Waveguide structure for estimating the electromagnetic characteristics of a dielectric or magnetic material

A device is provided for estimating the electromagnetic characteristics of a dielectric or magnetic material by means of a hyperfrequency electromagnetic wave which device comprises a coaxial line formed of a tubular body (2,3) and a core (1), which are rigid and coaxial with, at one end, a housing such that the external and internal peripheries of an annular sample piece (17) are respectively in tight friction contact with the inner surface of said tubular body and with the external surface of said core, said tubular body being formed of two parts which can be assembled together.

The present invention relates to a device for estimating the 
electromagnetic characteristics of a dielectric or magnetic material. 
In order to characterize the electromagnetic behavior of materials, it is 
necessary to know their relative permittivity and permeability. 
Furthermore, each of these electromagnetic characteristics (relative 
permittivity and permeability) comprises a real term and an imaginary 
term. 
Methods are already known, for example from the article "A complete 
analysis of the reflection and transmission methods for measuring the 
complex permeability and permittivity of materials at microwaves" 
published in ALTA FREQUENZA, vol 36, n.degree. 8, August 1967, pages 
757-764, MIlan, IT, G FRANCESCHETTI or else from the article "Time-domain 
measurements for determination of dielectric properties of agricultural 
materials" published in IEEE TRANSACTION ON INSTRUMENTATION AND 
MEASUREMENT, vol IM-28, n.degree. 2 June 1979, pages 109-112, IEEE, New 
York, U.S., B.P KWOW et al, either for directly measuring the reflection 
and/or transmission coefficients of dielectric samples, or for measuring 
the real and imaginary terms of the relative permittivity and permeability 
of a material. 
These methods are based on the fact that, if a dielectric or magnetic 
material receives an incident electromagnetic wave, it transmits and 
reflects this wave while attenuating it and phase-shifting it, the 
attenuation and the phase-shift undergone by the transmitted wave being 
functions of the permeability and the permittivity of said material and of 
the limit conditions imposed on the wall of the material. 
To put such methods into practice, U.S. Pat. No. 3,693,080 describes a 
device for estimating the electromagnetic characteristics of a dielectric 
or magnetic material, comprising : 
a rigid core made from an electrically conducting material, 
a rigid tubular body, also made from an electrically conducting material, 
coaxially surrounding said conducting core so that said core and said 
tubular body are able to form a rigid coaxial line; 
means for applying an incident hyperfrequency electromagnetic wave between 
said core and said body; 
a housing for an annular piece of uniform thickness formed from said 
material, this annular piece being such that, when it is disposed in said 
housing, its external and internal peripheries are respectively in tight 
friction contact with the inner surface of said tubular body and with the 
outer surface of said core; 
means for collecting the electromagnetic wave transmitted by said annular 
piece. 
Thus, in order to use this known device : 
an annular piece of uniform thickness is formed in said material; 
said annular piece is introduced in said coaxial line; 
an incident hyperfrequency electromagnetic wave is applied to said coaxial 
line; 
the amplitudes and/or the phases of the transmitted wave and of the 
reflected wave are examined by said tubular body. 
In this known device, in order to form a mathematical model of the 
measurement, a wave reflector element is provided in the coaxial line 
disposed downstream of said annular piece and the transmitted wave is 
collected from the side of said coaxial line to which the incident wave is 
applied. This wave reflector element is an electromagnetic element which 
forms a short circuit between the tubular body and the core of the coaxial 
line. 
In the device of U.S. Pat. No. 3,693,080, the housing for the annular piece 
of material to be studied is provided in an intermediate position in said 
coaxial line so that it is necessary to provide a removable cap in said 
rigid tubular body. In addition, it is also necessary for said annular 
piece to be fixed to a conducting core section which must be provided with 
means for fixing to the upstream and downstream portions of said core, so 
as to ensure the electric continuity thereof. 
Consequently, not only the sample of the material to be studied is complex 
to form, since besides said annular piece it must comprise a core section, 
but it must further have a removable cap in the rigid tubular body. The 
result is then a complex structure. Furthermore, in particular because of 
the manufacturing tolerances of said cap of said sample, it is practically 
impossible for said annular piece to be able to occupy a fixed reference 
position and for the relative positions of the tubular body and of the 
core to be invariable, during a series of measurement. This lack of 
positioning precision leads to measurement errors whose importance makes 
the measurements themselves illusory. 
