MEASURING DIELECTRIC PROPERTIES OF MATERIALS

Abstract: Apparatus for measuring the complex permittivity and conductivity, or a change in the complex permittivity and conductivity, of a dielectric material, comprises: a structure (1) which can be made to resonate at a radio or microwave frequency when in contact with or adjacent a dielectric material; first means (4), for providing a first indication, related to a change in resonant frequency of said structure when the latter is in contact with or adjacent said material as compared with a reference resonant frequency when not in contact with or adjacent said material; and second means (5, 6, 7, 8), for providing a second indication, related to the return loss when said structure is in contact with or adjacent said material, whereby the complex permittivity and conductivity, or a change in the complex permittivity and conductivity of said material can be calculated using said first and second indications.

Description: MEASURING DIELECTRIC PROPERTIES OF MATERIALS The present invention relates to measuring dielectric properties of materials, that is the permittivity and conductivity, at radio and microwave frequencies. Conventionally, dielectric properties of a material have been measured using a sample of the material prepared for insertion in, for example, a coaxial line or resonant cavity and observing changes in transmission or reflection characteristics. An alternative method uses an open-ended coaxial line in contact with the material whose properties are to be measured, an automatic network analyser being used to measure the complex impedance thus presented to the coaxial line (see IEE Electronics Letters, 15 February 1990, Vol. 24, No.4, pp 234-235, S Jenkins and A W Preece; and J. Phys. E: Sci. Instrum. 22 (1989), pp 757-770, J P Grant, et al). Such a method requires knowing the geometry of the open-ended coaxial line and is particularly suitable for liquids. There is a practical need for a simple means of measuring complex permittivity, or changes in complex permittivity, without using expensive equipment. Resonant structures for use as medical applicators for heating tissue are disclosed in Microwave Theory and Technique (IEEE) 35, pp 1317-1321, R H Johnson, et al and GB-2 193 099 A, but, deliberately, they are substantially insensitive to the dielectric properties, or changes in such properties, of material to be heated. According to the present invention from one aspect, there is provided apparatus for measuring the complex permittivity and conductivity, or a change in the complex permittivity and conductivity, of a dielectric material, the apparatus comprising: a structure which can be made to resonate at a radio or microwave frequency when in contact with or adjacent a dielectric material; first means, for providing a first indication, related to a change in resonant frequency of said structure when the latter is in contact with or adjacent said material as compared with a reference resonant frequency when not in contact with or adjacent said material; and second means, for providing a second indication, related to the return loss when said structure is in contact with or adjacent said material, whereby the complex permittivity and conductivity, or a change in the complex permittivity and conductivity, of said material can be calculated using said first and second indications. According to the present invention from another aspect, there is provided a method of measuring the complex permittivity and conductivity, or a change in the complex permittivity and conductivity, of a dielectric material, the method comprising: resonating a structure in contact with or adjacent said material at a radio or microwave frequency; providing a first indication, related to a change in resonant frequency of said structure when the latter is in contact with or adjacent said material as compared with a reference resonant frequency when not in contact with or adjacent said material; providing a second indication, related to the return loss when said structure is in contact with or adjacent said material; and calculating the complex permittivity and conductivity, or a change in the complex permittivity and conductivity of said material, using said first and second indications. The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is an equivalent circuit diagram of an example of a resonant structure for apparatus according to the present invention; Fig. 2 is a graph of return loss against frequency of an example of such a structure; Fig. 3 is a circuit diagram of apparatus for measuring return loss for apparatus according to the present invention; Fig. 4 is a diagram of one example of a resonant structure for apparatus according to the present invention, in the form of a resonant open-ended coaxial line; and Fig. 5 is a diagram of another example of a resonant structure for apparatus according to the present invention, in the form of a closed resonant structure. Referring to Fig. 1, a structure 1 is made to resonate by a microwave or radio source 2, being coupled to the latter by a coupling having a coupling reactance X c . The structure 1 is equivalent to an inductance L, a capacitance C and losses represented by a resistance R. A material whose dielectric properties are to be monitored is placed in contact with, or adjacent, the resonant structure 1, so inducing additional reactance X x and resistance R x . The resonant frequency and Q-factor are changed by an amount which is a function of the dielectric properties of the material. The change in resonant frequency can be measured, or the original resonant frequency restored by suitable modification of the resonant structure, for example by changing a dimension or varying a reactance. In one example, the behaviour of the resonant structure 1 can be calibrated using liquids or other material having accurately known dielectric properties. Alternatively, for a geometrically and electrically well defined structure 