring method consists in using a thermocouple, which is 
necessarily introduced inside the body the temperature of which one wishes 
to measure. However, in very many cases, the penetration of the body by 
the thermocouple represents a major drawback. 
To avoid such drawbacks, it is preferred to make use of the thermal noise 
signals emitted in the microwave frequency range, that is to say 
frequencies ranging approximately between 0.5 and 20 GHz. 
In this connection, there are known microwave radiometry devices in which 
the microwave radiation emitted via an antenna is picked up and the 
signals received are routed to signal processing means which enable the 
temperature of the object in question to be determined. 
However, on of the main problems encountered in microwave frequency 
radiometry resides in the matching of the antenna in respect of the 
material the temperature of which one wishes to know. Indeed, the antenna 
used has a reflection coefficient R.sub.o and, as a result, the antenna is 
never perfectly matched, given that the objects to be measured generally 
have different configurations, sizes and properties. 
Under these circumstances, the error made in measuring the temperature of 
the object, due to the fact that the coefficient .vertline.R.sub.o 
.vertline..sup.2 of the antenna is different from zero, has two 
implications, namely: on one hand, the emissivity of the object: 
=1-.vertline.R.sub.o .vertline..sup.2 is different from unity and, on the 
other hand, given the reflection coefficient of the antenna, a part of the 
noise emitted at the input of the signal processing means is reflected by 
the antenna, and then amplified by the said means, and thus unduly 
contributes to the signal measured at the output of the said means. 
To remedy these different drawbacks, different processes have been devised 
to enable the internal temperature of a body to be measured without 
thereby necessitating the introduction into this body of means to detect 
this temperature. 
Document FR-2,497,947, in fact, discloses a microwave thermography device 
and process based on the principle of the Dicke radiometer, using an 
antenna, a circulator, an auxiliary source of noise with known 
characteristics, an amplifier-receiver and a detector. In addition, 
according to this document, the use of a circulator is associated with a 
two-channel microwave frequency switch cyclically connecting the measuring 
line to the antenna or short circuiting the measuring line. 
Thus, this circulator-switch assembly, on one hand, enables the signal 
emitted at the input of the amplifier to be absorbed and, on the other 
hand, enables the antenna to be presented with a temperature load 
substantially equal to that of the material to be measured. Under these 
conditions, when the coefficient .vertline.R.sub.o .vertline..sup.2 is 
different from zero, the reduction in noise emitted by the material to be 
measured is compensated for by the noise emitted by the said load and 
reflected by the antenna. 
However, the process and device according to document FR-2,497,947 
necessitate the use of a circulator, which can be disadvantageous in 
certain cases. The circulator is, in fact, generally formed by a ferrite 
element determined in accordance with the frequency range and over the 
size and price of which one has no control. This naturally affects, 
therefore, the cost of the device and on its size, precluding any 
possibility of monolithically integrating the device. 
This being the case, document FR-2,561,769 discloses a process for 
controlling impedance matching in low noise reception chains and a 
miniature microwave thermometer for implementing this process. 
Such a device comprises an antenna, a temperature and impedance adjustable 
standard noise source, switching means connected to the antenna and to the 
standard noise source, an amplifier disposed downstream of the switch, 
supplying a signal the amplitude of which corresponds to the difference in 
level between the signals from the antenna and the standard noise source, 
a controlled additional impedance to be placed periodically at the 
amplifier input, and means for analyzing the divergence between the 
impedances presented by the antenna and the standard noise source, to 
adjust, by matching their impedances, either the antenna or the noise 
source, to equalize the influences exerted by the additional impedance on 
the antenna and on the noise source, and, in consequence, to equalize the 
impedances presented by the antenna and the standard noise source. 
The process according to FR-2,561,769 thus consists in attempting to use a 
reference noise source, the electronically adjustable reflection 
coefficient of which is made equal in modulus to that of the antenna 
placed in the presence of the object to be measured. For this purpose, the 
noise emitted by the amplifier input can be used to check the equality of 
the two reflection coefficients, and use is made, for this purpose, of a 
variable additional impedance the value of which can be electronically 
controlled, and which is placed at the input of the amplifier. 
