WO2008032293A2

WO2008032293A2 - Measuring the transmission of a waveguide partially filled with the laminated material using a network analyzer

Info

Publication number: WO2008032293A2
Application number: PCT/IB2007/053722
Authority: WO
Inventors: Lorenzo Carbonini; Enrico Gallina; Enrico Pagana; Luigi Volpi
Original assignee: Selex Communications S.P.A.
Priority date: 2006-09-14
Filing date: 2007-09-14
Publication date: 2008-03-20
Also published as: ITTO20060656A1; WO2008032293A3

Abstract

A system and a method for evaluating the relative dielectric constant of a laminated dielectric material are described, wherein: a waveguide device (26) is arranged to hold at least one slab (12) of dielectric material under test in a longitudinal arrangement, and a vector network analyzer (A) is coupled to the waveguide device (26) to provide measurement data of the transmission coefficient (S21 parameter) of the device (26), and wherein the relative dielectric constant value of the dielectric material under test is determined by comparison of phase difference data indicative of the difference between the phase of the transmission coefficient in a first propagation condition in an empty guide, that is without the slab (12), and in a second propagation condition with the guide loaded, that is with the slab (12), with corresponding phase difference data calculated in accordance with a predetermined reference model of propagation in the device.

Description

Test apparatus, a system and a method for the broad-band evaluation of the relative dielectric constant of laminated materials for microwaves

The present invention relates in general to techniques for evaluating the dielectric constants of materials for microwaves.

More specifically, the invention relates to a system for the broad-band evaluation of the real part of the permittivity or relative dielectric constant of a laminated dielectric material according to the preamble to Claim 1, an apparatus for implementing such a system, and a method according to the preamble to Claim 27.

In the most common applications, laminated materials for microwave circuits are subject to electric fields that are oriented in a direction (conventionally identified as the z axis) which is perpendicular to the surface of the plate. Producers of materials for microwaves, are consequently required to control the perpendicular dielectric constant ε_z very precisely, particularly by the technicians who are involved in the design and testing of microwave circuits. Techniques and standards have accordingly been developed for evaluating this parameter as accurately as possible.

JP 2002 214161 describes a method and a device for measuring the complex dielectric constant of a dielectric material with losses in the microwave and millimetre wave bands. A thick slab of material is arranged transversely on a rectangular, closed waveguide path and the dielectric constant is obtained by comparison of the reflection and transmission coefficient measured with an analytical model derived from the solution of Maxwell's equations for the configuration under test.

In practice, the method proposed in JP 2002 214161 enables solely the perpendicular dielectric constant ε_z of the material to be evaluated. Moreover, it produces results that are not sufficiently accurate to characterize a dielectric material of which samples are available solely in the form of thin slabs, which is particularly frequently the case in microwave electronics applications. There are, however, applications in which the effect of the anisotropy of a laminated material is very considerable and variations in the transverse dielectric constant parameters (ε_x, ε_y) affect the performance of circuits and antennae for which the electric field components parallel to the surface of the slab are not negligible; these variations may lead to considerable alterations in the performance of the devices.

To give an example, in a device constituted by an array of suspended micro-strip antennae operating at 13.3 GHz, in which the micro-strips are supported by a laminated dielectric material without an earth plane, with the use of a material with a nominal ε_z of about 6.1, it was found, theoretically and experimentally, that a difference in the transverse dielectric constant value (along the x or y axes) of between 7.5 and 8.2 would lead to a variation of about three degrees in the direction of the major lobe of the array as well as a variation in the directions of the side lobes.

It is therefore desirable to have available a technique for the accurate evaluation of the transverse dielectric constants of laminated dielectric materials in order to estimate more accurately the performance of circuit components made of these materials.

Techniques for characterizing the dielectric constants of dielectric materials based on the use of resonant circuits as required, for example, by standard IPC-TM-650, method 2.5.5.5, which is generally used by producers, are known in the literature.

According to these norms, it is necessary to provide resonant circuits that are specific to a particular dielectric material or apparatus with resonant cavities suitable for housing the dielectric material under test.

In both cases, the measurements are limited to a specific operative frequency.

In the second case, the dimensions of the resonant cavity may advantageously be modifiable but by mechanical operations to move the walls which take a long time and have to be performed by a skilled operator. US 6 472 885 describes a method and apparatus for characterizing the electrical properties of dielectric materials and in particular the complex permittivities of materials in solid, liquid or gaseous form. The apparatus includes a TEM or quasi-TEM transmission line (for example, a micro-strip line) including the dielectric material under test. The method is based on the measurement of reflection scattering parameters at each end of the line when the other end is connected to predetermined loads and the comparison of the values of the parameters measured with corresponding parameters calculated on the basis of an equivalent theoretical transmission-line model.