1, absolute measurement of complex permittivity can be made. If the reactance of L and C in parallel in the equivalent circuit in Fig. 1 is X, then series resonance will occur when X = -X c , and the impedance Z, looking into X c , will be a pure resistance. This impedance can be made any value, such as 50 ohm, by adjusting X c with consequent change in resonant frequency. If the reflection coefficient r is 0.001, the return loss is -60dB, and Fig. 2 shows an example of return loss as a function of frequency. When the resonant structure 1 is influenced by the lossy dielectric material, Z and r are complex and Z x = Z. l^T l+r but at resonance Z x becomes real. Note that Z F(X, X c , R) Z x = F(Z, X x , X c , R x ) R x = F(6,σ) X x = F(6,σ) where e = permittivity and σ = conductivity of the unknown dielectric. The relationship between e, σ and r,X x can be calculated or determined experimentally, depending on the resonant structure 1. Various methods are available for measuring resonance and return loss, and depend on the accuracy and simplicity required. An alternative to an automatic network analyser is a simple reflectometer arrangement shown in Fig. 3. Source 2 is a fixed frequency power source of 1 to 20mW and excites the resonant structure 1 through a directional coupler 3 (10 to 20dB) having good isolation, and a matching section 4. Any reflected signal passes into a calibrated attenuator 5 (0 to 60dB), an RF amplifier 6 (30 - 50dB gain), a detector 7 and a signal lever meter 8, reference numeral 9 designating a matched load. With the resonant structure 1 first replaced by a well-matched load, the matching section 4 is adjusted for minimum reflected signal. This matched load is then replaced by a short-circuit and the output of the detector 7 adjusted to a reference level by suitable setting of the power of source 2, gain of amplifier 6 and calibrated attenuator 5. Adjustment of the resonant structure 1 for resonance is by minimising the detected signal, and the return loss obtained from the difference in calibrated attenuator setting required to restore the signal to the reference level obtained with the short circuit. More sophisticated systems using isolators, mixers, modulated sub-carriers and linear detectors can be alternatives. The system illustrated in Fig. 3 is suitable for measuring a lightly damped resonance, so that the resonant structure 1 must not be tightly coupled to a lossy dielectric. Otherwise a frequency swept source is desirable for accurately determining resonance. In Fig. 4, one example of resonant structure 1 is a coaxial structure which comprises metallic tubes 11 and 12 providing two outer sections of a coaxial line of which the inner conductor is a continuous conductor 10 terminated by a sliding short-circuit 13 for tuning the structure. The latter is excited (e.g. at 500 MHz) through a coupling capacitance which is formed between the inner conductor 10 and the inner conductor of a coaxial input 14. Suitable means of fine adjustment of this capacitance is provided by changing the spacing between the inner conductors. A coupling section 15 of the coaxial line is terminated by an open-end 16 of the structure 1 and the inner conductor 10 is held in place by dielectric spacers 17 and 18. Dielectric spacer 17 has dimensions to minimise reflections at interfaces. A flange 19 of metallic tube 11 terminating the coaxial line enables the open-end impedance to be accurately calculated. If the coaxial line section 15 is exactly half a wavelength, the impedance at the open-end 16 is exactly transferred to a coupling plane 20 in a section 21 which joins tubes 11 and 12 and receives the input 14, and resonance occurs when the sliding short-circuit 13 adjusts the length of the coaxial line to be the appropriate amount less than one quarter wavelength. If the coaxial line section 15 is not half a wavelength, but its length is accurately known, then the impedance at the coupling plane 20 is changed but can be calculated. In use, the structure 1 is made to resonate when radiating into air and with the coupling capacitance adjusted by adjusting input 14 for the minimum necessary to obtain a measurable response on an automatic network analyser. The impedance transferred to the coupling plane 20 from the open-and short-circuit ends must now be equal. The coupling capacitance is now increased until the input impedance at input 14 is matched to the network analyser (usually 50 ohm) and the return loss is about 60dB. To maintain the same resonant frequency, the short-circuit length of the line must be readjusted and this change allows the coupling capacitance to be calculated. The open-end 16 is now placed in contact with or adjacent material with unknown dielectric properties and resonance at the measurement frequency is restored by adjustment of the short-circuit length of the coaxial line by sliding the short-circuit 13, the return loss also being measured. In the simplest example of a lossless line and an exact half-wavelength coupling section 15, the change in length of the short-circuited line gives the reactance of the input impedance from Z 0 tan (short-circuit length.