Such a technique makes it possible to dispense with the use of a 
circulator; unfortunately, however, it can only be used when the load 
presented by the antenna is resistive and if the length of line placed 
between the antenna and the amplifier is negligible. 
To the disadvantages of the known devices should be added other limitations 
for which it is not possible to find solutions. In particular, when a 
circulator is used, the size of the ferrite utilized is all the greater 
the lower the frequency at which one operates. For example, at 1 GHz, the 
size soon becomes prohibitive. Furthermore, when operating to determine 
the temperature with a zero method, one generally attempts to make nil a 
factor constituted by the difference between the temperature of the 
auxiliary reference source and the temperature of the body to be measured. 
It thus becomes a delicate matter to measure the temperature of a body 
with a temperature of less than 273.degree. K. (.degree.C.), which is 
disadvantageous in certain cases. 
Furthermore, in a measuring chain, the removal of the circulator element is 
not problem-free. Indeed, for an amplifier having a given gain g, the 
latter inevitably presents an input noise T.sub.e, as well as an amplifier 
noise T.sub.a, charactering its noise factor. Thus, in direct 
amplification circuits, allowances have to be made for a possible 
correlation between input noise T.sub.e and the output noise of the 
amplifier, T.sub.a when the load at the circuit input is not matched. 
The measurement errors due to the mismatch depend on the refection 
coefficient R.sub.o , and on the said noises T.sub.e and T.sub.a. 
To remedy this correlation phenomenon, it is known in the art to use in 
parallel on the amplifier input line aperiodic phase shifters, which make 
it possible to introduce randomly phases -.pi./2, +.pi./2, for example, 
the purpose of which is to preclude any possibility of coherent 
construction of the noises, and thus to cancel out any correlation between 
Te and T.sub.a. 
However, such phase shifters reduce the amplitude of the noise signals 
picked up and are, furthermore, of large dimensions, which render any 
monolithic integration of the radiometer impossible. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a process and a device 
for measuring the temperature of a material or of a given object using 
microwave radiation, which enable the aforementioned drawbacks to be 
overcome and make it possible to dispense with the use of a circulator 
while, at the same time, permitting precise temperature measurement. 
One of the objects of the present invention is to provide a process and a 
device for measuring radiometric signals which can, consequently, be 
designed using monolithic integration techniques and are thus 
miniaturizable and of low cost. 
Another object of the present invention is to provide a process for 
measuring radiometric signals which can be used when the antenna is not 
especially matched to the object to be measured, or if the length of line 
placed between the antenna and the amplifier is not negligible. 
Another object of the present invention is to provide a process and a 
device for measuring radiometric signals which make it possible to become 
free from the influence of any correlation between the input noise and the 
output noise of the amplifier when the load at the input is not matched. 
Another object of the present invention is to provide a process for 
measuring radiometric signals which does not use the principle of the 
Dicke radiometer, while producing a result that is equivalent, or even 
superior. 
Another object of the invention concerns, in fact, the application of the 
radiometric signal measuring process to determining the microwave 
frequency reflection coefficient of a material or a given object. 
Further objects and advantages of the present invention will emerge in the 
course of the following description which is, however, given only by way 
of illustration and is not intended to limit it. 
According to the invention, the process for measuring the temperature 
T.sub.x of a given material or object using microwave radiation, whereby 
the microwave radiation emitted via an antenna, having a reflection 
coefficient, R.sub.o x, is picked up, and the signals received are routed 
to signal processing means, is characterized by the fact that there is 
intercalated between the antenna and the said means a line of a given 
impedance, which is a function of the input impedance of the said means, 
having a length L that is very large in relation to the wavelength of the 
signals such that the correlation factor of the said means is negligible. 