The object of the present invention is to provide an easily implementable and effective technique for evaluating the real part of the dielectric constants of laminated dielectric materials for microwaves.

According to the present invention, this object is achieved by means of a system having the characteristics defined in Claim 1 and of a method having the characteristics defined in Claim 27.

A further subject of the invention is an apparatus for implementing a system of this type having the characteristics defined in Claim 22.

Specific embodiments of the invention are defined in the dependent claims.

In summary, the present invention is based upon the principle of the correlation of phase measurements of scattering parameters on samples of material arranged in a waveguide, which measurements are obtained by a vector network analyzer, with a reference model represented by analytical formulae or simulations, in which the sample of material in slab form is arranged longitudinally in the waveguide so as to bring about alterations in the propagation phase of an electromagnetic field which are dependent on the value of the transverse dielectric constant.

Preferably, the waveguide is a single-mode, rectangular or circular waveguide and the slab of dielectric material is arranged in the plane in which the electric field lines of greatest intensity of the mode being propagated lie; the slab may be supported in a pair of opposed slots formed in the internal surfaces of the waveguide walls.

The technique discussed is intrinsically a broad-band technique, unlike conventional techniques that are known in the literature.

The measurements can be made easily and the rectangular test samples of dielectric material, which do not have metallic coatings on either face, are easy to prepare. Neither specific circuits nor specialized technical competence are required to carry out the tests.

The technique of the invention advantageously permits both rapid checking of the dielectric properties of a material during its manufacture and subsequent analysis and evaluation in a laboratory for the design of circuits, for example, microwave circuits, prior to the use of such materials in the applications envisaged.

This technique can also be extended to the characterization of the perpendicular dielectric constant of the material with a suitable orientation of the slab of material in the waveguide, again in a longitudinal arrangement.

Further characteristics and advantages of the invention will be described in greater detail in the following specific description which is given by way of non-limiting example with reference to the appended drawings, in which:

Figure 1 is a block diagram of the system of the invention,

Figure 2 is an overall, perspective view of the apparatus of the invention,

Figures 3a and 3b are a perspective view and a cross-section, respectively, of a waveguide of the apparatus of Figure 2, in a first operative configuration,

Figures 4a and 4b are a perspective view and a cross-section, respectively, of a waveguide of the apparatus of Figure 2, in a second operative configuration,

Figures 5a-5e are views of a waveguide with dielectric material in the operative configuration of Figure 3, taken from different angles and at successive stages of preparation,

Figures 6a and 6b are overall views of the apparatus of the invention in two stages of preparation,

Figure 7 shows a plan view and a side view of a plate of dielectric material from which the test slabs are obtained,

Figures 8a and 8b show the results of a first measurement test and the respective technique for obtaining the test slabs from a sample of material in order to evaluate the transverse component of the dielectric constant of a material,

Figures 9a and 9b show the results of a second measurement test and the respective technique for obtaining the test slabs from a sample of material in order to evaluate the transverse component of the dielectric constant of a material,

Figures 10a and 10b show the results of a third measurement test and the respective technique for obtaining the test slabs from a sample of material in order to evaluate the transverse component of the dielectric constant of a material,

Figures 11a and l ib show the results of a fourth measurement test and the respective technique for obtaining the test slabs from a sample of material in order to evaluate the transverse component of the dielectric constant of a material,

Figures 12a and 12b show the results of a fifth measurement test and the respective technique for obtaining the test slabs from a sample of material in order to evaluate the transverse component of the dielectric constant of a material,

Figures 13a, 13b and 13c show the results of a measurement test, the respective technique for obtaining the test slabs from a sample of material, and a graph of comparison with a reference model, in order to evaluate the perpendicular component of the dielectric constant of a material, and

Figures 14a and 14b are graphs which show the relative dielectric constant values measured by the apparatus of the invention for two different materials, without and with suitable filtering in the time domain, respectively.

Figure 1 is a block diagram of the system of the invention in which an apparatus, generally indicated 10, for characterizing the relative dielectric constant of a dielectric material in the form of a thin slab 12 is coupled to a vector network analyzer A which in turn is connected to a computer unit C which may comprise a reference model M.

The evaluation of the dielectric constant is based on the measurement of the phase delay along a waveguide portion loaded with a slab of material under test in a longitudinal arrangement, as will be specified in detail below.

The apparatus 10 enables the transverse components of the dielectric constant, that is the components parallel to the surface of the laminate, to be measured with a high degree of accuracy. It is also suitable for evaluating the longitudinal (or perpendicular) component of the dielectric constant, that is, the component perpendicular to the surface, although with a lower degree of accuracy because of the limited thickness of the slab through which the electromagnetic field passes and consequently of the smaller phase difference that has to be measured.