An object of the present invention is to overcome these drawbacks. 
For this, in accordance with the present invention, the device for 
estimating the electromagnetic characteristics of a material, comprising : 
a rigid core made from an electrically conducting material; 
a rigid tubular body, also made from an electrically conducting material, 
surrounding said conducting core coaxially, so that said core and said 
tubular body are able to form a rigid coaxial line; 
means for applying an incident hyperfrequency electromagnetic wave between 
said core and said body; 
a housing for an annular piece of uniform thickness formed from said 
material, this annular piece being such that, when it is disposed in said 
housing, its external and internal peripheries are respectively in tight 
friction contact with the inner surface of said tubular body and with the 
outer surface of said core, and 
means for collecting the electromagnetic wave transmitted by said annular 
piece, 
is remarkable : 
in that said housing is provided at one end of said coaxial line; 
and in that said tubular body is made from two parts which can be assembled 
together. 
Thus, said sample may be formed only of said annular piece, which may be 
readily introduced through the open end of said line and there occupy a 
fixed position. 
Furthermore, because of the possibility of separating the tubular body into 
two parts, it is easy to remove said sample, for example so as to replace 
it by another. Furthermore, with this design, the respective positions of 
the core and of the tubular body remain invariable. 
It will be noted that the article in the review IEEE TRANSACTIONS ON 
INSTRUMENTATION AND MEASUREMENT mentioned above, already provides for 
disposing the sample at the end of the line. But then it is necessary to 
provide a removable portion of the central core, which raises electric 
coupling difficulties and does not allow ready removal of the sample. 
As will be explained hereafter, the reference position of the sample in the 
device of the present invention may be defined by cooperation of the wave 
reflector element with said coaxial line. Such an element may be a simple 
short circuit, but it may also be provided for introducing a calibrated or 
known load between said tubular body and said core. Such a reflector 
element may be disposed directly downstream of said annular piece, i.e. in 
contact therewith, or else at some distance from said annular piece, 
downstream thereof. 
Generally, the purpose of such a reflector element is to place the coaxial 
line in a given and reproducible electromagnetic condition. 
Preferably, a plurality of such different reflector elements are provided 
so as to be able to communicate to said coaxial line a plurality of given 
electromagnetic conditions which makes possible the calculation of 
corrector coefficients. 
Use of the device of the present invention may make it possible to 
determine, by comparison, if a material to be examined has electromagnetic 
characteristics different from those of a reference material. 
However, it is preferably used for measuring the relative permittivity and 
permeability of a dielectric or magnetic material. 
It will be noted that such a measurement requires four unknowns to be 
determined, namely the real and imaginary terms of the permittivity and of 
the permeability. It is therefore necessary to make at least two phase and 
modulus measurements. Such measurements may be made in different ways. For 
example, several pieces of different thicknesses of the same material may 
be examined, the examination being carried out by only studying the 
reflection coefficients (in phase and in amplitude). 
However, only a single annular piece of the material to be examined may be 
used and the reflection and transmission coefficients measured at the same 
time. In this case as many relationships are established as it is 
necessary for calculating the real and imaginary parts of the permittivity 
and of the permeability of the material forming the annular piece. 
It is clear form the foregoing that said means for collecting the 
transmitted wave may be disposed on the side of said coaxial line opposite 
the means for applying the incident wave. They may also be disposed on the 
same side of said coaxial line as the means for applying the incident 
wave. 
In a way known per se, in order to overcome the measurement errors which 
might be due to variations in the relative arrangements of the core and of 
the tubular body, the latter are securely fixed to each other by means of 
internal annular spacers. For the ease of manufacture, one of said spacers 
is preferably located in the vicinity of the connection between said 
assemblable parts of said tubular body. For the same purpose of ease of 
manufacture, at least one other spacer of the coaxial line is disposed in 
the vicinity of one end thereof. Preferably, each spacer is housed, at its 
internal and external peripheries, in facing grooves, formed respectively 
in said core and in said tubular body. 