Furthermore, according to the measuring process of the invention, the 
signals received by the antenna are routed, via the said length L of line, 
to the input of the said processing means, with a power gain .tau.g, with 
an input noise temperature T.sub.e and an output noise temperature T.sub.a 
such that the following output voltage is obtained at the output of the 
said processing means: 
EQU V.sub.s =g.gamma.[T.sub.x (1-.vertline.R.sub.o x .vertline..sup.2)+T.sub.e 
.vertline.R.sub.o x .vertline..sup.2 +T.sub.a ] 
then all the parameters are calculated periodically by modifying cyclically 
the structure of the said means. 
In this respect, to implement the process according to the invention, the 
measuring device has an intercalary line of a length L, with a given 
impedance that is a function of the input impedance of the signal 
processing means, placed between the latter and the antenna, the said 
length L being very large in relation to the wavelength of the signals 
processed. 
Furthermore, the said processing means ape constituted by a direct 
amplification microwave frequency receiver, followed by a square law 
detector, and preceded by a microwave frequency multi-way switch, suitable 
for modifying the structure of the said means cyclically.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention relates to a process and a device for measuring the 
temperature of a given material or object, using microwave radiation, as 
well as to an application of the measuring process for the purpose of 
determining the microwave frequency reflection coefficient of a given 
material or object. 
As mentioned earlier, the term "object" or "material" is to be interpreted 
broadly, to refer to the material of a body to be measured, a body which 
can, in particular, be material objects, substances or living tissues. 
Furthermore, it should also be pointed out that any material brought to a 
temperature T.sub.x, emits electromagnetic radiation the power of which, 
in the microwave range, is proportional to the temperature and to the 
pass-band of the measuring device. In particular, the power picked up by 
an antenna is given by the relation: 
EQU P=kT.sub.x .DELTA.f(1-.vertline.R.sub.o x .vertline..sup.2) 
wherein k is the Boltzmann's constant (1.38.10.sup.-23), T.sub.x the 
temperature of the material to be measured, .DELTA.f the pass-band, and 
.vertline.R.sub.o x .vertline..sup.2 the reflection coefficient of the 
antenna. 
When a direct amplification radiometer is produced, that is to say when an 
antenna is connected directly to the input of an amplifier-receiver A, and 
a square law detector D is connected to its output, without intercalating 
the circulator chain, the voltage V.sub.s at the output of the detector is 
given by the following relation: 
EQU V.sub.s =g.gamma.[T.sub.x (1-.vertline.R.sub.o x .vertline..sup.2)+T.sub.e 
.vertline.R.sub.o x .vertline..sup.2 +T.sub.a ] 
wherein g corresponds to the gain of the amplifier, .tau. corresponds to 
the conversion gain of the detector, T.sub.e corresponds to the input 
noise of the amplifier and T.sub.a corresponds to the noise of the 
amplifier, characterizing its noise factor. 
However, this relation V.sub.s has to be corrected for the radiometric 
measurement error caused by a possible correlation between the input noise 
T.sub.e and the output noise T.sub.a of the amplifier when the load at the 
input of the radiometer is not matched. 
The noise T.sub.e is linked to the physical temperature of the amplifier 
and the noise T.sub.a is linked to the image of the degradation brought by 
the amplifier to the input noise. There is said to be correlation between 
T.sub.e and T.sub.a. 
Thus, when a circulator is not used, the basic contribution, at a frequency 
f, of the correlation noise induced by the amplifier, is given by the 
following relation: 
##EQU1## 
wherein: k is the Boltzmann's constant, 
.PHI.is the phase difference between the input noise and the output noise 
of the receiver equal to 2.pi..L.sub.o.f/c., 
C is the speed of light, 
L.sub.o corresponds to the intrinsic length of the amplifier. 
The total contribution of the correlation noise is thus obtained by 
integrating the preceding equation in the passband of the amplifier, which 
gives: 
##EQU2## 
f.sub.2 and f.sub.1 being respectively the high and low cut-off 
frequencies of the amplifier, 
.DELTA.f being the pass-band of the amplifier, f.sub.2 -f.sub.1, 
L being the length of the line between the input of the amplifier and the 
antenna. 