In this description, particularly in the examples of measurement results that are given below, slabs of different materials with a nominal thickness of 25 mils (0.635 mm) were used, and were characterized at a frequency of 13 GHz.

Naturally, the following remarks can be extended to the evaluation of relative dielectric constants of other materials, on the basis of slabs of different thicknesses and/or at different frequencies, without thereby departing from the scope of the present invention.

The apparatus of the invention includes a flat metal support structure 20 to which a central upright member 22 and a pair of lateral upright members 24a, 24b are firmly secured, the upright members extending upwards from the surface to support, respectively, a waveguide 26 and a respective pair of end adaptors 28a, 28b which are used to couple the guide to coaxial cables 30a, 30b for connection to a vector network analyzer A.

In the embodiment described, the waveguide is a WR75 rectangular waveguide which defines a guide section adapted to propagate solely the non-degenerate fundamental TEio mode at the operative frequency for the characterization of the material.

The adaptors 28a, 28b are connected, by means of respective coupling flanges 32a, 32b, to respective ports Pl, P2 of the waveguide 26 and, on the opposite side, have respective connectors 34a, 34b, for example, female SMA connectors, for connection to the respective coaxial cables 30a, 30b so as to provide a matched cable guide transition for the coupling of an electromagnetic stimulation signal generated in the analyzer and for the extraction of an output signal at the ports of the guide.

The electromagnetic field (in the fundamental TE₁₀ mode) is excited in the waveguide in the conventional manner, for example, by using the central conductor of the coaxial cable which is extended inside the bodies of the adaptors 28a, 28b so as to constitute a rectilinear excitation probe.

The vector network analyzer (VNA) A is arranged for measuring the S₂₁direct transmission scattering parameter and in particular for evaluating the phase of the S₂₁ parameter. An Agilent Technologies Agilent 8510C vector analyzer may, for example but not exclusively, be used as the network analyzer.

The phase measurement is critical since it can be disturbed by small variations in the setting of the apparatus, for example, a slight movement of a cable.

For this reason, a firm engagement of the adaptors 28a, 28b on the corresponding support uprights 24a, 24b is provided for, to prevent any movement during operations to remove and position the waveguide.

Moreover, the cables 30a, 30b are secured to the metal support surface 20 by corresponding clamping bands or fasteners 40a, 40b which prevent twisting or other undesired movements thereof.

The waveguide 26 is connected releasably to the adaptors 28a, 28b by respective coupling flanges 36a, 36b adapted to be connected rigidly to the adaptor flanges 32a, 32b by means of captive screws.

Two possible operative configurations of a rectangular waveguide arranged for receiving, in its internal cavity, a slab of dielectric material under test are described with reference to Figures 3a, 3b and 4a, 4b. In the first configuration, shown in Figures 3 a and 3b, the waveguide 26 has a pair of opposed slots 50 which are formed, for a portion of its longitudinal extent, in the internal surface of the cavity, along the larger walls of the guide. These slots are suitable for receiving opposite edges of a dielectric slab 12 inserted in the cavity of the guide, as shown in Figure 3b. The slots are advantageously formed in a median longitudinal section plane (vertical in the orientation of the drawing) in which the electric field lines of greatest intensity of the mode being propagated lie.

In the second configuration, shown in Figures 4a and 4b, the waveguide 26 has a pair of opposed slots 52 which are formed, for a portion of its longitudinal extent, in the internal surface of the cavity, along the smaller walls of the guide. These slots are suitable for receiving opposite edges of a dielectric slab 12 inserted in the cavity of the guide, as shown in Figure 4b. The slots are, for example, formed in a median longitudinal section plane (horizontal in the orientation of the drawing), perpendicular to the electric field lines of the mode being propagated.

The embodiment shown in Figures 3 a, 3b is used for evaluating the transverse components of the dielectric constant, hereinafter indicated by the references ε_{r x} and ε_r_y, whereas the configuration of Figures 4a, 4b is used for evaluating the longitudinal (perpendicular) component ε_{r z} of the dielectric constant.

In a test apparatus comprising waveguide, adaptors and supports, two distinct WR75 waveguides, which differ in the different positions of the slots, are advantageously provided in the embodiments of Figure 3 a and of Figure 4a, respectively. Naturally, a single waveguide comprising both slots 50, 52 could be provided.

With the use of a circular waveguide, naturally, it would suffice to provide a single guide comprising a pair of diametrally opposed longitudinal slots formed in the internal surface of the cavity wall and to rotate the guide through 90° to obtain the two above-described configurations.

The technique used to evaluate the dielectric constant of the thin laminated material is based substantially upon the measurement of the phase difference between the direct transmission S₂1 coefficients in an empty waveguide portion and in a waveguide portion loaded with a sample of the material, and subsequent comparison with a reference model.