Each of these grooves is then defined between a face of said tubular body 
and an end piece able to be assembled with said body and said core is made 
from several interfitting parts. 
Thus it can be seen that, with the present invention, a rigid measuring 
device is obtained in which the annular dielectric or magnetic material 
piece is positioned accurately and is subjected to balanced pressures on 
both its faces.

The device of the invention, illustrated schematically in FIG. 1, comprises 
a central core 1 and a tubular body made from two assemblable parts 2, 3. 
The central core 1 and tubular body 2,3 are made from electrically 
conducting materials and have circular sections. The central core 1 and 
the tubular body portion 2 are disposed coaxially with respect to each 
other and are held mechanically together by means of annular spacers 4 
orthogonal to the longitudinal axis X--X common to said central core 1 and 
to said tubular body portion 2. 
At its two ends, the tubular body portion 2 ends in flat annular end faces, 
respectively 5 and 6, orthogonal to the axis X--X. Similarly, the central 
core 1 is defined by two flat end faces orthogonal to axis X--X, 
referenced respectively 7 and 8. The flat end face 7 of core 1 is coplanar 
with the end face 5 of the tubular body portion 2, whereas, on the other 
hand, the flat end face 8 of core 1 projects with respect to the 
corresponding face 6 of the tubular body portion 2. 
The tubular body portion 3 is defined by two annular flat end faces, 
respectively 9 and 10, orthogonal to its axis Y--Y. Vent holes 11 are 
provided in its sidewalls. 
At their ends 5, 6, 9 and 10, the tubular body portions 2 and 3 are 
provided with means, respectively 12, 13, 14 and 15, shown schematically 
in the form of flanges for fixing them to other elements. 
Portions 2 and 3 of the tubular body may be assembled rigidly together, 
using means 13 and 14, for example, associated with screws and nuts 16 
(see figure 2). When portions 2 and 3 are thus firmly fixed together, the 
axes X--X and Y--Y are merged together and the end face 9 of the body 
portion 3 is applied against the end face 6 of the body portion 2. In 
addition, in this position of said body portions 2 and 3, the end face 8 
of core 1 is in the plane of the end face 10 of body portion 3. 
Thus it can be seen that the device of the invention formed of core 1 and 
body 2, 3 forms a rigid coaxial line which may be connected, on the end 
face 5 and 7 side, to a hyperfrequency generator. 
In order to be able to examine the electromagnetic properties of a 
dielectric or magnetic material using the above described device 1-3, at 
least one piece 17, of annular shape, is machined from said material. The 
diameter of the central hole 18 in said annular piece 17 is such that it 
can be fitted on the central core 1 with a tight friction fit. Similarly, 
the diameter of the external periphery 19 of said annular piece 17 is such 
that it can be fitted in the body portion 3 also with a tight fit. Thus, 
when the body portions 2 and 3 are assembled together and when a piece 17 
is introduced into the body portion 3 through the end face 10, this piece 
is in excellent electric contact both with core 1 and with body 2, 3. 
Furthermore, piece 17 is defined by flat end faces 20 and 21 parallel to 
each other and orthogonal to the axis Z--Z of said piece. 
In FIG. 2, a piece 17 has been shown during positioning in the body portion 
3. This piece 17 is pushed by means of a tool 22 having a flat face 23 
applied to face 21 of said piece. Tool 22 comprises at least one vent hole 
24 for expelling the air imprisoned in hole 18 of piece 17, between the 
end face 8 of core 1 and said tool. It will be readily understood that air 
imprisoned inside body portion 3, between piece 17 and core 1, may escape 
through the vent holes 11 during introduction of said piece. 
Tool 22 comprises means 25 able to cooperate with the means 15 of body 
portion 3 of fixing to the latter. When the tool is assembled on body 
portion 3, it is certain that axis Z--Z of piece 17 merges with the axes 
X--X and Y--Y and that the flat annular face 21 is coplanar with the end 
faces 8 and 10 of core 1 and of the body portion 3. Tool 22 is then 
removed if necessary, unless it serves as short circuit accessory in 
addition, as will be described hereafter. 