As a function of the above, one of the characteristics of the present 
invention resides in the fact that there is intercalated between the 
antenna and the said signal processing means, in particular its amplifier, 
a line of given impedance, which is a function of the input impedance of 
the amplifier, having a length L which is very large in relation to the 
wavelength of the signals such that the correlation noise factor of the 
said means is negligible. 
In the preceding equation, S.sub.cor, the following quantity will thus be 
obtained: 
##EQU3## 
which tends towards zero, when .alpha. tends towards infinity. This 
condition is fulfilled when L is very large in relation to the wavelength 
of the signals processed, namely C/f.sub.2 -f.sub.1. 
The graph in FIG. 2 illustrates the influence of the correlation and, more 
precisely, the ratio V.sub.cor /V.sub.s as a function of the length L in 
centimeters between the antenna and the amplifier input. 
The ratio can be seen to tend towards zero when L is very large in relation 
to the wavelength. For example, in the case of an amplifier operating in a 
pass-band of 2 to 4 GHz, we begin to obtain results as from a length L in 
the order of 10 cm. For the sake of security, use will be made of a length 
of 40 cm, for example. 
Furthermore, the impedance of this line L has to correspond substantially 
to the input impedance of the amplifier to avoid a mismatch. In the 
microwave frequency, the amplifiers used generally have an input impedance 
in the order of 50 ohms, which is why a line section L will be chosen to 
conduct energy from the antenna to the amplifier with a characteristic 
impedance Zc of 50 ohms. 
This being the case, it can then be considered that the radiometric signal 
is given by the relation indicated previously, namely: 
EQU V.sub.s =g.gamma.[T.sub.x (1-.vertline.R.sub.o x .vertline..sup.2)+T.sub.e 
.vertline.R.sub.o x .vertline..sup.2 +T.sub.a ] 
Thus, according to another feature of the measuring process according to 
the present invention, the signals received by the antenna are routed, via 
the said length of line L, to the input of the processing means such that 
the said voltage V.sub.s is obtained at the output, and all the parameters 
are then calculated periodically by cyclically modifying the structure of 
the said means. 
FIG. 1 diagrammatically illustrates, by way of example, a device for 
measuring the temperature T.sub.x of given a material or object permitting 
implementation of the process according to the invention. 
This diagram shows an antenna 1 for receiving signals emitted by the object 
2, the temperature T.sub.x of which it is wished to determine, connected 
to processing means 3 via an intercalary line 4 having a length L such as 
defined above. 
More precisely, the processing means 3 are constituted by a direct 
amplification microwave frequency receiver 15, followed by a square law 
detector 6 and preceded by a microwave frequency multi-channel switch 8. 
As we have already seen, the microwave frequency receiver 15 can also be 
constituted by an amplifier A, with a microwave frequency pass-band, a 
large gain g, and a low noise factor. 
Furthermore, given the absence of the traditional amplifier, this amplifier 
can be produced using integrated monolithic technology. 
As to square law detector 8, with a conversion gain .tau., it will be 
formed advantageously by a Schottky detection diode, which is also easy to 
integrate. 
This being the case, according to the present invention, to determine the 
value of temperature T.sub.x, the equation to be solved will be: 
EQU V.sub.s =g.gamma.[T.sub.x (1-.vertline.R.sub.o x .vertline..sup.2)+T.sub.e 
.vertline.T.sub.o x .vertline..sup.2 +R.sub.a ] 
wherein the parameters to be determined are g.tau., T.sub.e, T.sub.a, 
.vertline.R.sub.o x .vertline..sup.2, and T.sub.x. 
We thus have an equation with five unknowns, and, with the process 
according to the invention, the structure of the signal processing means 
will thus be modified cyclically as many times as necessary to obtain as 
many equations as there are unknowns. 