The method of evaluating the dielectric constant is based upon the supposition that the velocity of propagation of an electromagnetic wave in a waveguide depends on the dielectric constant of the material which occupies the waveguide cavity, even partially.

Thus, if a dielectric slab is inserted in a waveguide as shown, for example, in Figures 5a- 5e, the propagation constant along the waveguide is different from that of the empty guide and depends on the dielectric constant of the slab. For greater efficiency of measurement, the slab is arranged parallel to the smaller walls of the waveguide in the region of the median section of the larger walls, where the electromagnetic field has the greatest intensity, upon the assumption that the fundamental TE_1O mode is propagated. The phase delay which can be measured between the ports Pl and P2 of the guide depends on the length of the slab, on its thickness, and on its relative dielectric constant. In particular, since the electromagnetic field in the waveguide is polarized along the y axis, a phase measurement of the S₂₁ parameter enables the relative dielectric constant component along the y axis to be characterized.

In the example described, the standard WR75 waveguide selected has the following characteristics: frequency band: 9.84 GHz - 15.00 GHz; internal transverse dimensions: 19.050 mm - 9.525 mm.

It is therefore adapted for the characterization of the dielectric constants of materials at around the central frequency of interest of 13.325 GHz (K band) for microwave applications.

If the waveguide portion is considered as a device with two ports, the dielectric constant of a slab of thin material can be calculated on the basis of the measurement of the difference between the phase of the S₂₁ parameter of the empty guide and the phase of the S₂₁ parameter of the guide loaded with the slab.

Specifically, for a low relative dielectric constant, in particular no greater than 4, its value can be obtained directly and with sufficient accuracy on the basis of the phase difference measured by a vector network analyzer A coupled to the apparatus 10 of the invention by comparison with an analytical reference model of an equivalent transmission line derived from well-known theoretical considerations which will be given only briefly below.

A waveguide with transverse dimensions a and b and length L is considered, in which solely the fundamental TE₁₀ mode is propagated.

The propagation constant at a given operative frequency f₀ is given by the following equation:

in which λo is the wavelength of the free space at the frequency f₀ and X₀ = 2a is the wavelength at the cut-off frequency f_c of the TE₁₀ mode.

The phase delay along the empty waveguide is:

Φ_o = K_Q L

If the waveguide is loaded with a slab of dielectric material arranged symmetrically in the waveguide as shown in Figures 5a-5e, the propagation constant of the loaded waveguide is:

in which the propagation velocity is

where:

t is the slab thickness, a is the larger dimension of the cross-section of the guide, and ε_r is the relative dielectric constant of the material.

The phase delay along the guide loaded with the slab is therefore:

Φ_s = k_s L

The phase difference between the two cases is determined as:

ΔΦ = Φ₀ -Φ_s = (A_o -A_s)-L

and, expressed in terms of ε_r:

The value of ε_r can therefore easily be determined on the basis of the measurement of the phase difference ΔΦ by means of the vector analyzer and by inversion of the equation given above.

Naturally, this equation is valid upon the assumption that solely the fundamental TE₁₀ mode is propagated, which is acceptable for low values of the dielectric constant ε_r. As ε_r increases, the configuration of the electromagnetic fields inside the waveguide may differ significantly from that of the fundamental mode and the approximations made may fail, rendering the above-described analytical method inapplicable.

However, for relative dielectric constant values greater than 4, an accurate evaluation can still be achieved by measuring the phase delay and comparing the values measured with reference scattering data obtained previously by computer simulations, for example, by executing a processing program which can simulate the propagation of a stimulation electromagnetic field in a theoretical model of the waveguide device, in the empty and loaded configurations, based on a finite element calculation method.

The simulations of propagation of a stimulation electromagnetic field in the waveguide are preferably carried out for different hypothetical values of the dielectric constant of the test material so as to collect a plurality of reference data.

The method of characterizing the dielectric constant is described below with reference to Figures 6a and 6b.

In a first operative step, the measurement apparatus 10 is prepared by arranging the set of components provided and connecting it to an available vector network analyzer A. The waveguide 26 is coupled to the analyzer without a dielectric slab. The waveguide is coupled to the respective adaptors 28a, 28b by clamping of the connecting flanges 32a, 36a and 32b, 36b by means of the captive screws, and the flexible cables 30a, 30b for connection to the network analyzer ports are connected to the adaptors 28a, 28b. The cables are then secured to the metal support structure 20 by means of the clamping bands 40a, 40b.

The network analyzer A is programmed to perform a series of frequency measurements within a predetermined frequency range, for example, of between 13.00 GHz and 13.50 GHz, at predetermined intervals, for example, of 0.010 GHz.

The network analyzer is set to measure the S₂₁ parameter and to display S₂1 phase measurement data as a function of the frequency.