In FIG. 3, piece 17 has been shown in position in device 1-3 and the plane 
common to faces 8, 10 and 21 has been designated by P. 
It will be readily understood that, if an incident hyperfrequency wave is 
applied between end face 5 of body 2, 3 and the end face 7 of core 1, this 
wave will propagate through line 1-3 as far as piece 17. Consequently, 
this incident wave will pass through piece 17. When passing through piece 
17, the electromagnetic wave has a propagation speed which, in a way known 
per se, is a function of the relative permittivity and of the relative 
permeability of the material forming piece 17. Since the thickness e of 
piece 17 is known, it is then sufficient to know the propagation time of 
the wave through said piece in order to obtain a relationship relating the 
relative permeability and permittivity of the material of piece 17. Now, 
this propagation time of the waves in the piece correpsonds to the 
phase-shift of the wave transmitted by said annular piece 17 with respect 
to the incident wave. In addition, the attenuation undergone by the 
electromagnetic wave on passing through piece 17 is also a function of the 
relative permeability and permittivity of the material of said piece. 
Thus, if several phase-shift and attenuation measurements are made with 
pieces made from the same material but of different thicknesses e, and/ or 
with different conditions for line 1-3, equations are obtained by means of 
which the relative permittivity and permeability of said material may be 
calculated. 
In FIG. 3, in addition to the coaxial line 1-3, different accessories have 
been shown for connecting said line to an electromagnetic wave generator 
and a receiver and imposing on said coaxial line a known and reproducible 
electromagnetic condition. 
It is a question essentially of : 
coaxial connectors 27 for connecting the ends of said line 1-3 both to an 
incident electromagnetic wave generator and to a receiver receiving the 
electromagnetic wave transmitted by said piece 17. Such coaxial connectors 
27 are preferably of the type described in the French patent application 
n.degree. 88- 11010 filed on Aug. 3, 1987 by the Applicant and entitled 
"Dispositif pour 1e raccord de deux structurtes pour hyperfrequences, 
coaxiales et de diametres differents"; 
short-circuits 28 for providing an electromagnetic short circuit between 
core 1 and the tubular body 2-3 at the level of plane P. Such short 
circuits may be identical to accessory 22. They are essentially formed by 
a plate of electrically conducting material having a flat face 29 (or 23) 
able to be applied simultaneously to the end faces 10 and 8 of the body 
2,3 of core 1, 
short circuits 30 providing an electromagnetic short circuit between core 1 
and the tubular body 2-3 in a plane P', offset by a fixed distance d 
downstream of plane P. They are formed essentially by a piece of 
electrically conducting material having a plate 31, extended in the 
direction of line 1-3, by a coaxial casing 32 and core 33. Plate 31 is 
orthogonal to said casing and to said core and the latter have faces 34 
and 35 which may be applied respectively agianst the face 10 of tubular 
body 2-3 and against face 8 of core 1, 
open circuits 36 formed of a casing 37 extending the tubular body 2,3 and 
an adjustable dielectric, magnetic or conducting core 38 sliding in said 
casing, 
calibrated loads, of known type, and for example of the order of 50 ohms 
able to be connected to line 1-3 thorough a coaxial connector 27. 
As shown schematically in FIG. 4, this instrumentation 1-3, 27, 28, 30, 36 
and 39 is completed by a network analysis and computation system, such for 
example as the one commercialized by HEWLETT KARD under the reference 
HP 8510, associated with a computer 41 (for example of type HP 9836). This 
unit receives data relative to the measurement system (FIG. 3) contained 
in memory 42 and a measurement programme and the mathematical processing 
43. 
It will be readily understood that the system shown in FIG. 4 may operate 
in different ways. 
First of all it is possible by fixing a coaxial connector to each end of 
line 1-3 to operate with direct transmission, from one end of the line to 
the other. In this case, the incident wave is applied to end 5, 7 of line 
1-3 and the transmitted wave is collected at the end 8,10 of this line. On 
the other hand, end 8, 10 of line 1-3 may be connected to one of the 
devices 28, 30, 36 or 39 so that in this case the electromagnetic wave is 
reflected and is collected at end 5, 7 at which the incident wave is 
applied. Similarly, the electromagnetic properties of two different 
materials may be compared directly by measuring the transmission 
coefficients or by measuring the reflection without measuring the relative 
permittivity and permeability, or else the system may be used for 
determining solely the attenuation produced by a given material. 