However, in order to facilitate this solution, according to the process of 
the invention, the said processing means are further subjected selectively 
and cyclically to the influence of a high impedance (Z&gt;&gt;Zc) noise source 
.DELTA.T.sub.B to avoid disturbing the measuring line, which changes the 
relationship giving the output voltage V.sub.s as follows: 
EQU V.sub.s =g.gamma..vertline.T.sub.x (1-.vertline.R.sub.o x 
.vertline..sup.2)+T.sub.e .vertline.R.sub.o x .vertline..sup.2 +T.sub.a 
+.DELTA.T.sub.B (1+.vertline.R.sub.o x .vertline..sup.2)] 
The device according to the present invention, and, more precisely, the 
said processing means 3, further comprise, as shown in FIG. 1, a noise 
source 9, suitable for reinjecting complementary noise .DELTA.T.sub.B, 
placed at the input to the said means, and, more precisely, at the input 
to amplifier 5. 
This noise source is an advantageous element which will make it easier to 
solve the system of equations. 
In this connection, the said switch 8 will have, advantageously, at least 
four microwave frequency channels which, as illustrated in FIG. 1, are 
connected to: 
a load 10, of known characteristics, having a temperature T.sub.1, 
a line of a length L, similar to intermediate line 4, this line being 
short-circuited, 
the said intermediate line 4, of a length L, connected to antenna 1, 
another load 12, of known characteristics, having a temperature T.sub.2, 
the said noise source 9.DELTA.T.sub.B. 
The four different channels of the switch are identified on the figure by a 
serial number, 1, 2, 3 and 4, surrounded by a circle; circled round serial 
number 5 corresponds to a control 5, in particular a logic control, 
activating or otherwise the said source .DELTA.T.sub.B. 
Furthermore, in order to command cyclically the said microwave frequency 
switch 8 and the said control 5, the device according to the present 
invention comprises a computing and synchronising unit 13, in order to 
permit, in cooperation with the said switch 8 and the said control 5, the 
mathematical real-time solution of a system of equations defined for each 
condition of the switch, with a view to determining at least the 
temperature T.sub.x of the body to be measured. 
More precisely, according to the measuring process of the present 
invention, the input of the said processing means, or more precisely the 
input of amplifier receiver 15, is connected cyclically to: 
the load 10 having a temperature T.sub.1, using the switch on channel 1, 
the said load 10 having a temperature T.sub.1 and the noise source 9, 
.DELTA.T.sub.B, using the switch on channel 1 and supplying the said 
source .DELTA.T.sub.B via control 5, 
ground via line 11, by placing the switch on channel 2, 
antenna 1 via line 4 L, by placing the switch on position 3, 
the said antenna 1 via the said line 4 and the said complementary noise 
source 9, by placing the switch on channel 3 and supplying the said source 
.DELTA.T.sub.B via control 5, 
load 12 having a temperature T.sub.2 by placing the switch on channel 4. 
Thus, at each cycle, we obtain the following system of equations: 
EQU V.sub.1 =.gamma.g[T.sub.1 +T.sub.a ] 
EQU V.sub.15 =.gamma.g[T.sub.1 +T.sub.a +.DELTA.T.sub.B ] 
EQU V.sub.2 =.gamma.g[T.sub.e +T.sub.a ] 
EQU V.sub.3 =.gamma.g[(1-.vertline.R.sub.o x .vertline..sup.2)T.sub.x 
+.vertline.R.sub.o x .vertline..sup.2 T.sub.e +T.sub.a ] 
EQU V.sub.35 =.gamma.g[(1-.vertline.R.sub.o x .vertline..sup.2)T.sub.x 
+.vertline.R.sub.o x .vertline..sup.2 T.sub.e +T.sub.a 
+(1+.vertline.R.sub.o x .vertline..sup.2 .DELTA.T.sub.B ] 
EQU V.sub.4 =.gamma.g[T.sub.2 +T.sub.a ] 
Such a system, with six equations and six unkowns, can be processed using 
conventional computing means, such as computing unit 13 organized around a 
microprocessor, an analog input and output interface board (analog 
digital/digital analog converter), a logic input and output board (PIA) 
and display means. 
Unit 13 will thus enable the six parameters to be determined and, in 
particular, the display of the temperature values T.sub.x and of the 
antenna reflection coefficient .vertline.R.sub.o .vertline..sup.2. 