π) half-wavelength where Z 0 is the line characteristic impedance. The conductivity or dielectric loss is calculated from the change in input impedance, and this is determined from the measured return loss. Return loss = 20 log p where p= reflection coefficient and input impedance= 50(l-p)/(l+p). Fig. 5 shows another resonant structure 1, which includes an inductance formed by a generally U-shaped, flat high conductivity strip 30, the ends of the U being coupled together by a capacitance formed by a plurality of conductive plates 31. Movement of a conductive disc 32 adjusts the capacitance between additional conductive half-plates 33 attached to the ends of the U-shaped conductive strip 30 in order to tune the resonant frequency. An insulating bush 34 isolates disc 32 from a micrometer assembly 35. Excitation of this resonant circuit is provided by a coaxial input 36 through the capacitance between a conductive plate 37 and one side of the U-shaped conductive strip 30. Fine adjustment of this coupling capacitance is provided by a suitable screw assembly 38 insulated from the conductive plate 37 by an insulating bush 39. The base of the U-shaped conductive strip 30 is secured by suitable means to a low-loss dielectric end cap 40 in such a way that the base of the U is uniformly covered by a relatively thin layer 41 of low-loss dielectric. The end cap 40 is made re-entrant for attachment to a screening container 42. It is important that end cap 40 should insulate the outside of the lower part of the screening container 42 to prevent contact with a liquid dielectric. The thickness of the dielectric layer 41 is chosen to suit the range of values of permittivity and conductivity to be measured. For example, if the conductivity to be measured is high, then damping of the resonant structure must be limited by providing sufficient thickness of the dielectric layer 41 to allow resonant frequency and return loss to be accurately measured. In its simplest application, the structure of Fig. 5 may be used with the measuring system shown in Fig. 3. The coupling capacitance between plate 37 and strip 30 is adjusted to match a 50 ohm input for resonance when radiating into air and the desired operating frequency set by adjustment of micrometer assembly 35. The structure is then applied in contact with or adjacent dielectrics having known characteristics of the order of those to be measured, and calibration of the relationship between permittivity and conductivity and micrometer movement and return loss respectively is established. Alternatively, the resonant structure can be coupled to the source and treated like a resonant cavity where calibration can be used to obtain the coefficients in the equations relating e and σ to frequency change and Q- factor (see Waldron, R A, 1969, "Theory of Electric Waves", van Nostrand Reinhold, London). No accurately prepared material samples are required. In contrast to the structures according to Figs. 4 and 5, applicators such as are mentioned in the introduction are insensitive to load, i.e. resonant frequency is not be much affected by small changes in proximity to load or minor load changes. Expressed another way, in such applicators, return loss should remain >10dB, i.e. standing wave ratio <2 or reflection coefficient <03; whereas the resonant structures according to Figs. 4 and 5 are responsive to load, with relatively large changes in resonant frequency and return loss. The resonant structure of Fig. 5 should have a relatively large inductance and small capacitance to make it sensitive to the proximity of a lossy dielectric. Such applicators as are mentioned in the introduction have relatively small inductance and high capacitance to reduce the effect of adjacent lossy material. However, one feature of these applicators is that they are intended to become mis¬ matched when radiating into air, i.e. become unloaded, and consequently they radiate inefficiently into air and this is intended as a safety feature. The ratio of inductance to capacitance of structures according to Figs. 4 and 5 cannot be increased too much because of losses in the conductor which limit the Q-factor at¬ tainable. Conversely, the ratio cannot be made too low, because the circuit becomes difficult to drive. At the aperture of the open-ended coaxial line structure according to Fig. 4, there is a small field at the open end which is confined to the volume surrounding the end of the inner conductor, whereas the resonant structure of Fig. 5 has a relatively much larger conductor area radiating into surrounding space.

Claims: CLAIMS 1. Apparatus for measuring the complex permittivity and conductivity, or a change in the complex permittivity and conductivity, of a dielectric material, the apparatus comprising: a structure (1) which can be made to resonate at a radio or microwave frequency when in contact with or adjacent a dielectric material; first means (4), for providing a first indication, related to a change in resonant frequency of said structure when the latter is in contact with or adjacent said material as compared with a reference resonant frequency when not in contact with or adjacent said material; and second means (5,6,7,8), for providing a second indication, related to the return loss when said structure is in contact with or adjacent said material, whereby the complex permittivity and conductivity, or a change in the complex permittivity and conductivity, of said material can be calculated using said first and second indications. 2. Apparatus according to claim 1, wherein said structure (1) comprises an adjustable open-ended co-axial s ructure (11-19). 3. Apparatus according to claim 1, wherein said structure (1) comprises an adjustable plate-form inductance and capacitance assembly (30-33). 4. A method of measuring the complex permittivity and conductivity, or a change in the complex permittivity and conductivity, of a dielectric material, the method comprising: resonating a structure (1) in contact with or adjacent said material at a radio or microwave frequency; providing a first indication, related to a change in resonant frequency of said structure when the latter is in contact with or adjacent said material as compar with a reference resonant frequency when not in conta with or adjacent said material; providing a second indication, related to the retu loss when said structure is in contact with or adjace said material; and calculating the complex permittivity a conductivity, or a change in the complex permittivity an conductivity of said material, using said first an second indications.

Inventors: JOHNSON REGINALD HARRY, PREECE ALAN WILLIAM, JOHNSON, REGINALD, HARRY, PREECE, ALAN, WILLIAM
Applicants: UNIV BRISTOL, THE UNIVERSITY OF BRISTOL
Application year: 1992
Main classification: G01R27
IPC codes: G01R27