However, in an advantageous embodiment of the system of FIG. 4 : 
the off-load coaxial line 1-3, i.e. not having any annular piece 17, is 
first of all calibrated, this line working either under direct 
transmission (a coaxial connector 27 at each end), or by reflection (a 
coaxial connector 26 connected to end 5, 7 and a device 28, 30, 36 or 27, 
39 connected to end 8, 10). This calibration operation makes it possible 
for system 40 to calculate the error coefficients required for precise 
measurement of the reflection and transmission coefficients of sample 17. 
then, an annular piece 17 made from a dielectric or magnetic material is 
positioned and the coaxial line 1-3 is given a configuration identical to 
that of the preceding calibration. In addition, the same incident 
electromagnetic wave is then applied to the coaxial line 1-3 as during 
calibration. The system 40 then determines the reflection and/or 
transmission coefficients which result therefrom. 
The system 40 determines the reflection and/or transmission coefficients 
proper to the annular piece 17 being examined, by using the error 
coefficients, measured during the calibration phase and calculated by 
system 40. 
The above succession of operations is repeated several times, with 
different line configurations, so as to obtain sufficient measurements for 
calculating and confirming the real and imaginary values of the 
permeability and permittivity. 
In FIG. 5, a practical embodiment has been shown of the rigid coaxial line 
1-3. It can be seen that the spacers 4 are disposed at the ends of the 
body portion 2 and are engaged, by their inner and outer peripheries, in 
grooves 44 and 45 formed respectively in core 1 and in the tubular body 2, 
3. For positioning these spacers, end pieces 46, 47 are provided fixed to 
said tubular body portion 2 and on which are fixed respectively a coaxial 
connector 27 and said tubular body portion 3. In addition, core 1 is made 
from several portions 1.1, 1.2 and 1.3 which can be assembled together and 
are provided with shoulders at their respective ends. The assembly of 
these different parts of core 1 may be obtained by any known means but, 
preferably, it is provided by means of the seal described in French patent 
n.degree. 88 11011 filed on 3 August 1987 by the Applicant and entitled 
"Systeme de liaison a joint pour elements travaillant en hyperfrequence". 
During a measuring procedure, said portions 1.1, 1.2 and 1.3 remain fixed 
together. 
The annular piece 17 is housed in the tubular body portion 3 and is held in 
position by device 27, 28, 30 or 36 (bearing the common reference 50 in 
FIG. 5) which is fixed to the tubular body portion 3. 
Guide and centering fingers 51 are provided between the different elements 
for facilitating assembly. Preferably, the different assemblies are formed 
by screwing. 
It will be noted that it is important for core 1 and the tubular body 2, 3 
to be fixed together by spacers 4. The latter make it possible to fix the 
relative positions of these elements and so to ensure repetitive 
positioning of said annular pieces 17 and further allow the tubular body 
2, 3 to be given a large diameter. Thus, it is possible to use pieces 17 
of large diameters, which is important for studying materials whose 
homogeneity is not microscopic. Of course, preliminary adjustments are 
made so that the spacers introduce a minimum of disturbance in the 
propagation of the electromagnetic waves. During these adjustments, 
material may be removed from or added to said spacers so that the parasite 
waves reflected by them have the minimum amplitude. 
With the structure of the coaxial line 2, 3 of the present invention, it 
can be seen : 
that the position of an annular piece is not altered when devices 27, 28, 
30, 36, 50 are exchanged with each other; 
that an annular piece 17 may be removed and replaced by another without 
changing the relative positions of core 1 and body 2, 3. In fact, after 
removal of part 3 for removing a piece 17 (portion 1.3 of core 1 remaining 
fixed to portion 1.2), this part 3 again takes up a position identical to 
the one it occupied before as soon as it is fixed again to part 2, and 
that all the successive pieces 17 occupy the same position inside the end 
of part 3 of the tubular body.