The guide 26 is then removed with great care to avoid any movement of other components of the measurement equipment, and is reconfigured by the introduction of the test dielectric slab 12 into the respective slots and then re-coupled to the adaptors. The phase of the S₂₁ parameter is measured again with a frequency sweep and the measurement data are saved.

The phase measurement data in the empty and loaded conditions of the guide are processed by the analyzer or, alternatively, are exported to an associated computer unit C, to produce phase difference data indicative of the phase difference of the transmission coefficient between the above-mentioned propagation conditions.

The relative dielectric constant value is determined, possibly by the computer unit C, by comparison of the phase difference data measured with corresponding values determined by a reference model calculated by the computer unit C or otherwise available, for example, by comparison with corresponding curves of phase difference with respect to frequency obtained by the resolution of analytical expressions or by simulation programs.

The measurement is performed in the frequency domain. If the phase data measured is represented with respect to frequency there is less ripple with a maximum deviation of 0.2° which is due to the electrical discontinuities along the path of the signal, particularly in the adaptors, which lead to imperfect adaptation at the ports of the network analyzer. This residual ripple may produce a slight error in the estimation of the dielectric constant if the phase measurement value is read at a single frequency and not averaged over the entire band of measurement frequencies. With the use of the option of measurement in the time domain, which is available in the network analyzer, the accuracy of the estimation can be improved by suitable filtering.

Figure 7 shows an example of the cutting of the slabs of dielectric material from a plate or strip of sample material. The slab is rectangular and has dimensions such as to have a length which is less than the length of the waveguide device in the operative condition. In particular, Figure 7 shows how it is possible to evaluate the dielectric constant along the two transverse axes x, y in two different measurement operations with the same configuration as Figures 3a, 3b, by placing under test two different slabs cut along two perpendicular axes, as indicated in the drawing. The measurement of the relative dielectric constant along the two transverse axes is advisable to check the hypothesis of isotropy of the material.

In the examples to which this description relates, the samples of laminated dielectric material, with a nominal thickness of 25 mils (0.635 mm), were cut to give rectangular slabs with the following dimensions:

- slab for evaluating the transverse dielectric constant (ε_{r x} or ε_r_y): transverse dimension equal to 10.43 mm, with a tolerance of 0.3 mm, and longitudinal dimension equal to 20 mm, with a tolerance of 0.1 mm;

- slab for evaluating the longitudinal dielectric constant (ε_{r z}): transverse dimension equal to 19.95 mm, with a tolerance of 0.3 mm, and longitudinal dimension of 50 mm, with a tolerance of 0.1 mm.

Measurements were performed on various slabs of different materials to estimate the error associated with the phase measurement and to check the repeatability of the measurements at different times. There was a maximum deviation of the order of ±0.1° in the acquisition of the phases, which corresponds to an uncertainty in the evaluation of the transverse dielectric constant |Δε_r__x,y| < 0.05. The deviation of the phase measurement is due basically to variations in the environmental conditions and, in particular, in temperature. The error was negligible when the measurements were carried out on the same sample with stable and unchanging environmental conditions.

The same accuracy can also be attributed to the tests for evaluating the longitudinal component of the dielectric constant. However, in this case, because of the small thickness of the sample, the associated error, expressed in terms of uncertainty in the dielectric constant value, corresponds to |Δε_{r z}| = 0.2.

Figures 8a, 8b, 9a, 9b, 10a, 10b, 11a, l ib, 12a and 12b show various results of measurements carried out with the apparatus of the invention. Specifically, the drawings numbered with the suffix "a" show the curve of the phase difference, expressed in degrees, with respect to the frequency, expressed in GHz, for a plurality of slabs of the same material. The drawings numbered with the suffix "b", on the other hand, show the technique for the cutting of the dielectric slab tested from an entire plate of sample material.

Figures 8a and 8b show the curve of the phase difference measured in the network analyzer for two slabs of dielectric material obtained from the opposite ends of a strip material sample, from which the homogeneity of the dielectric constant is evident.

Figures 9a and 9b show the curve of the phase difference measured in the network analyzer for two slabs of dielectric material obtained from the opposite ends of a strip material sample, from which a degree of non-homogeneity of the dielectric constant is evident.

Figures 10a and 10b show the results of a measurement of the phase difference with respect to frequency for three different slabs of dielectric material taken from a sample plate in three different, non-aligned sections of the plate, respectively.

Figures 11a and l ib show the results of a measurement of the phase difference with respect to frequency for three different slabs of dielectric material taken from a sample plate in three different, aligned sections of the plate, respectively.