Furthermore, if required, the parameters specific to the amplifier, 
.tau.g, T.sub.e and T.sub.a, .DELTA.T.sub.B, can be displayed. 
To conclude, the different parameters will be obtained from the following 
equations: 
##EQU4## 
This computing and synchronising unit is managed by a loop program 
comprising: 
initialization of the logic board, 
operation of the four channels of the microwave frequency switch and 
acquisition of the radiometric signals averaged over "n" samples, for 
example n=100, 
control 5 activating or not activating the said noise source 
.DELTA.T.sub.B, 
computing parameters T.sub.x, .vertline.R.sub.o x .vertline..sup.2, 
T.sub.e, T.sub.a, .tau.g, .DELTA.T.sub.B, 
averaging the parameters over "n" values, 
displaying the results, 
return to initialization. 
As to the structure of loads 10 and 12, use will be made advantageously of 
the loads the impedance of which is matched to that of the amplifier input 
and thus, in the present case, loads having an impedance of 50 ohms which 
will each be placed at a pre-established, known temperature, T.sub.1 
and/or T.sub.2. 
As to microwave frequency switch 8, use will made advantageously of an 
assembly of four-channel MES FET elements. 
By way of example, FIG. 3 shows such an arrangement of MES FET elements to 
form a high insulation two-channel microwave frequency switch. 
We thus have four MES FET elements 14 disposed in series, the gates of 
which are controlled two by two respectively at G.sub.1 and G.sub.2 from 
computing and synchronising unit 13. 
Between points ES.sub.1 and ES.sub.2, we then have the two desired 
channels, E being the common point of the switch. 
Such a technique is within the reach of one skilled in the art in question 
and will be extended to the production of a four channel switch. 
Finally, FIGS. 4 and 5 represent two forms of embodiment that can be 
contemplated for noise source 9. 
FIG. 4 shows such a complementary noise source 9 formed by a MES FET 
element 19 of which the Schottky contact is used reverse biased until 
avalanche conditions are obtained. The avalanche noise thus obtained is 
controlled by a current generator 18. 
On the other hand, FIG. 5 shows the use of an avalanche diode 16 placed in 
series with a resistor R.sub..rho. having a large ohmic value in relation 
to the input impedance of the said means, arranged at the input of the 
latter. 
This circuitry is also within the reach of a man of the art. However, the 
essential criterion to be kept in mind is to produce a source of noise 
with high impedance in relation to that of the amplifier input, to avoid 
mismatching the circuit. 
As to the antenna, use will be made of any device suitable for picking up 
microwave frequency radiation, such as any measuring cell, applicator or 
dipole. 
To determine the temperature values T.sub.1 or T.sub.2 of loads 10 or 12, 
use can be made of various methods, such as those illustrated in FIGS. 6 
and 7. 
In FIG. 6, AsGa planar resistors, constituting loads 10, 12, having a known 
temperature coefficient, are introduced into a Wheatstone bridge 23, which 
is, for example, supplied by a d.c. or a.c. generator 22 and outputs at 25 
a signal proportional to temperature T.sub.1 or T.sub.2, via an 
inductance-capacitance polarizing Tee, 20, 21. 
In FIG. 7, a resistive film 24 of nickel-chromium (NiCr) or tantalum 
nitride (NiTa), having a known temperature coefficient, is deposited on 
the AsGa planar resistors constituting loads 10, 12, previously insulated 
by a polymide element 26. These NiCr or NiTa resistors 24 are introduced 
into a measuring bridge, such as a Wheatstone bridge 23, for example, as 
described earlier. 
This being the case, the reasoning which has just been set out starts out 
from the principle that the microwave frequency switch 8 is loss-free. 
However, such a switch inevitably has a certain resistance, characterized 
by "a", which is an image of the transmission of one of the channels of 
the switch. 