Figures 12a and 12b, on the other hand, show the results of measurements performed on slabs of two types which are oriented in perpendicular directions for the evaluation of the transverse dielectric constant along the x axis and of the transverse dielectric constant along the y axis for the material, respectively. The results of the measurements are grouped in two distinct sets of curves indicative of an anisotropy of the material.

Figures 13 a- 13 c, however, show the results of a measurement of the phase difference with respect to frequency for the evaluation of the longitudinal dielectric constant ε_z. The measurement was performed on two slabs of the same sample, cut as shown in Figure 13b, by two different measurement procedures relating to a "through" calibration and a "full 2- port" calibration of the network analyzer. In Figure 13c the results of the measurements are compared with the phase difference with respect to frequency, calculated by a simulation program such as the CST microwave Studio 5.1 program, with reference to a standard WR75 waveguide loaded with a slab of dielectric material with a length of 50 mm and a thickness of 0.66 mm with hypothetical dielectric constant values ε_r equal to 0.95, 6.15 and 6.35.

Finally, Figures 14a and 14b are two graphs which show the curves of the transverse relative dielectric constant as a function of the frequency for two different materials, evaluated by a method without filtering in the time domain and by a method with filtering in the time domain, which eliminates the residual ripple. The values obtained are substantially in accordance (±0.15) with the test results obtained by a different known technique based on the bad effect in a resonant cavity.

In conclusion, the technique described and illustrated advantageously gives results comparable with those obtained by different and more complex methods. The measurement method according to the invention, however, can be performed very easily with the use of test apparatus as described and a conventional vector network analyzer, preferably with the option for filtering in the time domain, coupled to form a measurement system which can be installed easily for rapid checking of the dielectric constant of a material during its manufacture or for a subsequent evaluation of the dielectric constant of a material prior to use in the production of electronic microwave circuits.

The technique described is advantageously a broad-band technique and the measurements in a range of frequencies are performed automatically by a simple frequency sweep procedure carried out by the network analyzer, the measurement apparatus including no resonant or narrow-band circuits.

The preparation of the test samples is also facilitated since they have a simple rectangular shape and do not have to be treated; for example, the deposition of metallic layers on one or both faces for the production of a test micro-strip guide as in the prior art is not required.

Naturally, the principle of the invention remaining the same, the forms of embodiment and details of construction may be varied widely with respect to those described and illustrated purely by way of non-limiting example, without thereby departing from the scope of protection of the present invention defined by the appended claims.

Claims

1. A system for evaluating the real part of the permittivity of a laminated dielectric material, comprising:

- a waveguide device (26) adapted to propagate an electromagnetic field between an input port (Pl) and an output port (P2), and is arranged to hold at least one slab (12) of dielectric material under test in a longitudinal arrangement, and

- network analyzer means (A) which are operatively coupled to the input and output ports (Pl, P2) of the waveguide device (26) and which are adapted to provide measurement data of the scattering parameters of the device (26), the relative dielectric constant value of the dielectric material under test being determinable by comparison of the measurement data with corresponding scattering data calculated in accordance with a predetermined reference model of propagation in the device, characterized in that the system is arranged: to acquire vectorial measurement data of the transmission coefficient (S₂₁ parameter) of the waveguide device (26), and to determine phase difference data indicative of the difference between the phase of the transmission coefficient in a first propagation condition with the guide empty, that is without the slab (12), and in a second propagation condition with the guide loaded, that is with the slab (12), at least one predetermined operative frequency, evaluation of the relative dielectric constant value being possible by comparison of the said phase difference data with corresponding phase difference data calculated in accordance with the predetermined model.

2. A system according to Claim 1, in which the network analyzer means (A) are arranged to perform a series of vectorial measurements of the transmission coefficient at a plurality of frequencies of the stimulation electromagnetic field, within a predetermined range of operative frequencies.

3. A system according to Claim 1 or Claim 2, characterized in that the analyzer means (A) are arranged to provide the phase difference data that is indicative of the phase difference of the transmission coefficient between the propagation conditions.

4. A system according to any one of Claims 1, 2 and 3, characterized in that it comprises computer means (C) arranged to determine the relative dielectric constant value by comparison of the phase difference data provided by the analyzer means (A) with the phase difference data calculated in accordance with the predetermined model.

5. A system according to any one of the preceding claims, in which the waveguide device (26) is a waveguide section adapted to propagate solely a non-degenerate fundamental mode at the operative frequency/frequencies for the characterization of the material.

6. A system according to Claim 5, in which the waveguide device (26) is a closed, metal waveguide section.

7. A system according to Claim 6, in which the waveguide device (26) is a rectangular waveguide section.

8. A system according to Claim 5, in which, in the operative condition with the waveguide loaded, the longitudinal test slab (12) is arranged parallel to the smaller walls of the waveguide device (26) in the region of the median section of the larger walls, the above-described arrangement being adopted for the evaluation of the transverse relative dielectric constant of the slab (12).

9. A system according to Claim 5, in which, in the operative condition with the waveguide loaded, the longitudinal test slab (12) is arranged parallel to the larger walls of the waveguide device (26), the above-described arrangement being adopted for the evaluation of the longitudinal relative dielectric constant of the slab (12).

10. A system according to Claim 8, in which the waveguide device (26) comprises a pair of opposed longitudinal slots (50) which are formed in the internal surfaces of the larger walls in the region of the median section and are adapted to form seats for the support of the test slab (12).

11. A system according to Claim 9, in which the waveguide device (26) comprises a pair of opposite longitudinal slots (52) which are formed in the internal surfaces of the smaller walls in the region of the median section and are adapted to form seats for the support of the test slab (12).

12. A system according to Claim 6, in which the waveguide device (26) is a circular waveguide section.

13. A system according to Claim 12, in which, in the operative condition with the waveguide loaded, the longitudinal test slab (12) is arranged in the diametral plane in which the electric field lines of the propagation mode lie, the above-described arrangement being adopted for the evaluation of the transverse relative dielectric constant of the slab (12).

14. A system according to Claim 12, in which, in the operative condition with the waveguide loaded, the longitudinal test slab (12) is arranged in the diametral plane perpendicular to the electric field lines of the propagation mode, the above-described arrangement being adopted for the evaluation of the longitudinal relative dielectric constant of the slab (12).

15. A system according to Claim 13 or Claim 14, in which the waveguide device (26) comprises a pair of diametrally opposed longitudinal slots which are formed in the internal surface of the guide wall and are adapted to form seats for the support of the test slab (12).

16. A system according to any one of the preceding claims, in which the test slab (12) is rectangular and has dimensions such as to have a length less than the length of the waveguide device (26) in the operative condition.

17. A system according to any one of the preceding claims, in which the reference model is an analytical mathematical model of a transmission line equivalent to the waveguide device (26) in the empty and loaded configurations.

18. A system according to any one of the preceding claims, in which the reference model is a numerical simulation model comprising scattering data calculated by a processing program adapted to simulate the propagation of a stimulation electromagnetic field in a theoretical model of the waveguide device (26) in the empty and loaded configurations.

19. A system according to Claim 18, in which the simulation model comprises sets of scattering data obtained for different hypothetical values of the dielectric constant of the material under test.

20. A system according to Claim 18 or Claim 19, in which the simulation model is based on a finite element calculation method.

21. A system according to any one of the preceding claims, in which the computer means (C) are arranged to calculate the phase difference data in two different measurement conditions depending on the orientation of the dielectric material of the slab (12) under test.

22. Apparatus (10) for implementing a system for evaluating the relative dielectric constant of a laminated dielectric material according to any one of Claims 1 to 21, characterized in that it comprises, in combination:

- at least one waveguide device (26), adapted to propagate an electromagnetic field between an input port (Pl) and an output port (P2) and arranged to hold a test slab of dielectric material (12);

- a pair of adaptor devices (28a, 28b) adapted to couple the waveguide device (26), at its input and output ports (Pl, P2), to respective cables (30a, 30b) for connection to a network analyzer (A); and

- a structure for the support and stabilization of the waveguide device (26) and adaptor devices (28a, 28b), including: a metal support surface (20); a central upright member (22) arranged on the surface (20) for the support of the waveguide device (26); a pair of lateral upright members (24a, 24b) arranged on the surface (20) for the support of the adaptor devices (28a, 28b); and cable stabilizing means including fastener members (40a, 40b) for securing the connection cables (30a, 30b) to the metal support surface (20) in a firm position.

23. Apparatus (10) according to Claim 22, comprising a pair of waveguide devices (26) which are arranged to hold a slab (12) of dielectric material under test, each device having a pair of opposed longitudinal slots (50; 52) adapted to form seats for the support of the test slab (12), a first waveguide device being adapted to receive the slab in the arrangement adopted for the evaluation of the transverse relative dielectric constant, the second waveguide device being adapted to receive the slab in the arrangement adopted for the evaluation of the longitudinal relative dielectric constant.

24. Apparatus (10) according to Claim 22, comprising a waveguide device (26) which is arranged to hold a slab (12) of dielectric material under test and which has two pairs of opposed longitudinal slots (50, 52) adapted to form seats for the support of the test slab (12), a first pair of slots being adapted to receive the slab in the arrangement adapted for the evaluation of the transverse relative dielectric constant, the second pair of slots being adapted to receive the slab in the arrangement adapted for the evaluation of the longitudinal relative dielectric constant.

25. Apparatus (10) according to any one of Claims 22 to 24, in which the at least one waveguide device (26) is a rectangular, closed, metal waveguide section adapted to propagate solely a non-degenerate fundamental mode at the operative frequency/frequencies for the characterization of the material.