Thus, a part of the thermal noise power will be attenuated by the switch 
and we can consider that the switch used is equivalent to an attenuator 
brought to a temperature T.sub.com. Thus, the general relation for output 
voltage V.sub.s is written as follows: 
EQU V.sub.s =g.gamma.{[1-a(1-.vertline.R.sub.o x 
.vertline..sup.2)-.vertline.R.sub.o x .vertline..sup.2 a.sup.2 ]T.sub.com 
+ a.sup.2 .vertline.R.sub.o x .vertline..sup.2 T.sub.e + 
a(1-.vertline.R.sub.o x .vertline..sup.2)T.sub.x +T.sub.a +.DELTA.T.sub.B 
(1+a.sup.2 .vertline.R.sub.o x .vertline..sup.2)} 
It should be noted that T.sub.com can assume the value T.sub.1, just as 
long as switch 8 is placed in the immediate vicinity thanks to monolithic 
integration of the device. 
As to the other load 12, brought to temperature T.sub.2, it will be 
thermally insulated from the rest thanks to the heat sinks, which 
represent a technique well known to a man of the art. 
A new unknown is then introduced: "a"; it is thus appropriate to determine 
a new complementary equation. This is possible with the switch as 
described above, and following additional step will be effected, for 
example: 
switch placed on channel 2 and supplying the said source T.sub.B via 
control 5. 
The following relation will thus be obtained: 
EQU V.sub.25 =g.gamma.[(1-a.sup.2)T.sub.com +a.sup.2 T.sub.e +T.sub.a 
+(1+a.sup.2).DELTA.T.sub.B ] 
and by making: T.sub.com =T.sub.1, the relation becomes: 
EQU V.sub.25 =g.gamma.[(1-a.sup.2)T.sub.1 +a.sup.2 T.sub.e +T.sub.a 
+(1+a.sup.2).DELTA.T.sub.B ] 
Nonetheless, a calculation identical with the preceding one will enable us 
to arrive at relations determining the different variables if we do not 
neglect the losses at switch level, and if we fix T.sub.com =T.sub.1 
namely: 
EQU V.sub.1 =g.gamma.[(1-a)T.sub.1 +aT.sub.1 +T.sub.a ] 
EQU V.sub.15 =g.gamma.[(1-a)T.sub.1 +aT.sub.1 +T.sub.a +.DELTA.T.sub.B ] 
EQU V.sub.2 =g.gamma.[(1-a.sup.2)T.sub.1 +a.sup.2 T.sub.e +T.sub.a ] 
EQU V.sub.3 =g.gamma.{[1-a(1-.vertline.R.sub.o .vertline..sup.2)-a.sup.2 
.vertline.R.sub.o .vertline..sup.2 ]T.sub.1 + a.sup.2 .vertline.R.sub.o 
.vertline..sup.2 ]T.sub.e + (1-.vertline.R.sub.o .vertline..sup.2)aT.sub.x 
+T.sub.a } 
EQU V.sub.35 =g.gamma.{[1-a(1-.vertline.R.sub.o .vertline..sup.2)-a.sup.2 
.vertline.R.sub.o .vertline..sup.2 ]T.sub.1 + a.sup.2 .vertline.R.sub.o 
.vertline..sup.2 T.sub.e + (1-.vertline.R.sub.o .vertline..sup.2)aT.sub.x 
+T.sub.a +.DELTA.T.sub.B (1+a.sup.2 .vertline.R.sub.o .vertline..sup.2)} 
EQU V.sub.4 =g.gamma.[(1-a)T.sub.1 +aT.sub.2 +Ta] 
It seems obvious, of course, that one particular application would be that 
of temperature measurement in an industrial or medical environment or a 
home robotics application. 
Apart from this field of application, another application of the measuring 
process according to the present invention should be emphasized: that of 
determining the microwave frequency reflection coefficient of a given 
material or object. 
Through this expedient, it will then be possible to determine the 
dielectric or physical properties of a material, for example moisture 
content, structure, etc. 
Other embodiments of the present invention, within the grasp of a man of 
the art, could, of course, be contemplated without thereby departing from 
the scope of the present invention.