26. Apparatus (10) according to any one of Claims 22 to 24, in which the at least one waveguide device (26) is a circular, closed, metal waveguide section adapted to propagate solely a non-degenerate fundamental mode at the operative frequency/frequencies for the characterization of the material.

27. A method for the evaluation of the relative dielectric constant of a laminated dielectric material, comprising the steps of:

- providing at least one slab (12) of dielectric material under test,

- providing a waveguide device (26) adapted to hold the test slab (12) in a longitudinal arrangement,

- measuring the scattering parameters of the device (26) in order to acquire vectorial measurement data of the transmission coefficient (S₂₁ parameter) in a first propagation condition in an empty guide, that is without the slab (12), and in a second propagation condition in a loaded guide, that is with the slab (12), respectively,

- determining phase difference data indicative of the phase difference of the transmission coefficient between the above-mentioned propagation conditions, at least one predetermined operative frequency, and

- determining the relative dielectric constant value of the dielectric material under test by comparison of the said phase difference data with corresponding phase difference data calculated in accordance with a predetermined reference model of propagation in the device.

28. A method according to Claim 27, comprising the acquisition of a series of vectorial measurement data of the transmission coefficient at a plurality of frequencies of the stimulation electromagnetic field within a predetermined range of operative frequencies.

29. A method according to Claim 27 or Claim 28, comprising the provision, as waveguide device (26), of a rectangular, closed, metal waveguide section adapted to propagate solely a non-degenerate fundamental mode at the operative frequency/frequencies for the characterization of the material.

30. A method according to Claim 29, comprising the arrangement of the test slab (12) in the waveguide device (26) parallel to the smaller walls of the waveguide device in the region of the median section of the larger walls, the above-described arrangement being adopted for the evaluation of the transverse relative dielectric constant of the slab.

31. A method according to Claim 29, comprising the arrangement of the test slab (12) in the waveguide device (26) parallel to the larger walls of the waveguide device, the above-described arrangement being adopted for the evaluation of the longitudinal relative dielectric constant of the slab.

32. A method according to Claim 27 or Claim 28, comprising the provision, as waveguide device (26), of a circular, closed, metal waveguide section adapted to propagate solely a non-degenerate fundamental mode at the operative frequency/frequencies for the characterization of the material.

33. A method according to Claim 32, comprising the arrangement of the test slab (12) in the waveguide device (26) in the diametral plane in which the electric field lines lie, the above-mentioned arrangement being adopted for the evaluation of the transverse relative dielectric constant of the slab.

34. A method according to Claim 32, comprising the arrangement of the test slab (12) in the waveguide device (26) in the diametral plane perpendicular to the electric field lines, the above-described arrangement being adopted for the evaluation of the longitudinal relative dielectric constant of the slab.

35. A method according to any one of Claims 27 to 34, in which the provision of the slab (12) of dielectric material under test includes the production, from a sample of material, of a rectangular slab (12) having dimensions such as to have a length less than the dimension of the waveguide device (26), in the operative condition.

36. A method according to any one of Claims 27 to 35, comprising the calculation of phase difference data in accordance with an analytical mathematical reference model of a transmission line equivalent to the waveguide device (26) in the empty and loaded configurations.

37. A method according to any one of Claims 27 to 35, comprising the execution of a program for the simulation of the propagation of a stimulation electromagnetic field in a theoretical model of the waveguide device (26) in the empty and loaded configurations to obtain reference phase difference data.

38. A method according to Claim 37, in which the simulation program is executed for different hypothetical values of the dielectric constant of the material under test.

39. A method according to Claim 35, comprising the provision of a first test slab (12) obtained from a sample in a first orientation and the provision of a second test slab (12) obtained from the sample in a perpendicular orientation, the phase difference data being calculated in two different measurement conditions depending on the orientation of the dielectric material of the slab, for the evaluation of the relative dielectric constant along a pair of perpendicular transverse directions of the slab.

40. A method according to any one of Claims 27 to 39, comprising a step of the preparation of apparatus (10) for the evaluation of the dielectric constant, which includes:

- the provision of a pair of adaptor devices (28a, 28b) adapted to couple the waveguide device (26), at its input and output ports (Pl, P2), to respective cables (30a, 30b) for connection to a network analyzer (A),

- the arrangement of the waveguide device (26) on a central supporting and stabilizing upright member (22) which extends upwards from a metal support surface (20),

- the arrangement of the adaptor devices (28a, 28b) on a respective pair of lateral supporting and stabilizing upright members (24a, 24b) which extend upwards from the surface (20),

- the securing of the connecting cables (30a, 30b) to the metal support surface (20) in a firm position.